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James Gregory
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43-01.html
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43-01.html
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<meta name="vspubdate" content="07/01/97" />
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<meta name="vscategory" content="Web and Software Development: Game Development,Web and Software Development: Graphics and Multimedia Development" />
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<title>Michael Abrash's Graphics Programming Black Book Special Edition: Bit-Plane Animation</title><!-- HEADER -->
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<!-- Empty Reference Subhead -->
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<!--ISBN=1576101746//-->
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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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<!--PUBLISHER=The Coriolis Group, Inc.//-->
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<title>Michael Abrash's Graphics Programming Black Book Special Edition: Bit-Plane Animation</title>
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<meta name="chapter" content="43" />
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<meta name="pages" content="793-799" />
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<p><br /></p>
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<h2><a id="Heading1"></a>Chapter 43<br />
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<h2 id="Heading1">Chapter 43<br />
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Bit-Plane Animation</h2>
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<h3><a id="Heading2"></a>A Simple and Extremely Fast Animation Method for Limited Color</h3>
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<h3 id="Heading2">A Simple and Extremely Fast Animation Method for Limited Color</h3>
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<p>When it comes to computers, my first love is animation. There’s nothing quite like the satisfaction of fooling the eye and creating a miniature reality simply by rearranging a few bytes of display memory. What makes animation particularly interesting is that it has to happen fast (as measured in human time), and without blinking and flickering, or else you risk destroying the illusion of motion and solidity. Those constraints make animation the toughest graphics challenge—and also the most rewarding.</p>
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<p>It doesn’t much matter if bit-plane animation isn’t perfect for all applications, though. The real point of showing you bit-plane animation is to bring home the reality that the VGA is a complex adapter with many resources, and that you can do remarkable things if you understand those resources and come up with creative ways to put them to work at specific tasks.</p>
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<h3><a id="Heading3"></a>Bit-Planes: The Basics</h3>
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<h3 id="Heading3">Bit-Planes: The Basics</h3>
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<p>The underlying principle of bit-plane animation is extremely simple. The VGA has four separate bit planes in modes 0DH, 0EH, 10H, and 12H. Plane 0 normally contains data for the blue component of pixel color, plane 1 normally contains green pixel data, plane 2 red pixel data, and plane 3 intensity pixel data—but we’re going to mix that up a bit in a moment, so we’ll simply refer to them as planes 0, 1, 2, and 3 from now on.</p>
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<p>Take a good look at Figure 43.1. Any light bulbs going on over your head yet? If not, consider this. The general problem with VGA animation is that it’s complex and time-consuming to manipulate images that span the four planes (as most do), and that it’s hard to avoid interference problems when images intersect, since those images share the same bits in display memory. Since the four bit planes can be written to and read from independently, it should be apparent that if we could come up with a way to display images from each plane independently of whatever images are stored in the other planes, we would have four sets of images that we could manipulate very easily. There would be no interference effects between images in different planes, because images in one plane wouldn’t share bits with images in another plane. What’s more, since all the bits for a given image would reside in a single plane, we could do away with the cumbersome programming of the VGA’s complex hardware that is needed to manipulate images that span multiple planes.</p>
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<p><a id="Fig1"><!-- </A><A HREF="javascript:displayWindow('images/43-01.jpg',408,311 )"> --><img src="images/43-01.jpg" /><br />
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<BR><A HREF="javascript:displayWindow('images/43-01.jpg',408,311)"> --><b>Figure 43.1</b></a> <i>How 4 bits of video data become 6 bits of color.</i></p>
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<p><a id="Fig1"><img src="images/43-01.jpg" /><br />
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<b>Figure 43.1</b></a> <i>How 4 bits of video data become 6 bits of color.</i></p>
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<p>All in all, it would be a good deal if we could store each image in a single plane, as shown in Figure 43.2. However, a problem arises when images in different planes overlap, as shown in Figure 43.3. The combined bits from overlapping images generate new colors, so the overlapping parts of the images don’t look like they belong to either of the two images. What we really want, of course, is for one of the images to appear to be in front of the other. It would be better yet if the rearward image showed through any transparent (that is, background-colored) parts of the forward image. Can we do that?</p>
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<p>You bet.</p>
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<p><a id="Fig2"><!-- </A><A HREF="javascript:displayWindow('images/43-02.jpg',408,241 )"> --><img src="images/43-02.jpg" /><br />
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<BR><A HREF="javascript:displayWindow('images/43-02.jpg',408,241)"> --><b>Figure 43.2</b></a> <i>Storing images in separate planes.</i></p>
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<p><a id="Fig2"><img src="images/43-02.jpg" /><br />
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<b>Figure 43.2</b></a> <i>Storing images in separate planes.</i></p>
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<p><a id="Fig3"><!-- </A><A HREF="javascript:displayWindow('images/43-03.jpg',409,243 )"> --><img src="images/43-03.jpg" /><br />
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<!-- </A>
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<BR><A HREF="javascript:displayWindow('images/43-03.jpg',409,243)"> --><b>Figure 43.3</b></a> <i>The problem of overlapping colors.</i></p>
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<p><a id="Fig3"><img src="images/43-03.jpg" /><br />
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<b>Figure 43.3</b></a> <i>The problem of overlapping colors.</i></p>
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<p><br /></p>
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<div align="center">
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Graphics Programming Black Book © 2001 Michael Abrash
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</div><!-- all of the reference materials (books) have the footer and subfoot reveresed -->
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