78 lines
3.5 KiB
Markdown
78 lines
3.5 KiB
Markdown
---
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title: Michael Abrash's Graphics Programming Black Book, Special Edition
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author: Michael Abrash
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date: '1997-07-01'
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isbn: '1576101746'
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publisher: The Coriolis Group
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category: 'Web and Software Development: Game Development,Web and Software Development:
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Graphics and Multimedia Development'
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chapter: '43'
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pages: 811-813
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---
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Another limitation of bit-plane animation is that it's best if images
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stored in the same plane never cross each other. Why? Because when
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images do cross, the blank fringe
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| Palette Register | Register Setting |
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|------------------|------------------|
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| 0 | 00H (black) |
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| 1 | 01H (blue) |
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| 2 | 02H (green) |
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| 3 | 03H (cyan) |
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| 4 | 04H (red) |
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| 5 | 05H (magenta) |
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| 6 | 14H (brown) |
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| 7 | 07H (light gray) |
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| 8 | 3EH (yellow) |
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| 9 | 3EH (yellow) |
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| 10 | 3EH (yellow) |
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| 11 | 13EH (yellow) |
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| 12 | 3EH (yellow) |
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| 13 | 3EH (yellow) |
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| 14 | 3EH (yellow) |
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| 15 | 3EH (yellow) |
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Table: Table 43.2 Palette RAM settings for two-plane animation.
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around each image can temporarily erase the overlapped parts of the
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other image or images, resulting in momentary flicker. While that's not
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fatal, it certainly detracts from the rock-solid animation effect of
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bit-plane animation.
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Not allowing images in the same plane to overlap is actually less of a
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limitation than it seems. Run Listing 43.1 again. Unless you were
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looking for it, you'd never notice that images of the same color almost
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never overlap—there's plenty of action to distract the eye, and the
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trajectories of images of the same color are arranged so that they have
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a full range of motion without running into each other. The only
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exception is the chain of green images, which occasionally doubles back
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on itself when it bounces directly into a corner and reverses direction.
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Here, however, the images are moving so quickly that the brief moment
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during which one image's fringe blanks a portion of another image is
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noticeable only upon close inspection, and not particularly unaesthetic
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even then.
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When a technique has such tremendous visual and performance advantages
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as does bit-plane animation, it behooves you to design your animation
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software so that the limitations of the animation technique don't get in
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the way. For example, you might design a shooting gallery game with all
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the images in a given plane marching along in step in a continuous band.
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The images could never overlap, so bit-plane animation would produce
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very high image quality.
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### Shearing and Page Flipping {#Heading7}
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As Listing 43.1 runs, you may occasionally see an image shear, with the
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top and bottom parts of the image briefly offset. This is a consequence
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of drawing an image directly into memory as that memory is being scanned
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for video data. Occasionally the CRT controller scans a given area of
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display memory for pixel data just as the program is changing that same
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memory. If the CRT controller scans memory faster than the CPU can
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modify that memory, then the CRT controller can scan out the bytes of
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display memory that have been already been changed, pass the point in
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the image that the CPU is currently drawing, and start scanning out
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bytes that haven't yet been changed. The result: Mismatched upper and
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lower portions of the image.
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