View source for Classic Graphics Primitives

== Graphics Primitives ==

This section defines the display routines for use on Classic Amiga hardware. These routines show you how to form and manipulate a display, including the following aspects of display use:

* How to query the graphics system to find out what type of video monitor is attached and which graphics modes can be displayed on it.
* How to identify the memory area that you wish to have displayed.
* How to position the display area window to show only a certain portion of a larger drawing area.
* How to split the screen into as many vertically stacked slices as you wish.
* How to determine which horizontal and vertical resolution modes to use.
* How to determine the current correct number of pixels across and lines down for a particular section of the display.
* How to specify how many color choices per pixel are to be available in a specific section of the display.

== Components of a Display ==

In producing a display, you are concerned with two primary components: sprites and the playfield. Sprites are the easily movable parts of the display. The playfield is the static part of the display and forms a backdrop against which the sprites can move and with which the sprites can interact.

This chapter covers the creation of the background. Sprites are described in the “Graphics Sprites, Bobs and Animation” chapter.

=== Introduction to Raster Displays ===

There are three major television standards in common use around the world: NTSC, PAL, and SECAM. NTSC is used primarily in the United States and Japan; PAL and SECAM are used primarily in Europe. The Amiga currently supports both NTSC and PAL. The major differences between the two systems are refresh frequency and the number of scan lines produced. Where necessary, the differences will be described and any special considerations will be mentioned.

The Amiga produces its video displays on standard television or video monitors by using raster display techniques. The picture you see on the video display screen is made up of a series of horizontal video lines stacked one on top of another, as illustrated in the following figure. Each line represents one sweep of an electronic video beam, which “paints” the picture as it moves along. The beam sweeps from left to right, producing the full screen one line at a time. After producing the full screen, the beam returns to the top of the display screen.

[[File:LibFig27-1.png|frame|center|How the Video Display Picture Is Produced]]

The diagonal lines in the figure show how the video beam returns to the start of each horizontal line.

==== Effect of Display Overscan on the Viewing Area ====

To assure that the picture entirely fills the monitor (or television) screen, the manufacturer of the video display device usually creates a deliberate ''overscan''. That is, the video beam is swept across an area that is larger than the viewable region of the monitor.

The video beam actually covers 262 vertical lines (312 for PAL). The user, however, sees only the portion of the picture that is within the center region of the display, typically surrounded by a border as illustrated in the figure below. The center region is nominally about 200 lines high on an NTSC machine (256 lines for PAL). Overscan also limits the amount of video data that can appear on each display line. The width of the center region is nominally, about 320 pixels for both PAL and NTSC.

[[File:LibFig27-2.png|frame|center|Display Overscan Restricts Usable Picture Area]]

The flexibility of the Amiga graphics subsystem allows the overscan region, which normally forms the border of the display, to be used for application graphics instead. So the nominal dimensions given above can be enlarged.

The time during which the video beam is below the bottom line of the viewable region and above the first line is called the ''vertical blanking interval''. The recommended minimum to allow for this interval is 21 lines for NTSC (29 lines for PAL). So, for applications that take full advantage of the overscan area, a maximum of 241 usable lines in NTSC (283 in PAL) can be achieved. The display resolution can also be changed by changing the Amiga display mode as discussed in the sections below.

==== Color Information for the Video Lines ====

The hardware reads the system display memory to obtain the color information for each line. As the video display beam sweeps across the screen producing the display line, it changes color, producing the images you have defined. On the current generation of Amiga hardware, there are 4,096 possible colors.

=== Interlaced and Non-Interlaced Modes ===

In producing the complete display (262 lines in NTSC, 312 in PAL), the video display device produces the top line, then the next lower line, then the next, until it reaches the bottom of the screen. When it reaches the bottom, it returns to the top to start a new scan of the screen. Each complete set of lines is called a ''display field''. It takes about 1/60th of a second to produce a complete NTSC display field (1/50th of a second for PAL).

The Amiga has two vertical display modes: ''interlaced'' and ''non-interlaced''. In non-interlaced mode, the video display produces the same picture for each successive display field. A non-interlaced NTSC display normally has about 200 lines in the viewable area (up to a maximum of 241 lines with overscan) while a PAL display will normally show 256 lines (up to a maximum of 283 with overscan).

With interlaced mode, the amount of information in the viewable area can be doubled. On an NTSC display this amounts to 400 lines (482 with overscan), while on a PAL display it amounts to 512 lines (566 with overscan).

For interlaced mode, the video beam scans the screen at the same rate (1/60th of a second per complete NTSC video display field); however, it takes two display fields to form a complete video display picture and twice as much display memory to store line data. During the first of each pair of display fields, the system hardware shows the odd-numbered lines of an interlaced display (1, 3, 5, and so on). During the second display field, it shows the even-numbered lines (2, 4, 6 and so on). The second field is positioned slightly lower so that the lines in the second field are “interlaced” with those of the first field, giving the higher vertical resolution of this mode.

Interlaced Mode – Display Fields and Data in Memory

<table>
<tr class="odd">
<td align="left">'''Data as Displayed'''</td>
<td align="left">'''Data In Memory'''</td>
</tr>
<tr class="even">
<td align="left">Odd field - Line 1</td>
<td align="left">Line 1</td>
</tr>
<tr class="odd">
<td align="left">Even field - Line 1</td>
<td align="left">Line 2</td>
</tr>
<tr class="even">
<td align="left">Odd field - Line 2</td>
<td align="left">Line 3</td>
</tr>
<tr class="odd">
<td align="left">Even field - Line 2</td>
<td align="left">Line 4</td>
</tr>
<tr class="even">
<td align="left">. . .</td>
<td align="left">. . .</td>
</tr>
<tr class="odd">
<td align="left">Odd field - Line 200</td>
<td align="left">Line 399</td>
</tr>
<tr class="even">
<td align="left">Even field - Line 200</td>
<td align="left">Line 400</td>
</tr>
</table>

The following figure shows a display formed as display lines 1, 2, 3, 4, ... 400. The 400-line interlaced display uses the same physical display area as a 200-line non-interlaced display.

[[File:LibFig27-4.png|frame|center|Interlaced Mode Doubles Vertical Resolution]]

During an interlaced display, it appears that both display fields are present on the screen at the same time and form one complete picture. However, interlaced displays will appear to flicker if adjacent (odd and even) scan lines have contrasting brightness. Choosing appropriate colors for your display will reduce this flicker considerably. This phenomenon can also be reduced by using a long-persistence monitor, or alleviated completely with a hardware de-interlacer.

=== Low, High and Super-High Resolution Modes ===

The Amiga also has three horizontal display modes: low-resolution (or ''Lores''), high-resolution (''Hires'') and super-high-resolution (''SuperHires'').

Normally, these three horizontal display modes have a width of 320 for Lores, 640 for Hires or 1,280 for SuperHires on both PAL and NTSC machines. However, by taking full advantage of the overscan region, it is possible to create dispays up to 362 pixels wide in Lores mode, 724 pixels wide in Hires or 1,448 pixels wide in SuperHires. Usually, however, you should use the standard values (320, 640 or 1,280) for most applications.

In general, the number of colors available in each display mode decreases as the resolution increases. The Amiga has two special display modes that can be used to increase the number of colors available. HAM is ''Hold-And-Modify'' mode, EHB is ''Extra-Half-Brite'' mode.

Hold-And-Modify (HAM) allows you to display the entire palette of 4,096 colors on-screen at once with certain restrictions, explained later.

Extra-Half-Brite allows for 64 colors on-screen at once; 32 colors plus 32 additional colors that are half the intensity of the first 32. For example, if color 1 is defined as 0xFFF (white), then color 33 is 0x777 (grey).

==== Display Modes, Colors, and Requirements ====

The following chart lists all of the display modes that are available.

{| class="wikitable"
! 15 kHz Amiga Display Modes
! NTSC
! PAL
! Maximum Colors
! Supports HAM/EHB
|-
| Lores || 320x200 || 320x256 || 32 of 4,096 || Yes
|-
| Lores-Interlaced || 320x400 || 320x512 || 32 of 4,096 || Yes
|-
| Hires || 640x200 || 640x256 || 16 of 4,096 || No
|-
| Hires-Interlaced || 640x400 || 640x512 || 16 of 4,096 || No
|-
| SuperHires* || 1,280x200 || 1,280x256 || 4 out of 64 || No
|-
| SuperHires-Interlaced* || 1,280x400 || 1,280x512 || 4 out of 4,096 || No
|}

<nowiki>*</nowiki> Requires ECS

{| class="wikitable"
! 31 kHz Amiga Display Modes*
! Default Resolution
! Maximum Colors
! Supports HAM/EHB
|-
| VGA-ExtraLores
| 160x480
| 32 out of 4,096
| Yes
|-
| VGA-ExtraLores-Interlace
| 160x960
| 32 out of 4,096
| Yes
|-
| VGA-Lores
| 320x480
| 16 out of 4,096
| No
|-
| VGA-Lores-Interlace
| 320x960
| 16 out of 4,096
| No
|-
| Productivity
| 640x480
| 4 out of 64
| No
|-
| Productivity-Interlace
| 640x960
| 4 out of 64
| No
|}

<nowiki>*</nowiki> 31 kHz modes require ECS and either a bi-scan or multi-scan monitor

{| class="wikitable"
! A2024* Display Modes
! NTSC
! PAL
! Maximum Colors
|-
| A2024-10Hz || 1,008x800 || 1,008x1,024 || 4 out of 4 grey levels
|-
| A2024-15Hz || 1,008x800 || 1,008x1,024 || 4 out of 4 grey levels
|}

<nowiki>*</nowiki> A2024 modes require special hardware

=== About ECS ===

ECS stands for ''Enhanced Chip Set'', the latest version of the Amiga’s custom chips that provides for improved graphics capabilities. Some of the special features of the Amiga’s graphics sub-system such as the VGA, Productivity and SuperHires display modes require the ECS.

==== SuperHires (35 nanosecond) Pixel Resolutions ====

The enhanced version of the Denise chip can generate SuperHires pixels that are twice as fine as Hires pixels. It is convenient to refer to pixels here by their speed, rather than width, for reasons that will be explained below. They are approximately 35nS long, while Hires are 70nS, and Lores 140nS. In the absence of any other features, this can bring a new mode with nominal dimensions of 1,280x200 (NTSC) or 1,280x256 (PAL). This mode requires the ECS Agnus chip as well.

When Denise is generating these new fast pixels, simple bandwidth arithmetic indicates that at most two bitplanes can be supported. Also note that with two bitplanes, DMA bandwidth is saturated. The palette for SuperHires pixels is also restricted to 64 colors.

==== Productivity Mode ====

The enhanced version of the Denise chip can support monitor horizontal scan frequencies of 31kHz, twice the old 15.75kHz rate. This provides over 400 non-interlaced horizontal lines in a frame, but requires the use of a multiple scan rate, or multi-sync monitor.

This effect speeds up the video beam roughly by a factor of two, which has the side effect of doubling the width of a pixel emitted at a given speed. Thus, for a given Denise mode, pixels are twice as fat, and there are half as many on a given line.

The increased scan rate interacts with all of the Denise modes. So with both SuperHires (35nS) pixels and the double scan rate the display generated would be 640 pixels wide by more than 400 rows, non-interlaced, with up to four colors from a palette of 64. This combination is termed Productivity mode, and the default international height is 480 rows.

This conforms, in a general way, to the VGA Mode 3 Standard ''8514/A''.

The support in Agnus is actually more flexible, and gives the ability to conform to special-purpose modes, such as displays synchronized to motion picture cameras.

==== Selectable PAL/NTSC ====

The Enhanced Chip Set can be set to NTSC or PAL modes under software control. Its initial default behavior is determined by a jumper or trace on the system motherboard. This has no bearing on Productivity mode and other programmable scan operations, but the new system software can support displays in either mode.

==== Determining Chip Versions ====

It is possible to ascertain whether the ECS chips are in the machine at run time by looking in the ChipRevBits0 field of the GfxBase structure. If this field contains the flag for the chip you are interested in (as defined in the &lt;gfxbase.h&gt; include file), then that chip is present.

For example, if the C statement (GfxBase-&gt;ChipRevBits0 &amp; GFXF_HR_AGNUS) evaluates to non-zero, then the machine contains the ECS version of the Agnus chip and has advanced features such as the ability to handle larger rasters. Older Agnus chips were capable of handling rasters up to 1,024 by 1,024 pixels. The ECS Agnus can handle rasters up to 16,384 by 16,384 pixels.

If (GfxBase-&gt;ChipRevBits0 &amp; GFXF_HR_DENISE) is non-zero, then the ECS version of the Denise chip is present. Having both the ECS Agnus and ECS Denise present allows for the special SuperHires, VGA and Productivity display modes. For more information on ECS and the custom chips, refer to the ''Amiga Hardware Reference Manual''.

=== Forming an Image ===

To create an image, you write data (that is, you “draw”) into a memory area in the computer. From this memory area, the system can retrieve the image for display. You tell the system exactly how the memory area is organized, so that the display is correctly produced. You use a block of memory words at sequentially increasing addresses to represent a rectangular region of data bits. The following figure shows the contents of three example memory words: 0 bits are shown as blank rectangles, and 1 bits as filled-in rectangles.

[[File:LibFig27-5.png|frame|center|Sample Memory Words]]

The system software lets you define linear memory as rectangular regions, called ''bitplanes''. The figure below shows how the system would organize three sequential words in memory into a rectangular bitplane with dimensions of 16x3 pixels.

[[File:LibFig27-6.png|frame|center|A Rectangular Bitplane Made from 3 Memory Words]]

The following figure shows how 4,000 words (8,000 bytes) of memory can be organized to provide enough bits to define a single bitplane of a full-screen, low-resolution video display (320x200).

[[File:LibFig27-7.png|frame|center|Bitplane for a Full-screen, Low-resolution Display]]

Each memory data word contains 16 data bits. The color of each pixel on a video display line is directly related to the value of one or more data bits in memory, as follows:

* If you create a display in which each pixel is related to only one data bit, you can select from only two possible colors, because each bit can have a value of only 0 or 1.
* If you use two bits per pixel, there is a choice of four different colors because there are four possible combinations of the values of 0 and 1 from each of the two bits.
* If you specify three, four, or five bits per pixel, you will have eight, sixteen, or thirty-two possible choices of a color for a pixel.
* If you use six bits per pixel, then depending on the video mode (EHB or HAM), you will have sixty-four or 4,096 possible choices for a pixel.

To create multicolored images, you must tell the system how many bits are to be used per pixel. The number of bits per pixel is the same as the ''number of bitplanes'' used to define the image.

As the video beam sweeps across the screen, the system retrieves one data bit from each bitplane. Each of the data bits is taken from a different bitplane, and one or more bitplanes are used to fully define the video display screen. For each pixel, data-bits in the same x,y position in each bitplane are combined by the system hardware to create a binary value. This value determines the color that appears on the video display for that pixel.

[[File:LibFig27-8.png|frame|center|Bits from Each Bitplane Select Pixel Color]]

You will find more information showing how the data bits actually select the color of the displayed pixel in the section called “ViewPort Color Selection”.

=== Role of the Copper (Coprocessor) ===

The Amiga has a special-purpose coprocessor, called the ''Copper'', that can control nearly the entire graphics system. The Copper can control register updates, reposition sprites, change the color palette, and update the blitter. The graphics and animation routines use the Copper to set up lists of instructions for handling displays, and advanced programmers can create their own custom Copper lists.


== Display Routines and Structures ==

{{Note|title=Caution|text=This section describes the lowest-level graphics interface to the system hardware. If you use any of the routines and the data structures described in these sections, your program will essentially take over the entire display. In general, this is not compatible with Intuition’s multiwindow operating environment since Intuition calls these low-level routines for you.}}

The descriptions of the display routines, as well as those of the drawing routines, occasionally use the same terminology as that in the Intuition chapters. These routines and data structures are the same ones that Intuition software uses to produce its displays.

The computer produces a display from a set of instructions you define. You organize the instructions as a set of parameters known as the View structure (see the &lt;graphics/view.h&gt; include file for more information).

The following figure shows how the system interprets the contents of a View structure. This drawing shows a complete display composed of two different component parts, which could (for example) be a low-resolution, multicolored part and a high-resolution, two-colored part.

[[File:LibFig27-9.png|frame|center|The Display Is Composed of ViewPorts]]

A complete display consists of one or more ViewPorts, whose display sections are vertically separated from each other by at least one blank scan line (non-interlaced). (If the system must make many changes to the display during the transition from one ViewPort to the next, there may be two or more blank scanlines between the ViewPorts.)

The viewable area defined by each ViewPort is rectangular. It may be only a portion of the full ViewPort, it may be the full ViewPort, or it may be ''larger'' than the full ViewPort, allowing it to be moved within the limits of its DisplayClip (discussed later). You are essentially defining a display consisting of a number of stacked rectangular areas in which separate sections of graphics rasters can be shown.

=== Limitations on the Use of Viewports ===

The system software for defining ViewPorts allows only vertically stacked fields to be defined.

The following figure shows acceptable and unacceptable display configurations.

[[File:LibFig27-10.png|frame|center|Correct and Incorrect Uses of ViewPorts]]

A ViewPort is related to the custom screen option of Intuition. In a custom screen, you can split the screen into slices as shown in the “correct” illustration of the above figure. Each custom screen can have its own set of colors, use its own resolution, and show its own display area.

=== Characteristics of a ViewPort ===

To describe a ViewPort fully, you need to set the following parameters: height, width, depth and display mode.

In addition to these parameters, you must tell the system the location in memory from which the data for the ViewPort display should be retrieved (by associating with it a BitMap structure) and how to position the final ViewPort display on the screen. The ViewPort will take on the user’s default Workbench colors unless otherwise instructed with a ColorMap. See the section called “Preparing the ColorMap Structure” for more information.

=== ViewPort Size Specifications ===

The following figure illustrates that the variables DHeight, and DWidth specify the size of a ViewPort.

[[File:LibFig27-11.png|frame|center|Size Definition for a ViewPort]]

==== ViewPort Height ====

The DHeight field of the ViewPort structure determines how many video lines will be reserved to show the height of this display segment. The size of the actual segment depends on whether you define a non-interlaced or an interlaced display. An interlaced ViewPort displays twice as many lines as does a non-interlaced ViewPort in the same physical height.

For example, a complete View consisting of two ViewPorts might be defined as follows:

<ul>
<li><p>ViewPort</p>
<p>1 is 150 lines, high-resolution mode (uses the top three-quarters of the display).</p></li>
<li><p>ViewPort</p>
<p>2 is 49 lines of low-resolution mode (uses the bottom quarter of the display and allows the space for the required blank line between ViewPorts).</p></li></ul>

Initialize the height directly in DHeight. Nominal height for a non-interlaced display is 200 lines for NTSC, 256 for PAL. Nominal height for an interlaced display is 400 lines for NTSC, 512 for PAL.

To set your ViewPort to the maximum supported (displayable) height, use the following code fragment:

<pre>struct DimensionInfo querydims;
struct Rectangle *oscan;
struct ViewPort viewport;

if (GetDisplayInfoData( NULL,(UBYTE *)&amp;querydims, sizeof(struct DimensionInfo),
                        DTAG_DIMS, modeID ))
{
    /* Use StdOScan instead of MaxOScan to get standard overscan */
    /* dimensions as set by the user in Overscan Preferences     */
    oscan = &amp;querydims.MaxOScan;
    viewPort-&gt;DHeight = oscan-&gt;MaxY - oscan-&gt;MinY + 1;
}</pre>

==== ViewPort Width ====

The DWidth variable in the ViewPort structure determines how wide, in pixels, the display segment will be. To set your ViewPort to the maximum supported (displayable) NTSC high-resolution width, use the following fragment:

<pre>
struct DimensionInfo querydims;
struct Rectangle *oscan;
struct ViewPort viewport;

/* Use PAL_MONITOR_ID instead of NTSC_MONITOR_ID to get PAL dimensions */
if (GetDisplayInfoData( NULL,(UBYTE *)&amp;querydims, sizeof(querydims),
    DTAG_DIMS, NTSC_MONITOR_ID|HIRES_KEY ))
{
    /* Use StdOScan instead of MaxOScan to get standard overscan */
    /* dimensions as set by the user in Overscan Preferences     */
    oscan = &amp;querydims.MaxOScan;
    viewPort-&gt;DWidth = oscan-&gt;MaxX - oscan-&gt;MinX + 1;
}
</pre>

You may specify a smaller value of pixels per line to produce a narrower display segment or simply set ViewPort.DWidth to the nominal value for this resolution.

Although the system software allows you define low-resolution displays as wide as 362 pixels and high-resolution displays as wide as 724 pixels, you should use caution in exceeding the normal values of 320 or 640, respectively. Because display overscan varies from one monitor to another, many video displays will not be able to show all of a wider display, and sprite display may also be affected. However, if you use the standard overscan values (DimensionInfo.StdOScan) provided by the function GetDisplayInfoData() as shown above, the user’s preference for the size of the display will be satisfied.

If you are using hardware sprites or VSprites with your display, and you specify ViewPort widths exceeding 320 or 640 pixels (for low or high-resolution, respectively), it is likely that some hardware sprites will not be properly rendered on the screen.

These sprites may not be rendered because playfield DMA (direct memory access) takes precedence over sprite DMA when an extra-wide display is produced. See the ''Amiga Hardware Reference Manual'' for a more complete description of this phenomenon.

=== ViewPort Color Selection ===

The maximum number of colors that a ViewPort can display is determined by the depth of the BitMap that the ViewPort displays. The depth is specified when the BitMap is initialized.

See the section below called “Preparing the BitMap Structure.”

Depth determines the number of bitplanes used to define the colors of the rectangular image you are trying to build (the raster image) and the number of different colors that can be displayed at the same time within a ViewPort.

For any single pixel, the system can display any one of 4,096 possible colors.

The following table shows depth values and the corresponding number of possible colors for each value.

{| class="wikitable"
|+ Depth Values and Number of Colors in the ViewPort
! Colors !! Depth Value !! Notes
|-
| 2 || 1
|-
| 4 || 2
|-
| 8 || 3 || 1
|-
| 16 || 4 || 1, 2
|-
| 32 || 5 || 1, 2, 3
|-
| 16 || 6 || 1, 4
|-
| 64 || 6 || 1, 2, 3, 5
|-
| 4,096 || 6 || 1, 2, 3, 6
|}

Notes:

# Not available for SUPERHIRES.
# Single-playfield mode only - DUALPF ''not'' one of the ViewPort’s attributes.
# Low-resolution mode only - neither HIRES nor SUPERHIRES one of the ViewPort attributes.
# Dual Playfield mode - DUALPF is an attribute of this ViewPort. Up to eight colors (in three planes) for each playfield.
# Extra-Half-Brite mode - EXTRA_HALFBRITE is an attribute of this ViewPort.
# Hold-And-Modify mode only - HAM is an attribute of this ViewPort.

The color palette used by a ViewPort is specified in a ColorMap. See the section called “Preparing the ColorMap” for more information.

Depending on whether single- or dual-playfield mode is used, the system will use different color register groupings for interpreting the on-screen colors. The table below details how the depth and the different ViewPort modes affect the registers the system uses.

{| class="wikitable"
|+ Color Registers Used in Single-playfield Mode
! Color !! Depth !! Registers Used
|-
| 1 || 0,1
|-
| 2 || 0-3
|-
| 3 || 0-7
|-
| 4 || 0-15
|-
| 5 || 0-31 || (if EXTRA_HALFBRITE is an attribute of this ViewPort.)
|-
| 6 || 0-31 || (if HAM is an attribute of this ViewPort.)
|}

The following table shows the five possible combinations when DUALPF ''is'' an attribute of the ViewPort.

{| class="wikitable"
|+ Color Registers Used in Dual-playfield Mode
! Depth (PF-1} || Color Registers !! Depth (PF-2) || Color Registers
|-
| 1 || 0,1 || 1 || 8,9
|-
| 2 || 0-3 || 1 || 8,9
|-
| 2 || 0-3 || 2 || 8-11
|-
| 3 || 0-7 || 2 || 8-11
|-
| 3 || 0-7 || 3 || 8-15
|}

=== ViewPort Display Modes ===

The system has many different display modes that you can specify for each ViewPort. Under 1.3, the eight constants that control the modes are DUALPF, PFBA, HIRES, SUPERHIRES, LACE, HAM, SPRITES, and EXTRA_HALFBRITE. Some, but not all of the modes can be combined in a ViewPort. HIRES and LACE combine to make a high-resolution, interlaced ViewPort, but HIRES and SUPERHIRES conflict, and cannot be combined.

Set the display mode for a ViewPort by using the VideoControl() function as described in the section on “Monitors, Modes and the Display Database” later in this chapter.

The DUALPF and PFBA modes are related. DUALPF tells the system to treat the raster specified by this ViewPort as the first of two independent and separately controllable playfields. It also modifies the manner in which the pixel colors are selected for this raster (see the above table).

When PFBA is specified, it indicates that the second playfield has video priority over the first one. Playfield relative priorities can be controlled when the playfield is split into two overlapping regions. Single-playfield and dual-playfield modes are discussed in “Advanced Topics” below.

HIRES tells the system that the raster specified by this ViewPort is to be displayed with (nominally) 640 horizontal pixels, rather than the 320 horizontal pixels of Lores mode.

SUPERHIRES tells the system that the raster specified by this ViewPort is to be displayed with (nominally) 1,280 horizontal pixels. This can be used with 31 kHz scan rates to provide the VGA and Productivity modes. SUPERHIRES modes require ECS. See the section on “Determining Chip Versions” earlier in this chapter for an explanation of how to find out if the ECS is present.

LACE tells the system that the raster specified by this ViewPort is to be displayed in interlaced mode. If the ViewPort is non-interlaced and the View is interlaced, the ViewPort will be displayed at its specified height and will look only slightly different than it would look when displayed in a non-interlaced View (this is handled by the system automatically). See “Interlaced Mode vs. Non-interlaced Mode” below for more information.

HAM tells the system to use “hold-and-modify” mode, a special mode that lets you display up to 4,096 colors on screen at the same time. It is described in the “Advanced Topics” section.

SPRITES tells the system that you are using sprites in this display (either VSprites or Simple Sprites). The system will load color registers for the sprites. Note that since the mouse pointer is a sprite, omitting this mode will prevent the mouse pointer from being displayed when this ViewPort is frontmost.

See the “Graphics Sprites, Bobs and Animation” chapter for more information about sprites.

EXTRA_HALFBRITE tells the system to use the Extra-Half-Brite mode, a special mode that allows you to display up to 64 colors on screen at the same time. It is described in the “Advanced Topics” section.

If you peruse the &lt;graphics/view.h&gt; include file you will see another flag, EXTENDED_MODE. Never set this flag yourself; it is used by the system to control more advanced mode features.

Be sure to read the section on “Monitors, Modes and the Display Database” for additional information about the ViewPort mode.

==== Single-playfield Mode vs. Dual-playfield Mode ====

When you specify single-playfield mode you are asking that the system treat all bitplanes as part of the definition of a single playfield image. Each of the bitplanes defined as part of this ViewPort contributes data bits that determine the color of the pixels in a single playfield.

[[File:LibFig27-12.png|frame|center|A Single-playfield Display]]

If you use dual-playfield mode, you can define two independent, separately controllable playfield areas as shown below.

[[File:LibFig27-13.png|frame|center|A Dual-playfield Display]]

In the previous figure, PFBA was included in the display mode. If PFBA had not been included, the relative priorities would have been reversed; playfield 2 would have appeared to be behind playfield 1.

=== Low-resolution Mode vs. High-resolution Mode ===

In LORES mode, horizontal lines of 320 pixels fill most of the ordinary viewing area. The system software lets you define a screen segment width up to 362 pixels in this mode, or you can define a screen segment as narrow as you desire (minimum of 16 pixels). In HIRES mode, 640 pixels fill a horizontal line. In this mode you can specify any width from 16 to 724 pixels. In SUPERHIRES mode, 1,280 pixels fill a horizontal line. In this mode you can specify any width from 16 to 1,448 pixels. The fact that many monitor manufacturers set their monitors to overscan the video display normally limits you to showing only 16 to 320 pixels per line in LORES, 16 to 640 pixels per line in HIRES, or 16 to 1,280 pixels per line in SUPERHIRES. The user can set the monitor’s viewable screen size with the Preferences Overscan editor.

[[File:LibFig27-14.png|frame|center|How HIRES and SUPERHIRES Affect the Width of Pixels]]

==== Interlaced Mode vs. Non-interlaced Mode ====

In interlaced mode, there are twice as many lines available as in non-interlaced mode, providing better vertical resolution in the same display area.

[[File:LibFig27-15.png|frame|center|How LACE Affects Vertical Resolution]]

If the View structure does not specify LACE, and the ViewPort specifies LACE, only the top half of the ViewPort data will be displayed.

If the View structure specifies LACE and the ViewPort is non-interlaced, the same ViewPort data will be repeated in both fields. The height of the ViewPort display is the height specified in the ViewPort structure.

If both the View and the ViewPort are interlaced, the ViewPort will be built with double the normal vertical resolution. That means it will need twice as much data space in memory as a non-interlaced picture to fill the display.

==== ViewPort Display Memory ====

The picture you create in memory can be larger than the screen image that can be displayed within your ViewPort. This big picture (called a raster and represented by the BitMap structure) can have a maximum size dependent upon the version of the Agnus chip in the Amiga. The ECS Agnus can handle rasters up to 16,384x16,384 pixels. Older Agnus chips are limited to rasters up to 1,024x1,024 pixels. The section on [[#Determining Chip Versions|Determining Chip Versions]] explains how to find out which Agnus is installed.

The example in the following figure introduces terms that tell the system how to find the display data and how to display it in the ViewPort. These terms are RHeight, RWidth, RyOffset, RxOffset, DHeight, DWidth, DyOffset and DxOffset.

[[File:LibFig27-16.png|frame|center|ViewPort Data Area Parameters]]

The terms RHeight and RWidth do not appear in actual system data structures. They refer to the dimensions of the raster and are used here to relate the size of the raster to the size of the display area.

RHeight is the number of rows in the raster and RWidth is the number of bytes per row times 8. The raster shown in the figure is too big to fit entirely in the display area, so you tell the system which pixel of the raster should appear in the upper left corner of the display segment specified by your ViewPort. The variables that control that placement are RyOffset and RxOffset.

To compute RyOffset and RxOffset, you need RHeight, RWidth, DHeight, and DWidth. The DHeight and DWidth variables define the height and width in pixels of the portion of the display that you want to appear in the ViewPort. The example shows a full-screen, low-resolution mode (320-pixel), non-interlaced (200-line) display formed from the larger overall picture.

Normal values for RyOffset and RxOffset are defined by the formulas:

 0 < = RyOffset < = (RHeight - DHeight)
 0 < = RxOffset < = (RWidth - DWidth)

Once you have defined the size of the raster and the section of that raster that you wish to display, you need only specify where to put this ViewPort on the screen. This is controlled by the ViewPort variables DyOffset and DxOffset. These are offsets relative to the View.DxOffset and DyOffset.

Possible NTSC values for DyOffset range from -23 to +217 (-46 to +434 if the ViewPort is interlaced), PAL values range from -15 to +267 (-30 to +534 for interlaced ViewPorts). Possible values for DxOffset range from -18 to +362 (-36 to +724 if the ViewPort is Hires, -72 to +1,448 if SuperHires), when the View is in its default, initialized position.

The parameters shown in the figure above are distributed in the following data structures:

* View (information about the whole display) includes the variables that you use to position the whole display on the screen. The View structure contains a Modes field used to determine if the whole display is to be interlaced or non-interlaced. It also contains pointers to its list of ViewPorts and pointers to the Copper instructions produced by the system to create the display you have defined.

* ViewPort (information about this segment of the display) includes the values DxOffset and DyOffset that are used to position this portion relative to the overall View. The ViewPort also contains the variables DHeight and DWidth, which define the size of this display segment; a Modes variable; and a pointer to the local ColorMap. The VideoControl() function and its various tags are used to manipulate the ColorMap and ViewPort.Modes. Each ViewPort also contains a pointer to the next ViewPort. You create a linked list of ViewPorts to define the complete display.

* RasInfo (information about the raster) contains the variables RxOffset and RyOffset. It also contains pointers to the BitMap structure and to a companion RasInfo structure if this is a dual playfield.

* BitMap (information about memory usage) tells the system where to find the display and drawing area memory and shows how this memory space is organized, including the display’s depth.

You must allocate enough memory for the display you define. The memory you use for the display may be shared with the area control structures used for drawing. This allows you to draw into the same areas that you are currently displaying on the screen.

As an alternative, you can define two BitMaps. One of them can be the active structure (that being displayed) and the other can be the inactive structure. If you draw into one BitMap while displaying another, the user cannot see the drawing taking place. This is called ''double-buffering'' of the display. See “Advanced Topics” below for an explanation of the steps required for double-buffering. Double-buffering takes twice as much memory as single-buffering because two full displays are produced.

To determine the amount of required memory for each ViewPort for single-buffering, you can use the following formula.

<pre>
#include &lt;graphics/gfx.h&gt;

/* Depth, Width, and Height get set to something reasonable. */
UBYTE Depth, Width, Height;

/* Calculate resulting VP size. */
bytes_per_ViewPort = Depth * RASSIZE(Width, Height);
</pre>

RASSIZE() is a system macro attuned to the current design of the system memory allocation for display rasters. See the &lt;graphics/gfx.h&gt; include file for the formula with which RASSIZE() is calculated.

For example, a 32-color ViewPort (depth = 5), 320 pixels wide by 200 lines high currently uses 40,000 bytes. A 16-color ViewPort (depth = 4), 640 pixels wide by 400 lines high currently uses 128,000 bytes.

=== Forming a Basic Display ===

Here are the data structures that you need to define to create a basic display:

<pre>
struct View view;               /* These get used in all versions of the OS */
struct ViewPort viewPort;
struct BitMap bitMap;
struct RasInfo rasInfo;
struct ColorMap *cm;

struct ViewExtra *vextra;       /* Extra View data */
struct ViewPortExtra *vpextra;  /* Extra ViewPort data */
struct MonitorSpec *monspec;    /* Monitor data */
struct DimensionInfo dimquery;  /* Display dimension data */
</pre>

ViewExtra and ViewPortExtra are data structures used to hold extended data about their corresponding parent structure. ViewExtra contains information about the video monitor being used to render the View. ViewPortExtra contains information required for clipping of the ViewPort.

GfxNew() is used to create these extended data structures and GfxAssociate() is used to associate the extended data structure with an appropriate parent structure. Although GfxAssociate() can associate a ViewPortExtra structure with a ViewPort, it is better to use VideoControl() with the VTAG_VIEWPORTEXTRA_SET tag instead. Keep in mind that GfxNew() allocates memory for the resulting data structure which must be returned using GfxFree() before the application exits. The function GfxLookUp() will find the address of an extended data structure from the address of its parent.

==== Preparing the View Structure ====

The following code prepares the View structure for further use:

<pre>
InitView(&amp;view);      /*  Initialize the View.  */
view.Modes |= LACE;   /*  Only interlaced, 1.3 displays require this */
</pre>

A ViewExtra structure must also be created with GfxNew() and associated with this View with GfxAssociate() as shown in the example programs RGBBoxes.c and WBClone.c.

<pre>
/* Form the ModeID from values in &lt;displayinfo.h&gt; */
modeID=DEFAULT_MONITOR_ID | HIRESLACE_KEY;

/* Make the ViewExtra structure */
if( vextra=GfxNew(VIEW_EXTRA_TYPE) )
    {
    /* Attach the ViewExtra to the View */
    GfxAssociate(&amp;view , vextra);
    view.Modes |= EXTEND_VSTRUCT;

    /* Initialize the MonitorSpec field of the ViewExtra */
    if( monspec=OpenMonitor(NULL,modeID) )
        vextra-&gt;Monitor=monspec;
    else
        fail(&quot;Could not get MonitorSpec\n&quot;);
    }
 else fail(&quot;Could not get ViewExtra\n&quot;);
</pre>

==== Preparing the BitMap Structure ====

The BitMap structure tells the system where to find the display and drawing memory and how this memory space is organized. The following code section prepares a BitMap structure, including allocation of memory for the bitmap. This is done with two functions, InitBitMap() and AllocRaster(). InitBitMap() takes four arguments–a pointer to a BitMap and the depth, width, and height of the desired bitmap. Once the bitmap is initialized, memory for its bitplanes must be allocated. AllocRaster() takes two arguments–width and height. Here is a code section to initialize a bitmap:

<pre>
/*  Init BitMap for RasInfo.  */
InitBitMap(&amp;bitMap, DEPTH, WIDTH, HEIGHT);

/* Set the plane pointers to NULL so the cleanup routine will
   know if they were used. */
for(depth=0; depth&lt;DEPTH; depth++)
    bitMap.Planes[depth] = NULL;

/*  Allocate space for BitMap.  */
for(depth=0; depth&lt;DEPTH; depth++)
    {
    bitMap.Planes[depth] = (PLANEPTR)AllocRaster(WIDTH, HEIGHT);
    if (bitMap.Planes[depth] == NULL)
        cleanExit(RETURN_WARN);
    }
</pre>

This code allocates enough memory to handle the display area for as many bitplanes as the depth you have defined.

==== Preparing the RasInfo Structure ====

The RasInfo structure provides information to the system about the location of the BitMap as well as the positioning of the display area as a window against a larger drawing area.

Use the following steps to prepare the RasInfo structure:

<pre>
/* Initialize the RasInfos. */
rasInfo.BitMap = &amp;bitMap;   /* Attach the corresponding BitMap.          */
rasInfo.RxOffset = 0;       /* Align upper left corners of display       */
rasInfo.RyOffset = 0;       /*   with upper left corner of drawing area. */
rasInfo.Next = NULL;        /* for a single playfield display, there
                             * is only one RasInfo structure present     */
</pre>

The system may be made to reinterpret the RxOffset and RyOffset values in a ViewPort’s RasInfo structure by calling ScrollVPort() with the address of the ViewPort. Changing one or both offsets and calling ScrollVPort() has the effect of scrolling the ViewPort.

==== Preparing the ViewPort Structure ====

To prepare the ViewPort structure for further use, you call InitVPort() and initialize certain fields as follows:

<pre>
InitVPort(&amp;viewPort);            /* Initialize the ViewPort.             */
viewPort.RasInfo = &amp;rasInfo;     /* The rasInfo must also be initialized */
viewPort.DWidth  = WIDTH;
viewPort.DHeight = HEIGHT;

/* Under 1.3, you should set viewPort.Modes here to select a display mode. */
/* Under Release 2, use VideoControl() with VTAG_NORMAL_DISP_SET to select */
/* a display mode by attaching a DisplayInfo structure to the ViewPort.    */
</pre>

The InitVPort() routine presets certain default values in the ViewPort structure. The defaults include:

* Modes variable set to zero–this means you select a low-resolution display. (To alter this, use VideoControl() with the VTAG_NORMAL_DISP_SET tag as explained below.)

* Next variable set to NULL–no other ViewPort is linked to this one. If you want a display with multiple ViewPorts, you must fill in the link yourself.

If you want to create a View with two or more ViewPorts you must declare and initialize the ViewPorts as above. Then link them together using the ViewPort.Next field with a NULL link for the ViewPort at the end of the chain:

<pre>
viewPortA.Next = &amp;viewPortB;    /* Tell first one the address of the second. */
viewPortB.Next = NULL;          /* There are no others after this one. */
</pre>

Once a ViewPort has been prepared, a ViewPortExtra structure must also be created with GfxNew(), initialized, and associated with the ViewPort via the VideoControl() function. In addition, a DisplayInfo for this mode must be attached to the ViewPort. The fragment below shows how to do this. For complete examples, refer to the program listings of RGBBoxes.c and WBClone.c.

<pre>
struct TagItem vcTags[] =              /* These tags will be passed to the  */
{                                      /* VideoControl() function to set up */
    { VTAG_ATTACH_CM_SET, NULL },      /* the extended ViewPort structures  */
    { VTAG_VIEWPORTEXTRA_SET, NULL },  /* required in Release 2.  The NULL  */
    { VTAG_NORMAL_DISP_SET, NULL },    /* ti_Data field of these tags must  */
    { VTAG_END_CM, NULL }              /* be filled in before making the    */
};                                     /* call to VideoControl().           */

struct DimensionInfo dimquery; /* Release 2 structure for display size data */

/* Make a ViewPortExtra and get ready to attach it */
if( vpextra = GfxNew(VIEWPORT_EXTRA_TYPE) )
    {
    vcTags[1].ti_Data = (ULONG) vpextra;

    /* Initialize the DisplayClip field of the ViewPortExtra structure */
    if( GetDisplayInfoData( NULL , (UBYTE *) &amp;dimquery ,
                           sizeof(struct dimquery) , DTAG_DIMS, modeID) )
        {
        vpextra-&gt;DisplayClip = dimquery.Nominal;

        /* Make a DisplayInfo and get ready to attach it */
        if( !(vcTags[2].ti_Data = (ULONG) FindDisplayInfo(modeID)) )
            fail(&quot;Could not get DisplayInfo\n&quot;);
        }
    else fail(&quot;Could not get DimensionInfo\n&quot;);
    }
else fail(&quot;Could not get ViewPortExtra\n&quot;);

/* This is for backwards compatibility with, for example,   */
/* a 1.3 screen saver utility that looks at the Modes field */
viewPort.Modes = (UWORD) (modeID &amp; 0x0000ffff);
</pre>

==== Preparing the ColorMap Structure ====

When the View is created, Copper instructions are generated to change the current contents of each color register just before the topmost line of a ViewPort so that this ViewPort’s color registers will be used for interpreting its display. To set the color registers you create a ColorMap for the ViewPort with GetColorMap() and call SetRGB4(). Here are the steps used in 1.3 to initialize a ColorMap:

<pre>
if( view.ColorMap=GetColorMap( 4L ) )
    LoadRGB4((&amp;viewPort, colortable, 4);
</pre>

A ColorMap is attached to the View–usually along with DisplayInfo and ViewExtra–by calling the VideoControl() function.

<pre>
/*  RGB values for the four colors used.  */
#define BLACK 0x000
#define RED   0xf00
#define GREEN 0x0f0
#define BLUE  0x00f

/*  Define some colors in an array of UWORDS.  */
static UWORD colortable[] = { BLACK, RED, GREEN, BLUE };

/* Fill the TagItem Data field with the address of the properly initialized
   (including ViewPortExtra) structure to be passed to VideoControl().      */
vc[0].ti_Data = (ULONG)viewPort;

/* Init ColorMap.  2 planes deep, so 4 entries (2 raised to #planes power). */
if(cm = GetColorMap( 4L ) )
{
    /* For applications that must be compatible with 1.3, replace the next 2 */
    /* lines with: viewPort.ColorMap=cm;                                     */
    if( VideoControl( cm , vcTags ) )
        fail(&quot;Could not attach extended structures\n&quot;);

    /*  Change colors to those in colortable.  */
    LoadRGB4(&amp;viewPort, colortable, 4);
}
</pre>

{{Note|title=The 4 Is For Bits, Not Entries.|text=The 4 in the name LoadRGB4() refers to the fact that each of the red, green, and blue values in a color table entry consists of four bits. It has nothing to do with the fact that this particular color table contains four entries. The call GetRGB4() returns the RGB value of a single entry of a ColorMap. SetRGB4CM() allows individual control of the entries in the ColorMap before or after linking it into the ViewPort.}}

The LoadRGB4() call above could be replaced with the following:

<pre>
register USHORT entry;

/*  Operate on the same four ColorMap entries as above.  */
for (entry = 0; entry &lt; 4; entry++)
    {
    /*  Call SetRGB4CM() with the address of the ColorMap, the entry to be
        changed, and the Red, Green, and Blue values to be stored there.
    */
    SetRGB4CM(viewPort.ColorMap, entry,
    /*  Extract the three color values from the one colortable entry.  */
        ((colortable[entry] &amp; 0x0f00) &gt;&gt; 8),
            ((colortable[entry] &amp; 0x00f0) &gt;&gt; 4),
                (colortable[entry] &amp; 0x000f));
    }
</pre>

Notice above how the four bits for each color are masked out and shifted right to get values from 0 to 15.

{{Note|title=WARNING!|text=It is important to use ''only'' the standard system ColorMap-related calls to access the ColorMap entries. These calls will remain compatible with recent and future enhancements to the ColorMap structure.}}

You might need to specify more colors in the color map than you think. If you use a dual playfield display (covered later in this chapter) with a depth of 1 for each of the two playfields, this means a total of four colors (two for each playfield). However, because playfield 2 uses color registers starting from number 8 on up when in dual-playfield mode, the color map must be initialized to contain at least 10 entries.

That is, it must contain entries for colors 0 and 1 (for playfield 1) and color numbers 8 and 9 (for playfield 2). Space for sprite colors must be allocated as well. For Amiga system software version 1.3 and earlier, ''when in doubt'', allocate a ColorMap with 32 entries, just in case.

==== Creating the Display Instructions ====

Now that you have initialized the system data structures, you can request that the system prepare a set of display instructions for the Copper using these structures as input data.

During the one or more blank vertical lines that precede each ViewPort, the Copper is busy changing the characteristics of the display hardware to match the characteristics you expect for this ViewPort.

This may include a change in display resolution, a change in the colors to be used, or other user-defined modifications to system registers.

Here is the code that creates the display instructions:

<pre>
/*  Construct preliminary Copper instruction list.  */
MakeVPort( &amp;view, &amp;viewPort );
</pre>

In this line of code, &amp;view is the address of the View structure and &amp;viewPort is the address of the first ViewPort structure. Using these structures, the system has enough information to build the instruction stream that defines your display.

MakeVPort() creates a special set of instructions that controls the appearance of the display.

If you are using animation, the graphics animation routines create a special set of instructions to control the hardware sprites and the system color registers.

In addition, the advanced user can create special instructions (called user Copper instructions) to change system operations based on the position of the video beam on the screen.

All of these special instructions must be merged together before the system can use them to produce the display you have designed. This is done by the system routine MrgCop() (which stands for “Merge Coprocessor Instructions”). Here is a typical call:

<pre>
/*  Merge preliminary lists into a real Copper list in the view structure.  */
MrgCop( &amp;view );
</pre>

=== Loading and Displaying the View ===

To display the View, you need to load it using LoadView() and turn on the direct memory access (DMA). A typical call is shown below.

<pre>
LoadView(&amp;view);
</pre>

The &amp;view argument is the address of the View structure defined in the example above.

There are two macros, defined in &lt;graphics/gfxmacros.h&gt;, that control display DMA: ON_DISPLAY and OFF_DISPLAY. They simply turn the display DMA control bit in the DMA control register on or off.

If you are drawing to the display area and do not want the user to see intermediate steps in the drawing, you can turn off the display. Because OFF_DISPLAY shuts down the display DMA and possibly speeds up other system operations, it can be used to provide additional memory cycles to the blitter or the ''68000''. The distribution of system DMA, however, allows four-channel sound, disk read/write, and a sixteen-color, low-resolution display (or four-color, high-resolution display) to operate at the same time with no slowdown (7.1 MHz effective rate) in the operation of the ''68000''. Using OFF_DISPLAY in a multitasking environment may, however, be an unfriendly thing to do to the other running processes. Use OFF_DISPLAY with discretion.

==== A Custom ViewPort Example ====

The following example creates a View consisting of one ViewPort set to an NTSC, high-resolution, interlaced display mode of nominal dimensions. This example shows both the old 1.3 way of setting up the ViewPort and the new method used in Release 2.

<pre>
;/*  RGBBoxes.c simple ViewPort example -- works with 1.3 and Release 2
LC -b1 -cfistq -v -y -j73 RGBBoxes.c
Blink FROM LIB:c.o,RGBBoxes.o TO RGBBoxes LIBRARY LIB:LC.lib,LIB:Amiga.lib
quit
*/

#include &lt;exec/types.h&gt;
#include &lt;graphics/gfx.h&gt;
#include &lt;graphics/gfxbase.h&gt;
#include &lt;graphics/gfxmacros.h&gt;
#include &lt;graphics/copper.h&gt;
#include &lt;graphics/view.h&gt;
#include &lt;graphics/displayinfo.h&gt;
#include &lt;graphics/gfxnodes.h&gt;
#include &lt;graphics/videocontrol.h&gt;
#include &lt;libraries/dos.h&gt;
#include &lt;utility/tagitem.h&gt;

#include &lt;clib/graphics_protos.h&gt;
#include &lt;clib/exec_protos.h&gt;
#include &lt;clib/dos_protos.h&gt;

#include &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;

#define DEPTH 2     /*  The number of bitplanes.  */
#define WIDTH 640   /*  Nominal width and height  */
#define HEIGHT 400  /*  used in 1.3.              */

#ifdef LATTICE
int CXBRK(void)    { return(0); }  /* Disable Lattice CTRL/C handling */
int chkabort(void) { return(0); }  /* really */
#endif

VOID drawFilledBox(WORD , WORD );  /* Function prototypes */
VOID cleanup(int );
VOID fail(STRPTR);

struct GfxBase *GfxBase = NULL;

/*  Construct a simple display.  These are global to make freeing easier.   */
struct View view, *oldview=NULL;  /*  Pointer to old View we can restore it.*/
struct ViewPort viewPort = { 0 };
struct BitMap bitMap = { 0 };
struct ColorMap *cm=NULL;

struct ViewExtra *vextra=NULL;      /* Extended structures used in Release 2 */
struct MonitorSpec *monspec=NULL;
struct ViewPortExtra *vpextra=NULL;
struct DimensionInfo dimquery = { 0 };

UBYTE *displaymem = NULL;     /*  Pointer for writing to BitMap memory.  */

#define BLACK 0x000           /*  RGB values for the four colors used.   */
#define RED   0xf00
#define GREEN 0x0f0
#define BLUE  0x00f

/*
 * main():  create a custom display; works under either 1.3 or Release 2
 */
VOID main(VOID)
{
WORD depth, box;
struct RasInfo rasInfo;
ULONG modeID;

struct TagItem vcTags[] =
{
    {VTAG_ATTACH_CM_SET, NULL },
    {VTAG_VIEWPORTEXTRA_SET, NULL },
    {VTAG_NORMAL_DISP_SET, NULL },
    {VTAG_END_CM, NULL }
};

/*  Offsets in BitMap where boxes will be drawn.  */
static SHORT boxoffsets[] = { 802, 2010, 3218 };

static UWORD colortable[] = { BLACK, RED, GREEN, BLUE };

/* Open the graphics library */
GfxBase = (struct GfxBase *)OpenLibrary(&quot;graphics.library&quot;, 33L);
if(GfxBase == NULL)
    fail(&quot;Could not open graphics library\n&quot;);

/*  Example steals screen from Intuition if Intuition is around.      */
oldview = GfxBase-&gt;ActiView;   /* Save current View to restore later. */

InitView(&amp;view);           /*  Initialize the View and set View.Modes.     */
view.Modes |= LACE;        /*  This is the old 1.3 way (only LACE counts). */

if(GfxBase-&gt;LibNode.lib_Version &gt;= 36)
    {
    /* Form the ModeID from values in &lt;displayinfo.h&gt; */
    modeID=DEFAULT_MONITOR_ID | HIRESLACE_KEY;

    /*  Make the ViewExtra structure   */
    if( vextra=GfxNew(VIEW_EXTRA_TYPE) )
        {
        /* Attach the ViewExtra to the View */
        GfxAssociate(&amp;view , vextra);
        view.Modes |= EXTEND_VSTRUCT;

        /* Create and attach a MonitorSpec to the ViewExtra */
        if( monspec=OpenMonitor(NULL,modeID) )
            vextra-&gt;Monitor=monspec;
        else
            fail(&quot;Could not get MonitorSpec\n&quot;);
        }
     else fail(&quot;Could not get ViewExtra\n&quot;);
     }


/*  Initialize the BitMap for RasInfo.  */
InitBitMap(&amp;bitMap, DEPTH, WIDTH, HEIGHT);

/* Set the plane pointers to NULL so the cleanup routine */
/* will know if they were used.                          */
for(depth=0; depth&lt;DEPTH; depth++)
    bitMap.Planes[depth] = NULL;

/*  Allocate space for BitMap.             */
for (depth=0; depth&lt;DEPTH; depth++)
    {
    bitMap.Planes[depth] = (PLANEPTR)AllocRaster(WIDTH, HEIGHT);
    if (bitMap.Planes[depth] == NULL)
        fail(&quot;Could not get BitPlanes\n&quot;);
    }

rasInfo.BitMap = &amp;bitMap;       /*  Initialize the RasInfo.  */
rasInfo.RxOffset = 0;
rasInfo.RyOffset = 0;
rasInfo.Next = NULL;

InitVPort(&amp;viewPort);           /*  Initialize the ViewPort.  */
view.ViewPort = &amp;viewPort;      /*  Link the ViewPort into the View.  */
viewPort.RasInfo = &amp;rasInfo;
viewPort.DWidth = WIDTH;
viewPort.DHeight = HEIGHT;

/* Set the display mode the old-fashioned way */
viewPort.Modes=HIRES | LACE;

if(GfxBase-&gt;LibNode.lib_Version &gt;= 36)
{
    /* Make a ViewPortExtra and get ready to attach it */
    if( vpextra = GfxNew(VIEWPORT_EXTRA_TYPE) )
        {
        vcTags[1].ti_Data = (ULONG) vpextra;

        /* Initialize the DisplayClip field of the ViewPortExtra */
        if( GetDisplayInfoData( NULL , (UBYTE *) &amp;dimquery ,
                                sizeof(dimquery) , DTAG_DIMS, modeID) )
            {
            vpextra-&gt;DisplayClip = dimquery.Nominal;

            /* Make a DisplayInfo and get ready to attach it */
            if( !(vcTags[2].ti_Data = (ULONG) FindDisplayInfo(modeID)) )
                fail(&quot;Could not get DisplayInfo\n&quot;);
             }
        else fail(&quot;Could not get DimensionInfo \n&quot;);
        }
    else fail(&quot;Could not get ViewPortExtra\n&quot;);

    /* This is for backwards compatibility with, for example,   */
    /* a 1.3 screen saver utility that looks at the Modes field */
    viewPort.Modes = (UWORD) (modeID &amp; 0x0000ffff);
    }

/*  Initialize the ColorMap.  */
/*  2 planes deep, so 4 entries (2 raised to the #_planes power).  */
cm = GetColorMap(4L);
if(cm == NULL)
    fail(&quot;Could not get ColorMap\n&quot;);

if(GfxBase-&gt;LibNode.lib_Version &gt;= 36)
    {
    /* Get ready to attach the ColorMap, Release 2-style */
    vcTags[0].ti_Data = (ULONG) &amp;viewPort;

    /* Attach the color map and Release 2 extended structures */
    if( VideoControl(cm,vcTags) )
        fail(&quot;Could not attach extended structures\n&quot;);
    }
else
    /* Attach the ColorMap, old 1.3-style */
    viewPort.ColorMap = cm;

LoadRGB4(&amp;viewPort, colortable, 4);  /* Change colors to those in colortable. */

MakeVPort( &amp;view, &amp;viewPort ); /* Construct preliminary Copper instruction list.    */

/* Merge preliminary lists into a real Copper list in the View structure. */
MrgCop( &amp;view );

/* Clear the ViewPort */
for(depth=0; depth&lt;DEPTH; depth++)
    {
    displaymem = (UBYTE *)bitMap.Planes[depth];
    BltClear(displaymem, (bitMap.BytesPerRow * bitMap.Rows), 1L);
    }

LoadView(&amp;view);

/*  Now fill some boxes so that user can see something.          */
/*  Always draw into both planes to assure true colors.          */
for (box=1; box&lt;=3; box++)  /* Three boxes; red, green and blue. */
    {
    for (depth=0; depth&lt;DEPTH; depth++)        /*  Two planes.   */
        {
        displaymem = bitMap.Planes[depth] + boxoffsets[box-1];
        drawFilledBox(box, depth);
        }
    }

Delay(10L * TICKS_PER_SECOND);   /*  Pause for 10 seconds.                */
LoadView(oldview);               /*  Put back the old View.               */
WaitTOF();                       /*  Wait until the the View is being     */
                                 /*    rendered to free memory.           */
FreeCprList(view.LOFCprList);    /*  Deallocate the hardware Copper list  */
if(view.SHFCprList)              /*    created by MrgCop().  Since this   */
    FreeCprList(view.SHFCprList);/*    is interlace, also check for a     */
                                 /*    short frame copper list to free.   */
FreeVPortCopLists(&amp;viewPort);    /*  Free all intermediate Copper lists   */
                                 /*    from created by MakeVPort().       */
cleanup(RETURN_OK);              /*  Success.                             */
}


/*
 * fail():  print the error string and call cleanup() to exit
 */
void fail(STRPTR errorstring)
{
printf(errorstring);
cleanup(RETURN_FAIL);
}

/*
 * cleanup():  free everything that was allocated.
 */
VOID cleanup(int returncode)
{
WORD depth;

/*  Free the color map created by GetColorMap().  */
if(cm) FreeColorMap(cm);

/* Free the ViewPortExtra created by GfxNew() */
if(vpextra) GfxFree(vpextra);

/*  Free the BitPlanes drawing area.  */
for(depth=0; depth&lt;DEPTH; depth++)
    {
    if (bitMap.Planes[depth])
        FreeRaster(bitMap.Planes[depth], WIDTH, HEIGHT);
    }

/* Free the MonitorSpec created with OpenMonitor() */
if(monspec) CloseMonitor( monspec );

/* Free the ViewExtra created with GfxNew() */
if(vextra) GfxFree(vextra);

/* Close the graphics library */
CloseLibrary((struct Library *)GfxBase);

exit(returncode);
}


/*
 * drawFilledBox(): create a WIDTH/2 by HEIGHT/2 box of color
 *                  &quot;fillcolor&quot; into the given plane.
 */
VOID drawFilledBox(WORD fillcolor, WORD plane)
{
UBYTE value;
WORD boxHeight, boxWidth, width;

/*  Divide (WIDTH/2) by eight because each UBYTE that */
/* is written stuffs eight bits into the BitMap.      */
boxWidth = (WIDTH/2)/8;
boxHeight = HEIGHT/2;

value = ((fillcolor &amp; (1 &lt;&lt; plane)) != 0) ?  0xff : 0x00;

for( ; boxHeight; boxHeight--)
    {
    for(width=0 ; width &lt; boxWidth; width++)
        *displaymem++ = value;

    displaymem += (bitMap.BytesPerRow - boxWidth);
    }
}
</pre>

==== Exiting Gracefully ====

The preceding sample program provides a way of exiting gracefully with the cleanup() subroutine. This function returns to the memory manager all dynamically-allocated memory chunks. Notice the calls to FreeRaster() and FreeColorMap(). These calls correspond directly to the allocation calls AllocRaster() and GetColorMap() located in the body of the program. Now look at the calls within cleanup() to FreeVPortCopLists() and FreeCprList(). When you call MakeVPort(), the graphics system dynamically allocates some space to hold intermediate instructions from which a final Copper instruction list is created. When you call MrgCop(), these intermediate Copper lists are merged together into the final Copper list, which is then given to the hardware for interpretation. It is this list that provides the stable display on the screen, split into separate ViewPorts with their own colors and resolutions and so on.

When your program completes, you must see that it returns all of the memory resources that it used so that those memory areas are again available to the system for reassignment to other tasks. Therefore, if you use the routines MakeVPort() or MrgCop(), you must also arrange to use FreeCprList() (pointing to each of those lists in the View structure) and FreeVPortCopLists() (pointing to the ViewPort that is about to be deallocated). If your View is interlaced, you will also have to call FreeCprList(&amp;view.SHFCprList) because an interlaced view has a separate Copper list for each of the two fields displayed. Do not confuse FreeVPortCopLists() with FreeCprList(). The former works on intermediate Copper lists for a specific ViewPort, the latter directly on a hardware Copper list from the View.

As a final caveat, notice that when you do free everything, the memory manager or other programs may immediately change the contents of the freed memory. Therefore, if the Copper is still executing an instruction stream (as a result of a previous LoadView()) when you free that memory, the display will malfunction. Once another View has been installed via LoadView(), do a WaitTOF() for the new View to begin displaying, and then you can begin freeing up your resources. WaitTOF() waits for the vertical blanking period to begin and all vertical blank interrupts to complete before returning to the caller. The routine WaitBOVP() (for “WaitBottomOfViewPort”) ''busy'' waits until the vertical beam reaches the bottom of the specified ViewPort before returning to the caller. This means no other tasks run until this function returns.

=== Custom ViewPort Example ===

The following program will create a display with the same attributes as the user’s Workbench screen. It does this by first inquiring as to those attributes, duplicating them, and then creating a similar display.

<pre>
/**********************************************************************/
/*                                                                    */
/*       WBClone.c: To clone the Workbench using graphics calls        */
/*                                                                    */
/*       Compile : SAS/C 5.10a LC -b1 -cfist -L -v -y                 */
/*                                                                    */
/**********************************************************************/

#include &lt;exec/types.h&gt;
#include &lt;exec/exec.h&gt;
#include &lt;clib/exec_protos.h&gt;
#include &lt;intuition/screens.h&gt;
#include &lt;intuition/intuition.h&gt;
#include &lt;intuition/intuitionbase.h&gt;
#include &lt;clib/intuition_protos.h&gt;
#include &lt;graphics/gfx.h&gt;
#include &lt;graphics/gfxbase.h&gt;
#include &lt;graphics/view.h&gt;
#include &lt;graphics/gfxnodes.h&gt;
#include &lt;graphics/videocontrol.h&gt;
#include &lt;clib/graphics_protos.h&gt;

#include &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;

#define INTUITIONNAME &quot;intuition.library&quot;

#ifdef LATTICE
int CXBRK(void)    { return(0); }  /* Disable Lattice CTRL/C handling */
int chkabort(void) { return(0); }  /* really */
#endif


/*********************************************************************/
/*                            GLOBAL VARIABLES                       */
/*********************************************************************/

struct IntuitionBase *IntuitionBase = NULL ;
struct GfxBase *GfxBase = NULL ;

/**********************************************************************/
/*                                                                    */
/* VOID Error (char *String)                                          */
/*                                                                    */
/* Print string and exit                                              */
/*                                                                    */
/**********************************************************************/

VOID Error (char *String)
{
        VOID CloseAll (VOID) ;

        printf (String) ;

        CloseAll () ;
        exit(0) ;
}


/**********************************************************************/
/*                                                                    */
/* VOID Init ()                                                       */
/*                                                                    */
/* Opens all the required libraries allocates all memory, etc.        */
/*                                                                    */
/**********************************************************************/

VOID Init ( VOID )
{
        /* Open the intuition library.... */
        if ((IntuitionBase = (struct IntuitionBase *)OpenLibrary (INTUITIONNAME, 37L)) == NULL)
                Error (&quot;Could not open the Intuition.library&quot;) ;

        /* Open the graphics library.... */
        if ((GfxBase = (struct GfxBase *)OpenLibrary (GRAPHICSNAME, 36L)) == NULL)
                Error (&quot;Could not open the Graphics.library&quot;) ;
}

/**********************************************************************/
/*                                                                    */
/* VOID CloseAll ()                                                   */
/*                                                                    */
/* Closes and tidies up everything that was used.                     */
/*                                                                    */
/**********************************************************************/

VOID CloseAll ( VOID )
{
        /* Close everything in the reverse order in which they were opened */

        /* Close the Graphics Library */
        if (GfxBase)
                CloseLibrary ((struct Library *) GfxBase) ;

        /* Close the Intuition Library */
        if (IntuitionBase)
                CloseLibrary ((struct Library *) IntuitionBase) ;
}
/**********************************************************************/
/*                                                                    */
/* VOID DestroyView(struct View *view)                                */
/*                                                                    */
/* Close and free everything to do with the View                      */
/*                                                                    */
/**********************************************************************/

VOID DestroyView(struct View *view)
{
        struct ViewExtra *ve;

        if (view)
        {
                if (ve = (struct ViewExtra *)GfxLookUp(view))
                {
                        if (ve-&gt;Monitor)
                                CloseMonitor(ve-&gt;Monitor);

                        GfxFree((struct ExtendedNode *)ve);
                }

                /* Free up the copper lists */
                if (view-&gt;LOFCprList)
                        FreeCprList(view-&gt;LOFCprList);

                if (view-&gt;SHFCprList)
                        FreeCprList(view-&gt;SHFCprList);

                FreeVec(view);
        }
}

/**********************************************************************/
/*                                                                    */
/* struct View *DupView(struct View *v, ULONG ModeID)                 */
/*                                                                    */
/* Duplicate the View.                                                */
/*                                                                    */
/**********************************************************************/

struct View *DupView(struct View *v, ULONG ModeID)
{
        /* Allocate and init a View structure.  Also, get a ViewExtra
         * structure and attach the monitor type to the View.
         */

        struct View *view = NULL;
        struct ViewExtra *ve = NULL;
        struct MonitorSpec *mspc = NULL;

        if (view = AllocVec(sizeof(struct View), MEMF_PUBLIC | MEMF_CLEAR))
        {
                if (ve = GfxNew(VIEW_EXTRA_TYPE))
                {
                        if (mspc = OpenMonitor(NULL, ModeID))
                        {
                                InitView(view);
                                view-&gt;DyOffset = v-&gt;DyOffset;
                                view-&gt;DxOffset = v-&gt;DxOffset;
                                view-&gt;Modes = v-&gt;Modes;
                                GfxAssociate(view, (struct ExtendedNode *)ve);
                                ve-&gt;Monitor = mspc;
                        }
                        else printf(&quot;Could not open monitor\n&quot;);
                }
                else printf(&quot;Could not get ViewExtra\n&quot;);
        }
        else printf(&quot;Could not create View\n&quot;);

        if (view &amp;&amp; ve &amp;&amp; mspc)
                return(view);
        else
        {
                DestroyView(view);
                return(NULL);
        }
}


/**********************************************************************/
/*                                                                    */
/* VOID DestroyViewPort(struct ViewPort *vp)                          */
/*                                                                    */
/* Close and free everything to do with the ViewPort.                 */
/*                                                                    */
/**********************************************************************/

VOID DestroyViewPort(struct ViewPort *vp)
{
        if (vp)
        {
                /* Find the ViewPort's ColorMap. From that use VideoControl
                 *  to get the ViewPortExtra, and free it.
                 * Then free the ColorMap, and finally the ViewPort itself.
                 */
                struct ColorMap *cm = vp-&gt;ColorMap;
                struct TagItem ti[] =
                {
                        {VTAG_VIEWPORTEXTRA_GET, NULL}, /* &lt;-- This field will be filled in */
                        {VTAG_END_CM, NULL}
                };

                if (cm)
                {
                        if (VideoControl(cm, ti) == NULL)
                                GfxFree((struct ExtendedNode *)ti[0].ti_Data);
                        else
                                printf(&quot;VideoControl error in DestroyViewPort()\n&quot;);

                        FreeColorMap(cm);
                }
                else
                {
                        printf(&quot;Could not free the ColorMap\n&quot;);
                }

                FreeVPortCopLists(vp);

                FreeVec(vp);
        }
}

/**********************************************************************/
/*                                                                    */
/* struct ViewPort *DupViewPort(struct ViewPort *vp, ULONG ModeID)    */
/*                                                                    */
/* Duplicate the ViewPort.                                            */
/*                                                                    */
/**********************************************************************/

struct ViewPort *DupViewPort(struct ViewPort *vp, ULONG ModeID)
{
        /* Allocate and initialise a ViewPort. Copy the ViewPort width and
         * heights, offsets, and modes values.  Allocate and initialize a
         * ColorMap.
         *
         * Also, allocate a ViewPortExtra, and copy the TextOScan values of the
         * ModeID from the database into the ViewPortExtra.
         */

        #define COLOURS 32
        struct ViewPort *Myvp;
        struct ViewPortExtra *vpe;
        struct ColorMap *cm;
        struct TagItem ti[] =                   /* to attach everything */
        {
                {VTAG_ATTACH_CM_SET, NULL},     /* these NULLs will be replaced in the code */
                {VTAG_VIEWPORTEXTRA_SET, NULL},
                {VTAG_NORMAL_DISP_SET, NULL},
                {VTAG_END_CM, NULL}
        };
        struct DimensionInfo query = {0};
        UWORD colour;
        int c;
        ULONG gotinfo = NULL;

        if (Myvp = AllocVec(sizeof(struct ViewPort), MEMF_CLEAR | MEMF_PUBLIC))
        {
                if (vpe = (struct ViewPortExtra *)GfxNew(VIEWPORT_EXTRA_TYPE))
                {
                        if (cm = GetColorMap(32))
                        {
                                if (gotinfo = GetDisplayInfoData(NULL, (APTR)&amp;query,
                                                                 sizeof(query), DTAG_DIMS, ModeID))
                                {
                                        InitVPort(Myvp);

                                        /* duplicate the ViewPort structure */
                                        Myvp-&gt;DWidth = vp-&gt;DWidth;
                                        Myvp-&gt;DHeight = vp-&gt;DHeight;
                                        Myvp-&gt;DxOffset = vp-&gt;DxOffset;
                                        Myvp-&gt;DyOffset = vp-&gt;DyOffset;
                                        Myvp-&gt;Modes = vp-&gt;Modes;
                                        Myvp-&gt;SpritePriorities = vp-&gt;SpritePriorities;
                                        Myvp-&gt;ExtendedModes = vp-&gt;ExtendedModes;

                                        /* duplicate the Overscan values */
                                        vpe-&gt;DisplayClip = query.TxtOScan;

                                        /* attach everything together */
                                        ti[0].ti_Data = (ULONG)Myvp;
                                        ti[1].ti_Data = (ULONG)vpe;
                                        ti[2].ti_Data = (ULONG)FindDisplayInfo(ModeID);
                                        if (VideoControl(cm, ti) != NULL)
                                        {
                                                printf(&quot;VideoControl error in CreateViewPort()\n&quot;);
                                        }

                                        /* copy the colours from the workbench */
                                        for (c = 0; c &lt; COLOURS; c++)
                                        {
                                                if ((colour = GetRGB4(vp-&gt;ColorMap, c)) != -1)
                                                {
                                                SetRGB4CM(cm, c, (colour &gt;&gt; 8),
                                                          ((colour &gt;&gt; 4) &amp; 0xf), (colour &amp; 0xf));
                                                }
                                        }
                                }
                                else printf(&quot;Database error\n&quot;);
                        }
                        else printf(&quot;Could not get the ColorMap\n&quot;);
                }
                else printf(&quot;Could not get the ViewPortExtra\n&quot;);
        }
        else printf(&quot;Could not get the ViewPort\n&quot;);

        if (Myvp &amp;&amp; vpe &amp;&amp; cm &amp;&amp; gotinfo)
                return(Myvp);
        else
        {
                DestroyViewPort(vp);
                return(NULL);
        }
}


/***********************************************************************************/
/*                                                                                 */
/* VOID DestroyBitMap(struct BitMap *Mybm, SHORT width, SHORT height, SHORT depth) */
/*                                                                                 */
/* Close and free everything to do with the BitMap                                 */
/*                                                                                 */
/***********************************************************************************/

VOID DestroyBitMap(struct BitMap *Mybm, SHORT width, SHORT height, SHORT depth)
{
        int i;

        if (Mybm)
        {
                for (i = 0; (i &lt; depth); i++)
                {
                        if (Mybm-&gt;Planes[i])
                                FreeRaster(Mybm-&gt;Planes[i], width, height);
                }
                FreeVec(Mybm);
        }
}


/***********************************************************************/
/*                                                                     */
/* struct BitMap *CreateBitMap(SHORT width, SHORT height, SHORT depth) */
/*                                                                     */
/* Create the BitMap.                                                  */
/*                                                                     */
/***********************************************************************/

struct BitMap *CreateBitMap(SHORT width, SHORT height, SHORT depth)
{
        /* Allocate a BitMap structure, initialise it, and allocate each plane. */

        struct BitMap *Mybm;
        PLANEPTR allocated = (PLANEPTR) 1;
        int i;

        if (Mybm = AllocVec(sizeof(struct BitMap), MEMF_CLEAR | MEMF_PUBLIC))
        {
                InitBitMap(Mybm, depth, width, height);
                for (i = 0; ((i &lt; depth) &amp;&amp; (allocated)); i++)
                        allocated = (Mybm-&gt;Planes[i] = AllocRaster(width, height));

                if (allocated == NULL)
                {
                        printf(&quot;Could not allocate all the planes\n&quot;);
                        DestroyBitMap(Mybm, width, height, depth);
                        Mybm = NULL;
                }
        }
        else printf(&quot;Could not get BitMap\n&quot;);

        return(Mybm);
}

/********************************************************************************/
/*                                                                              */
/* VOID ShowView(struct View *view, struct ViewPort *vp, struct BitMap *bm,    */
/*                                                SHORT width, SHORT height)    */
/*                                                                              */
/* Assemble and display the View.                                               */
/*                                                                              */
/********************************************************************************/

VOID ShowView(struct View *view, struct ViewPort *vp, struct BitMap *bm,
                                               SHORT width, SHORT height)
{
        /* Attach the BitMap to the ViewPort via a RasInfo.  Attach the ViewPort
         * to the View.  Clear the BitMap, and draw into it by attaching the BitMap
         * to a RastPort.  Then MakeVPort(), MrgCop() and LoadView().
         * Just wait for the user to press &lt;RETURN&gt; before returning.
         */

        struct RastPort *rp;
        struct RasInfo *ri;

        if (rp = AllocVec(sizeof(struct RastPort), MEMF_CLEAR | MEMF_PUBLIC))
        {
                if (ri = AllocVec(sizeof(struct RasInfo), MEMF_CLEAR | MEMF_PUBLIC))
                {
                        InitRastPort(rp);
                        ri-&gt;BitMap = rp-&gt;BitMap = bm;
                        vp-&gt;RasInfo = ri;
                        view-&gt;ViewPort = vp;

                        /* render */
                        SetRast(rp, 0);         /* clear the background */
                        SetAPen(rp, ((1 &lt;&lt; bm-&gt;Depth) - 1));    /* use the last pen */
                        Move(rp, 0, 0);
                        Draw(rp, width, 0);
                        Draw(rp, width, height);
                        Draw(rp, 0, height);
                        Draw(rp, 0, 0);

                        /* display it */
                        MakeVPort(view, vp);
                        MrgCop(view);
                        LoadView(view);

                        getchar();

                        /* bring back the system */
                        RethinkDisplay();

                        FreeVec(ri);
                }
                else printf(&quot;Could not get RasInfo\n&quot;);

                FreeVec(rp);
        }
        else printf(&quot;Could not get RastPort\n&quot;);

}


/**********************************************************************/
/*                                                                    */
/* VOID main (int argc, char *argv[])                                 */
/*                                                                    */
/* Clone the Workbench View using Graphics Library calls.             */
/*                                                                    */
/**********************************************************************/

VOID main (int argc, char *argv[])
{
        struct Screen *wb;
        struct View *Myview;
        struct ViewPort *Myvp;
        struct BitMap *Mybm;
        ULONG ModeID;
        ULONG IbaseLock;

        Init () ;               /* to open the libraries */

        /* To clone the Workbench using graphics calls involves duplicating
         * the Workbench ViewPort, ViewPort mode, and Intuition's View.
         * This also involves duplicating the DisplayClip for the overscan
         * value, the colours, and the View position.
         *
         * When this is all done, the View, ViewPort, ColorMap and BitMap
         * (and ViewPortExtra, ViewExtra and RasInfo) all have to be linked
         * together, and the copperlists made to create the display.
         *
         * This is not as difficult as it sounds (trust me!)
         */

        /* First, lock the Workbench screen, so no changes can be made to it
         * while we are duplicating it.
         */
        if (wb = LockPubScreen(&quot;Workbench&quot;))
        {
                /* Find the Workbench's ModeID. This is a 32-bit number that
                 * identifies the monitor type, and the display mode of that monitor.
                 */
                ModeID = GetVPModeID(&amp;wb-&gt;ViewPort);

                /* We need to duplicate Intuition's View structure, so lock IntuitionBase
                 * to prevent the View changing under our feet.
                 */
                IbaseLock = LockIBase(0);
                if (Myview = DupView(&amp;IntuitionBase-&gt;ViewLord, ModeID))
                {
                        /* The View has been cloned, so we don't need to keep it locked. */
                        UnlockIBase(IbaseLock);

                        /* Now duplicate the Workbench's ViewPort. Remember, we still have
                         * the Workbench locked.
                         */
                        if (Myvp = DupViewPort(&amp;wb-&gt;ViewPort, ModeID))
                        {
                                /* Create a BitMap to render into. This will be of the
                                 * same dimensions as the Workbench.
                                 */
                                if (Mybm = CreateBitMap(wb-&gt;Width, wb-&gt;Height, wb-&gt;BitMap.Depth))
                                {
                                        /* Now we have everything copied, show something */
                                        ShowView(Myview, Myvp, Mybm, wb-&gt;Width-1, wb-&gt;Height-1);

                                        /* Now free up everything we have allocated */
                                        DestroyBitMap(Mybm, wb-&gt;Width, wb-&gt;Height, wb-&gt;BitMap.Depth);
                                }
                                DestroyViewPort(Myvp);
                        }
                        DestroyView(Myview);
                }
                else
                {
                        UnlockIBase(IbaseLock);
                }
                UnlockPubScreen(NULL, wb);
        }
        CloseAll () ;
}
</pre>

== Advanced Topics ==

This section covers advanced display topics such as dual-playfield mode, double-buffering, EHB mode and HAM mode.

=== Creating a Dual-Playfield Display ===

In dual-playfield mode, you have two separately controllable playfields. You specify dual-playfield by using any ModeID that includes DPF in its name as listed in &lt;graphics/displayinfo.h&gt;.

In dual-playfield mode, you always define two RasInfo data structures. Each of these structures defines one of the playfields. There are five different ways you can configure a dual-playfield display, because there are five different distributions of the bitplanes which the system hardware allows.

{| class="wikitable"
|+ Bitplane Assignment in Dual-playfield Mode
! Number of Bitplanes !! Playfield 1 Depth !! Playfield 2 Depth
|-
| 2 || 1 || 1
|-
| 3 || 2 || 1
|-
| 4 || 2 || 2
|-
| 5 || 3 || 2
|-
| 6 || 3 || 3
|}

If the ModeID includes DPF2 in its name, then the playfield priorities are swapped and playfield 2 will be displayed in front of playfield 1. In this way, you can get more bitplanes in the background playfield than you have in the foreground playfield.

The playfield priority affects only one ViewPort at a time. If you have multiple ViewPorts with dual-playfields, the playfield priority is set for each one individually.

Here’s a summary of the steps you need to take to create a dual-playfield display:

# Allocate one View structure and one ViewPort structure.
# Allocate two BitMap structures. Allocate two RasInfo structures (linked together), each pointing to a separate BitMap. The two RasInfo structures are linked together as follows: <syntaxhighlight>
struct RasInfo playfield1, playfield2;

playfield1.Next = &playfield2;
playfield2.Next = NULL;
</syntaxhighlight>
# Initialize each BitMap structure to describe one playfield, using one of the permissible bitplane distributions shown in the above table and allocate memory for the bitplanes themselves. Note that BitMap 1 and BitMap 2 need ''not'' be the same width and height.
# Initialize the ViewPort structure. Specify dual-playfield mode by selecting a ModeID that includes DPF (or DPF2) in its name as listed in &lt;graphics/displayinfo.h&gt;. Set the ViewPort.RasInfo field to the address of the playfield 1 RasInfo.
# Set up the ColorMap information
# Call MakeVPort(), MrgCop() and LoadView() to display the newly created ViewPort.

For display purposes, each of the two BitMaps is assigned to a separate ViewPort. To ''draw'' separately into the BitMaps, you must also assign these BitMaps to two separate RastPorts. The section called “Initializing a RastPort Structure” shows you how to use a RastPort data structure to control your drawing routines.

=== Creating a Double-Buffered Display ===

To produce smooth animation or similar effects, it is occasionally necessary to double-buffer your display. To prevent the user from seeing your graphics rendering while it is in progress, you will want to draw into one memory area while actually displaying a different area.

There are two methods of creating and displaying a double-buffered display. The simplest method is to create two complete Views and switch back and forth between them with LoadView() and WaitTOF().

The second method consists of creating two separate display areas and two sets of pointers to those areas for a single View. This is more complicated but takes less memory.

<ul>
<li><p>Allocate one ViewPort structure and one View structure.</p></li>
<li><p>Allocate two BitMap structures and one RasInfo structure. Initialize each BitMap structure to describe one drawing area and allocate memory for the bitplanes themselves. Initialize the RasInfo structure, setting the RasInfo.BitMap field to the address of one of the two BitMaps you created.</p>
<p></p></li>
<li><p>Call MakeVPort(), MrgCop() and LoadView(). When you call MrgCop(), the system uses the information you have provided to create a Copper instruction list for the Copper to execute. The system allocates memory for a long-frame (LOF) Copper list and, if this is an interlaced display, a short-frame (SHF) Copper list as well. The system places a pointer to the long-frame Copper list in View.LOFCprList and a pointer to a short-frame Copper list (if this is an interlaced display) in View.SHFCprList. The Copper instruction stream referenced by these pointers applies to the first BitMap.</p></li>
<li><p>Save the values in View.LOFCprList and View.SHFCprlist and reset these fields to zero. Place a pointer to the second BitMap structure in the RasInfo.BitMap field. Next call MakeVPort() and MrgCop().</p></li>
<li><p>When you perform MrgCop() with the Copper instruction list fields of the View set to zero, the system automatically allocates and fills in a new list of instructions for the Copper. Now you have created two sets of instruction streams for the Copper, one that works with data in the first BitMap and the other that works with data in the second BitMap.</p></li>
<li><p>You can save pointers to the second list of Copper instructions as well. Then, to perform the double-buffering, alternate between the two Copper lists. The code for the double-buffering loop would be as follows: call WaitTOF(), change the Copper instruction list pointers in the View, call LoadView() to show one of the BitMaps while drawing into the other BitMap, and repeat.</p></li></ul>

Remember that you will have to call FreeCprList() on both sets of Copper lists when you have finished.

=== Extra-Half-Brite Mode ===

In the Extra-Half-Brite mode you can create a single-playfield, low-resolution display with up to 64 colors, double the normal maximum of 32. This requires your ViewPort to be defined with six bitplanes. You specify EHB by selecting any ModeID which includes EXTRAHALFBRITE in its name as defined in the include file &lt;graphics/displayinfo.h&gt;.

When setting up the color palette for an EHB display, you only specify values for registers 0 to 31. If you draw using color numbers 0 through 31, the pixel you draw will be the color specified in that particular system color register. If you draw using a color number from 32 to 63, then the color displayed will be half the intensity value of the corresponding color register from 0 to 31. For example, if color register 0 is set to 0xFFF (white), then color number 32 would be half this value or 0x777 (grey).

EHB mode uses all six bitplanes. The color register (0 through 31) is obtained from the bit combinations from planes 5 to 1, in that order of significance. Plane 6 is used to determine whether the full intensity (bit value 0) color or half-intensity (bit value 1) color is to be displayed.

=== Hold-And-Modify Mode ===

In hold-and-modify mode you can create a single-playfield, low-resolution display in which 4,096 different colors can be displayed simultaneously. This requires your ViewPort to be defined with six bitplanes. You specify HAM by selecting any ModeID which includes HAM in its name as defined in &lt;graphics/displayinfo.h&gt;.

When you draw into the BitMap associated with this ViewPort, you can choose colors in one of four different ways. If you draw using color numbers 0 to 15, the pixel you draw will appear in the color specified in that particular system color register. If you draw with any other color value (16 to 63) the color displayed depends on the color of the pixel that is to the immediate left of this pixel on the screen. To see how this works, consider how the bitplanes are used in HAM.

Hold-and-modify mode requires six bitplanes. Planes 5 and 6 are used to modify the way bits from planes 1 through 4 are treated, as follows:

* If the bit combination from planes 6 and 5 for any given pixel is 00, normal color selection procedure is followed. Thus, the bit combinations from planes 4 to 1, in that order of significance, are used to choose one of 16 color registers (registers 0 through 15).
* If the bit combination in planes 6 and 5 is 01, the color of the pixel immediately to the left of this pixel is duplicated and then modified. The bit combinations from planes 4 through 1 are used to replace the four bits representing the blue value of the preceding pixel color. (No color registers are changed.)
* If the bit combination in planes 6 and 5 is 10, then the color of the pixel immediately to the left of this pixel is duplicated and modified. The bit combinations from planes 4 through 1 are used to replace the four bits representing the red value of the preceding pixel color.
* If the bit combination in planes 6 and 5 is 11, then the color of the pixel immediately to the left of this pixel is duplicated and modified. The bit combinations from planes 4 through 1 are used to replace the four bits representing the green value of the preceding pixel color.

You can use just five bitplanes in HAM mode. In that case, the data for the sixth plane is automatically assumed to be 0. Note that for the first pixel in each line, hold-and-modify begins with the background color. The color choice does ''not'' carry over from the preceding line.

{{Note|text=Since a typical hold-and-modify pixel only changes one of the three RGB color values at a time, color selection is limited. HAM mode does allow for the display of 4,096 colors simultaneously, but there are only 64 color options for any given pixel (not 4,096). The color of a pixel depends on the color of the preceding pixel.}}

== User Copper Lists ==

The Copper coprocessor allows you to produce mid-screen changes in certain hardware registers in addition to changes that the system software already provides. For example, it is the Copper that allows the Amiga to split the viewing area into multiple draggable screens, each with its own independent set of colors.

To create your own mid-screen effects on the system hardware registers, you provide “user Copper lists” that can be merged into the system Copper lists.

In the ViewPort data structure there is a pointer named UCopIns. If this pointer value is non-NULL, it points to a user Copper list that you have dynamically allocated and initialized to contain your own special hardware-stuffing instructions.

You allocate a user Copper list by an instruction sequence such as the following:

<pre>struct UCopList *uCopList = NULL;

/* Allocate memory for the Copper list.  Make certain that the initial */
/* memory is cleared.                                                  */
uCopList = (struct UCopList *)
        AllocMem(sizeof(struct UCopList), MEMF_PUBLIC|MEMF_CLEAR);

if (uCopList == NULL)
        return(FALSE);</pre>
<sub>b</sub>oxNote:User Copper lists do ''not'' have to be in Chip RAM.

=== Copper List Macros ===

Once this pointer to a user Copper list is available, you can use it with system macros (&lt;graphics/gfxmacros.h&gt;) to instruct the system what to add to its own list of things for the Copper to do within a specific ViewPort. The file &lt;graphics/gfxmacros.h&gt; provides the following five macro functions that implement user Copper instructions.

initializes the Copper list buffer. It is used to specify how many instructions are going to be placed in the Copper list. It is called as follows.

CINIT(uCopList, num_entries);

The uCopList argument is a pointer tot he user Copper list and num_entries is the number of entries in the list.

waits for the video beam to reach a particular horizontal and vertical position. Its format is:

CWAIT(uCopList, v, h)

Again, uCopList is the pointer to the Copper list. The v argument is the vertical position for which to wait, specified relative to the top of the ViewPort. The legal range of values (for both NTSC and PAL) is from 0 to 255; h is the horizontal position for which to wait. The legal range of values (for both NTSC and PAL) is from 0 to 226.

installs a particular value into a specified system register. Its format is:

CMOVE(uCopList, reg, value)

Again, uCopList is the pointer to the Copper list. The reg argument is the register to be affected, specified in this form: custom.''register-name'' where the ''register-name'' is one of the registers listed in the Custom structure in &lt;hardware/custom.h&gt;. The value argument to CMOVE is the value to place in the register.

increments the user Copper list pointer to the next position in the list. It is usually invoked for the programmer as part of the macro definitions CWAIT or CMOVE. Its format is:

CBump(uCopList)

where uCopList is the pointer to the user Copper list.

terminates the user Copper list. Its format is:

CEND(uCopList)

where uCopList is the pointer to the user Copper list.

Executing any of the user Copper list macros causes the system to dynamically allocate special data structures called intermediate Copper lists that are linked into your user Copper list (the list to which uCopList points) describing the operation. When you call the function MrgCop(&amp;view) as shown in the section called “Forming A Basic Display,” the system uses all of its intermediate Copper lists to sort and merge together the real Copper lists for the system (LOFCprList and SHFCprList).

When your program exits, you must return to the system all of the memory that you allocated or caused to be allocated. This means that you must return the intermediate Copper lists, as well as the user Copper list data structure. Here are two different methods for returning this memory to the system.

<pre>/* Returning memory to the system if you have NOT
 * obtained the ViewPort from Intuition.  */
FreeVPortCopLists(viewPort);

/* Returning memory to the system if you HAVE
 * obtained the ViewPort from Intuition.  */
CloseScreen(screen);    /* Intuition only */</pre>


User Copper lists may be clipped, under Release 2 and later, to ViewPort boundaries if the appropriate tag (VTAG_USERCLIP_SET) is passed to VideoControl(). Under earlier releases, the user Copper list would “leak” through to lower ViewPorts.

=== Copper List Example ===

The example program below shows the use of user Copper lists under Intuition.



<pre>/*  UserCopperExample.c
    User Copper List Example
    For SAS/C 5.10a,
    compile with:  LC -b1 -cfist -L -v -y UserCopperExample.c
    link with lc.lib and amiga.lib
*/

#include &lt;exec/types.h&gt;
#include &lt;exec/memory.h&gt;
#include &lt;graphics/gfxbase.h&gt;
#include &lt;graphics/gfxmacros.h&gt;
#include &lt;graphics/copper.h&gt;
#include &lt;graphics/videocontrol.h&gt;
#include &lt;intuition/intuition.h&gt;
#include &lt;intuition/preferences.h&gt;
#include &lt;hardware/custom.h&gt;
#include &lt;libraries/dos.h&gt;

#include &lt;clib/exec_protos.h&gt;           /*  Prototypes.  */
#include &lt;clib/graphics_protos.h&gt;
#include &lt;clib/intuition_protos.h&gt;
#include &lt;clib/dos_protos.h&gt;

#include &lt;stdlib.h&gt;

/*  Use this structure to gain access to the custom registers.  */
extern struct Custom far custom;

/*  Global variables.  */
struct GfxBase        *GfxBase = NULL;
struct IntuitionBase  *IntuitionBase = NULL;
struct Screen         *screen = NULL;
struct Window         *window = NULL;

VOID main( VOID ), cleanExit( WORD );
WORD openAll( VOID ), loadCopper( VOID );


/*
 *   The main() routine -- just calls subroutines
 */
VOID main( VOID )
{
WORD ret_val;
struct IntuiMessage     *intuiMessage;

        /*  Open the libraries, a screen and a window.  */
        ret_val = openAll();
        if (RETURN_OK == ret_val)
        {
                /*  Create and attach the user Copper list.  */
                ret_val = loadCopper();
                if (RETURN_OK == ret_val)
                {
                        /*  Wait until the user clicks in the close gadget.  */
                        (VOID) Wait(1&lt;&lt;window-&gt;UserPort-&gt;mp_SigBit);

                        while (intuiMessage = (struct IntuiMessage *)GetMsg(window-&gt;UserPort))
                                ReplyMsg((struct Message *)intuiMessage);
                }
        }
        cleanExit(ret_val);
}


/*
 * openAll() -- opens the libraries, screen and window
 */
WORD openAll( VOID )
{
#define MY_WA_WIDTH 270 /*  Width of window.  */

        WORD ret_val = RETURN_OK;

        /*  Prepare to explicitly request Topaz 60 as the screen font.  */
        struct TextAttr topaz60 =
        {
                (STRPTR)&quot;topaz.font&quot;,
                (UWORD)TOPAZ_SIXTY, (UBYTE)0, (UBYTE)0
        };

        GfxBase = (struct GfxBase *)OpenLibrary(&quot;graphics.library&quot;, 37L);
        if (GfxBase == NULL)
                ret_val = ERROR_INVALID_RESIDENT_LIBRARY;
        else
        {
                IntuitionBase = (struct IntuitionBase *)
                        OpenLibrary(&quot;intuition.library&quot;, 37L);

                if (IntuitionBase == NULL)
                        ret_val = ERROR_INVALID_RESIDENT_LIBRARY;
                else
                {
                        screen = OpenScreenTags( NULL,
                                 SA_Overscan, OSCAN_STANDARD,
                                 SA_Title,    &quot;User Copper List Example&quot;,
                                 SA_Font,     (ULONG)&amp;topaz60,
                                 TAG_END);

                        if (NULL == screen)
                                ret_val = ERROR_NO_FREE_STORE;
                        else
                        {
                                window = OpenWindowTags( NULL,
                                         WA_CustomScreen, screen,
                                         WA_Title,        &quot;&lt;- Click here to quit.&quot;,
                                         WA_IDCMP,        CLOSEWINDOW,
                                         WA_Flags,        WINDOWDRAG|WINDOWCLOSE|INACTIVEWINDOW,
                                         WA_Left,         (screen-&gt;Width-MY_WA_WIDTH)/2,
                                         WA_Top,          screen-&gt;Height/2,
                                         WA_Height,       screen-&gt;Font-&gt;ta_YSize + 3,
                                         WA_Width,        MY_WA_WIDTH,
                                         TAG_END);

                                if (NULL == window)
                                        ret_val = ERROR_NO_FREE_STORE;
                        }
                }
        }

        return(ret_val);
}


/*
 * loadCopper() -- creates a Copper list program and adds it to the system
 */
WORD loadCopper( VOID )
{
register USHORT   i, scanlines_per_color;
         WORD     ret_val    = RETURN_OK;
struct   ViewPort *viewPort;
struct   UCopList *uCopList  = NULL;
struct   TagItem  uCopTags[] =
          {
                { VTAG_USERCLIP_SET, NULL },
                { VTAG_END_CM, NULL }
          };

UWORD    spectrum[] =
          {
                0x0604, 0x0605, 0x0606, 0x0607, 0x0617, 0x0618, 0x0619,
                0x0629, 0x072a, 0x073b, 0x074b, 0x074c, 0x075d, 0x076e,
                0x077e, 0x088f, 0x07af, 0x06cf, 0x05ff, 0x04fb, 0x04f7,
                0x03f3, 0x07f2, 0x0bf1, 0x0ff0, 0x0fc0, 0x0ea0, 0x0e80,
                0x0e60, 0x0d40, 0x0d20, 0x0d00
          };

#define NUMCOLORS 32

        /*  Allocate memory for the Copper list.  */
        /*  Make certain that the initial memory is cleared.  */
        uCopList = (struct UCopList *)
                AllocMem(sizeof(struct UCopList), MEMF_PUBLIC|MEMF_CLEAR);

        if (NULL == uCopList)
                ret_val = ERROR_NO_FREE_STORE;
        else
        {
                /*  Initialize the Copper list buffer.  */
                CINIT(uCopList, NUMCOLORS);

                scanlines_per_color = screen-&gt;Height/NUMCOLORS;

                /*  Load in each color.  */
                for (i=0; i&lt;NUMCOLORS; i++)
                        {
                        CWAIT(uCopList, (i*scanlines_per_color), 0);
                        CMOVE(uCopList, custom.color[0], spectrum[i]);
                        }

                CEND(uCopList); /*  End the Copper list  */

                viewPort = ViewPortAddress(window);     /*  Get a pointer to the ViewPort.  */
                Forbid();       /*  Forbid task switching while changing the Copper list.  */
                viewPort-&gt;UCopIns=uCopList;
                Permit();       /*  Permit task switching again.  */

                /*  Enable user copper list clipping for this ViewPort.  */
                (VOID) VideoControl( viewPort-&gt;ColorMap, uCopTags );

                RethinkDisplay();       /*  Display the new Copper list.  */

                return(ret_val);
        }
}


/*
 *  cleanExit() -- returns all resources that were used.
 */
VOID cleanExit( WORD retval )
{
struct ViewPort *viewPort;

if (NULL != IntuitionBase)
{
        if (NULL != screen)
        {
                if (NULL != window)
                {
                        viewPort = ViewPortAddress(window);
                        if (NULL != viewPort-&gt;UCopIns)
                        {
                                /*  Free the memory allocated for the Copper.  */
                                FreeVPortCopLists(viewPort);
                                RemakeDisplay();
                        }
                        CloseWindow(window);
                }
                CloseScreen(screen);
        }
        CloseLibrary((struct Library *)IntuitionBase);
}

if (NULL != GfxBase)
        CloseLibrary((struct Library *)GfxBase);

exit((int)retval);
}
</pre>

== ECS and Genlocking Features ==

The Enhanced Chip Set (ECS) Denise chip (''8373-R2a''), coupled with the Release 2 graphics library, opens up a whole new set of genlocking possibilities. Unlike the old Denise, whose only genlocking ability allowed keying on color register zero, the ECS Denise allows keying on any color register. Also, the ECS Denise allows keying on any bitplane of the ViewPort being genlocked. With the ECS Denise, the border area surrounding the display can be made transparent (always passes video) or opaque (overlays using color 0). All the new features are set individually for each ViewPort. These features can be used in conjunction with each other, making interesting scenarios possible.

=== Genlock Control ===

Using VideoControl(), a program can enable, disable, or obtain the state of a ViewPort’s genlocking features. It returns NULL if no error occurred. The function uses a tag based interface:

<pre>error = BOOL VideoControl( struct ColorMap *cm, struct TagItem *ti );</pre>
The ti argument is a list of video commands stored in an array of TagItem structures. The cm argument specifies which ColorMap and, indirectly, which ViewPort these genlock commands will be applied to. The possible commands are:

<pre>VTAG_BITPLANEKEY_GET, _SET, _CLR
VTAG_CHROMA_PLANE_GET, _SET
VTAG_BORDERBLANK_GET, _SET, _CLR
VTAG_BORDERNOTRANS_GET, _SET, _CLR
VTAG_CHROMAKEY_GET, _SET, _CLR
VTAG_CHROMAPEN_GET, _SET, _CLR</pre>
This section covers only the genlock VideoControl() tags. See &lt;graphics/videocontrol.h&gt; for a complete list of all the available tags you can use with VideoControl().

VTAG_BITPLANEKEY_GET

is used to find out the status of the bitplane keying mode. VTAG_BITPLANEKEY_SET and VTAG_BITPLANEKEY_CLR activate and deactivate bitplane keying mode. If bitplane key mode is on, genlocking will key on the bits set in a specific bitplane from the ViewPort (the specific bitplane is set with a different tag). The data portion of these tags is NULL.

For inquiry commands like VTAG_BITPLANEKEY_GET (tags ending in _GET), VideoControl() changes the _GET tag ID (ti_Tag) to the corresponding _SET or _CLR tag ID, reflecting the current state of the genlock mode. For example, when passed the following tag array:

<pre>struct TagItem videocommands[] =
{
    {VTAG_BITPLANEKEY_GET, NULL},
    {VTAG_END_CM, NULL}
};</pre>
VideoControl()

changes the VTAG_BITPLANEKEY_GET tag ID (ti_Tag) to VTAG_BITPLANEKEY_SET if bitplane keying is currently on, or to VTAG_BITPLANEKEY_CLR if bitplane keying is off. In both of these cases, VideoControl() only uses the tag’s ID, ignoring the tag’s data field (ti_Data).

The VTAG_CHROMA_PLANE_GET tag returns the number of the bitplane keyed on when bitplane keying mode is on. VideoControl() changes the tag’s data value to the bitplane number. VTAG_CHROMA_PLANE_SET sets the bitplane number to the tag’s data value.

VTAG_BORDERBLANK_GET

is used to obtain the border blank mode status. This tag works exactly like VTAG_BITPLANEKEY_GET. VideoControl() changes the tag’s ID to reflect the current border blanking state. VTAG_BORDERBLANK_SET and VTAG_BORDERBLANK_CLR activate and deactivate border blanking. If border blanking is on, the Amiga will not display anything in its display border, allowing an external video signal to show through the border area. On the Amiga display, the border appears black. The data portion of these tags is NULL.

The VTAG_BORDERNOTRANS_GET, _SET and _CLR tags are used, respectively, to obtain the status of border-not-transparent mode, and to activate and to deactivate this mode. If set, the Amiga display’s border will overlay external video with the color in register 0. Because border blanking mode takes precedence over border-not-transparent mode, setting border-not-transparent has no effect if border blanking is on. The data portion of these tags is NULL.

The VTAG_CHROMAKEY_GET, _SET and _CLR tags are used, respectively, to obtain the status of chroma keying mode, and to activate and deactivate chroma keying mode. If set, the genlock will key on colors from specific color registers (the specific color registers are set using a different tag). If chroma keying is not set, the genlock will key on color register 0. The data portion of these tags is NULL.

VTAG_CHROMAPEN_GET

obtains the chroma keying status of an individual color register. The tag’s ti_Data field contains the register number. Like the other _GET tags, VideoControl() changes the tag ID (ti_Tag) to one that reflects the current state of the mode. VTAG_CHROMAPEN_SET and VTAG_CHROMAPEN_CLR activate and deactivate chroma keying for each individual color register. Chroma keying can be active for more than one register. By turning off border blanking and activating chroma keying mode, but turning off chroma keying for each color register, a program can overlay every part of an external video source, completely blocking it out.

After using VideoControl() to set values in the ColorMap, the corresponding ViewPort has to be rebuilt with MakeVPort(), MrgCop() and LoadView(), so the changes can take effect. A program that uses a screen’s ViewPort rather than its own ViewPort should use the Intuition functions MakeScreen() and RethinkDisplay() to make the display changes take effect.

The following code fragment shows how to access the genlock modes.

<pre>struct Screen *genscreen;
struct ViewPort *vp;
struct TagItem vtags [24];

        /* The complete example opened a window, rendered some colorbars,  */
        /* and added gadgets to allow the user to turn the various genlock */
        /* modes on and off.                                               */

        vp = &amp;(genscreen-&gt;ViewPort);

        /* Ascertain the current state of the various modes. */

        /* Is borderblanking on? */
        vtags[0].ti_Tag = VTAG_BORDERBLANK_GET;
        vtags[0].ti_Data = NULL;

        /* Is bordertransparent set? */
        vtags[1].ti_Tag = VTAG_BORDERNOTRANS_GET;
        vtags[1].ti_Data = NULL;

        /* Key on bitplane? */
        vtags[2].ti_Tag = VTAG_BITPLANEKEY_GET;
        vtags[2].ti_Tag = NULL;

        /* Get plane which is used to key on */
        vtags[3].ti_Tag = VTAG_CHROMA_PLANE_GET;
        vtags[3].ti_Data = NULL;

        /* Chromakey overlay on? */
        vtags[4].ti_Tag = VTAG_CHROMAKEY_GET;
        vtags[4].ti_Data = NULL;

        for (i = 0; i &lt; 16; i++)
        {
                /* Find out which colors overlay */
                vtags[i + 5].ti_Tag = VTAG_CHROMA_PEN_GET;
                vtags[i + 5].ti_Data = i;
        }

        /* Indicate end of tag array */
        vtags[21].ti_Tag = VTAG_END_CM;
        vtags[21].ti_Data = NULL;

        /* And send the commands. On return the Tags themselves will
        * indicate the genlock settings for this ViewPort's ColorMap.
        */
        error = VideoControl(vp-&gt;ColorMap, vtags);

        /* The complete program sets gadgets to reflect current states. */

        /* Will only send single commands from here on. */
        vtags[1].ti_Tag = VTAG_END_CM;

        /* At this point the complete program gets an input event and sets/clears the
           genlock modes as requested using the vtag list and VideoControl().
        */

        /* send video command */
        error = VideoControl(vp-&gt;ColorMap, vtags);

        /* Now use MakeScreen() and RethinkDisplay() to make the VideoControl()
        *  changes take effect.  If we were using our own ViewPort rather than
        * borrowing one from a screen, we would instead do:
        *
        *   MakeVPort(ViewAddress(),vp);
        *   MrgCop(ViewAddress());
        *   LoadView(ViewAddres());
        */
        MakeScreen(genscreen);
        RethinkDisplay();

/* The complete program closes and frees everything it had opened or allocated. */


/* The complete example calls the CheckPAL function, which is included below in its
   entirety for illustrative purposes.
*/

BOOL CheckPAL(STRPTR screenname)
{
        struct Screen *screen;
        ULONG modeID = LORES_KEY;
        struct DisplayInfo displayinfo;
        BOOL IsPAL;

        if (GfxBase-&gt;LibNode.lib_Version &gt;= 36)
        {
                /*
                * We got at least V36, so lets use the new calls to find out what
                * kind of videomode the user (hopefully) prefers.
                */

                if (screen = LockPubScreen(screenname))
                {
                        /*
                        * Use graphics.library/GetVPModeID() to get the ModeID of the specified screen.
                        * Will use the default public screen (Workbench most of the time) if NULL It is
                        * _very_ unlikely that this would be invalid, heck it's impossible.
                        */
                        if ((modeID = GetVPModeID(&amp;(screen-&gt;ViewPort))) != INVALID_ID)
                        {
                                /*
                                * If the screen is in VGA mode, we can't tell whether the system is PAL
                                * or NTSC. So to be foolproof we fall back to the displayinfo of the default
                                * monitor by inquiring about just the LORES_KEY displaymode if we don't know.
                                * The default.monitor reflects the initial video setup of the system, thus
                                * for either ntsc.monitor or pal.monitor.  We only use the displaymode of the
                                * is an alias specified public screen if it's display mode is PAL or NTSC and
                                * NOT the default.
                                */
                                if (!((modeID &amp; MONITOR_ID_MASK) == NTSC_MONITOR_ID ||
                                (modeID &amp; MONITOR_ID_MASK) == PAL_MONITOR_ID))
                                modeID = LORES_KEY;
                        }
                        UnlockPubScreen(NULL, screen);
                } /* if fails modeID = LORES_KEY. Can't lock screen, so fall back on default monitor. */

                if (GetDisplayInfoData(NULL, (UBYTE *) &amp; displayinfo,
                sizeof(struct DisplayInfo), DTAG_DISP, modeID))
                {
                        if (displayinfo.PropertyFlags &amp; DIPF_IS_PAL)
                                IsPAL = TRUE;
                        else
                                IsPAL = FALSE;
                        /* Currently the default monitor is always either PAL or NTSC. */
                }
        }
        else
                /* &lt; V36. The enhancements to the videosystem in V36 (and above) cannot be better
                * expressed than with the simple way to determine PAL in V34.
                */
                IsPAL= (GfxBase-&gt;DisplayFlags &amp; PAL) ? TRUE : FALSE;

        return(IsPAL);
}
</pre>

== Accessing the Blitter Directly ==

To use the blitter directly, you must first be familiar with how its registers control its operation. This topic is covered thoroughly in the ''Amiga Hardware Reference Manual'' and is not repeated here. There are two basic approaches you can take to perform direct programming of the blitter: synchronous and asynchronous.

* Synchronous programming of the blitter is used when you want to do a job with the blitter right away. For synchronous programming, you first get exclusive access to the blitter with OwnBlitter(). Next call WaitBlit() to ensure that any previous blitter operation that might have been in progress is completed. Then set up your blitter operation by programming the blitter registers. Finally, start the blit and call DisownBlitter().

* Asynchronous programming of the blitter is used when the blitter operation you want to perform does not have to happen immediately. In that case, you can use the QBlit() and QBSBlit() functions in order to queue up requests for the use of the blitter on a non-exclusive basis. You share the blitter with system tasks.

Whichever approach you take, there is one rule you should generally keep in mind about using the blitter directly:

{{Note|title=Don’t Tie Up The Blitter|text=The system uses the blitter extensively for disk and display operation. While your task is using the blitter, many other system processes will be locked out. Therefore, use it only for brief periods and relinquish it as quickly as possible.}}

To use QBlit() and QBSBlit(), you must create a data structure called a bltnode (blitter node) that contains a pointer to the blitter code you want to execute. The system uses this structure to link blitter usage requests into a first-in, first-out (FIFO) queue. When your turn comes, your own blitter routine can be repeatedly called until your routine says it is finished using the blitter.

Two separate blitter queues are maintained. One queue is for the QBlit() routine. You use QBlit() when you simply want something done and you do not necessarily care when it happens. This may be the case when you are moving data in a memory area that is not currently being displayed.

The second queue is maintained for QBSBlit(). QBS stands for “queue-beam-synchronized”. QBSBlit() requests form a beam-synchronized FIFO queue. When the video beam gets to a predetermined position, your blitter routine is called. Beam synchronization takes precedence over the simple FIFO. This means that if the beam sync matches, the beam-synchronous blit will be done before the non-synchronous blit in the first position in the queue. You might use QBSBlit() to draw into an area of memory that is currently being displayed to modify memory that has already been “passed-over” by the video beam. This avoids display flicker as an area is being updated.

The sole input to both QBlit() and QBSBlit() is a pointer to a bltnode data structure, defined in the include file &lt;hardware/blit.h&gt;. Here is a copy of the structure, followed by details about the items you must initialize:

<pre>
struct bltnode
{
    struct  bltnode *n;
    int     (*function)();
    char    stat;
    short   blitsize;
    short   beamsync;
    int     (*cleanup)();
};
</pre>

This is a pointer to the next bltnode, which, for most applications will be zero. You should not link bltnodes together. This is to be performed by the system in a separate call to QBlit() or QBSBlit().

This is the address of your blitter function that the blitter queuer will call when your turn comes up. Your function must be formed as a subroutine, with an RTS instruction at the end. Follow Amiga programming conventions by placing the return value in D0 (or in C, use return(value)).

If you return a nonzero value, the system will call your routine again next time the blitter is idle until you finally return 0. This is done so that you can maintain control over the blitter; for example, it allows you to handle all five bitplanes if you are blitting an object with 32 colors. For display purposes, if you are blitting multiple objects and then saving and restoring the background, you must be sure that all planes of the object are positioned before another object is overlaid. This is the reason for the lockup in the blitter queue; it allows all work per object to be completed before going on to the next one.

{{Note|text=Not all C compilers can handle (*function)() properly! The system actually tests the processor ''status codes'' for a condition of equal-to-zero (Z flag set) or not-equal-to-zero (Z flag clear) when your blitter routine returns. Some C compilers do not set the processor status code properly (i.e., according to the value returned), thus it is not possible to use such compilers to write the (*function)() routine. In that case assembly language should be used. Blitter functions are normally written in assembly language anyway so they can take advantage of the ability of QBlit() and QBSBlit() to pass them parameters in processor registers.}}

The register passing conventions for these routines are as follows. Register A0 receives a pointer to the system hardware registers so that all hardware registers can be referenced as an offset from that address. Register A1 contains a pointer to the current bltnode. You may have queued up multiple blits, each of which perhaps uses the same blitter routine. You can access the data for this particular operation as an offset from the value in A1. For instance, a typical user of these routines can precalculate the blitter register values to be placed in the blitter registers and, when the routine is called, simply copy them in. For example, you can create a new structure such as the following:

<pre>
    INCLUDE &quot;exec/types.i&quot;
    INCLUDE &quot;hardware/blit.i&quot;

    STRUCTURE mybltnode,0
                      ; Make this new structure compatible with a bltnode
                      ; by making the first element a bltnode structure.
    STRUCT bltnode,bn_SIZEOF
            UWORD   bltcon1         ; Blitter control register 1.
            UWORD   fwmask          ; First and last word masks.
            UWORD   lwmask
            UWORD   bltmda          ; Modulos for sources a, b,and c.
            UWORD   bltmdb
            UWORD   bltmdc
            UWORD   any_more_data   ; add anything else you want
    LABEL mbn_SIZEOF
</pre>

Other forms of data structures are certainly possible, but this should give you the general idea.

Tells the system whether or not to execute the clean-up routine at the end. This byte should be set to CLEANUP (0x40) if cleanup is to be performed. If not, then the bltnode cleanup variable can be zero.

The value that should be in the VBEAM counter for use during a beam-synchronous blit before the function() is called.

The system cooperates with you in planning when to start a blit in the routine QBSBlit() by not calling your routine until, for example, the video beam has already passed by the area on the screen into which you are writing. This is especially useful during single buffering of your displays. There may be time enough to write the object between scans of the video display. You will not be visibly writing while the beam is trying to scan the object. This avoids flicker (part of an old view of an object along with part of a new view of the object).

The address of a routine that is to be called after your last return from the QBlit() routine. When you finally return a zero, the queuer will call this subroutine (ends in RTS or return()) as the clean-up. Your first entry to the function may have dynamically allocated some memory or may have done something that must be undone to make for a clean exit. This routine must be specified.