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<meta name="vspubdate" content="07/01/97" />
<meta name="vscategory" content="Web and Software Development: Game Development,Web and Software Development: Graphics and Multimedia Development" />
<title>Michael Abrash's Graphics Programming Black Book Special Edition: Video Est Omnis Divisa</title><!-- HEADER -->
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<title>Michael Abrash's Graphics Programming Black Book Special Edition: Video Est Omnis Divisa</title>
<meta name="chapter" content="30" />
<meta name="pages" content="574-575" />
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<p><br /></p>
<h3><a id="Heading8"></a>Split Screen and Panning</h3>
<h3 id="Heading8">Split Screen and Panning</h3>
<p>Back in Chapter 23, I presented a program that performed smooth horizontal panning. Smooth horizontal panning consists of two parts: byte-by-byte (8-pixel) panning by changing the start address and pixel-by-pixel intrabyte panning by setting the Pel Panning register (AC register 13H) to adjust alignment by 0 to 7 pixels. (IBM prefers its own jargon and uses the word &ldquo;pel&rdquo; instead of &ldquo;pixel&rdquo; in much of their documentation, hence &ldquo;pel panning.&rdquo; Then there&rsquo;s DASD, a.k.a. Direct Access Storage Device&mdash;IBM-speak for hard disk.)</p>
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<p>On the VGA, there is recourse. A VGA-only bit, bit 5 of the AC Mode Control register (AC register 10H), turns off pel panning in the split screen. In other words, when this bit is set to 1, pel panning is reset to zero before the first line of the split screen, and remains zero until the end of the frame. This doesn&rsquo;t allow you to pan the split screen horizontally, mind you&mdash;there&rsquo;s no way to do that&mdash;but it does let you pan the normal screen while the split screen stays rock-solid. This can be used to produce an attractive &ldquo;streaming tape&rdquo; effect in the normal screen while the split screen is used to display non-moving information.</p>
<h4 align="left"><a id="Heading9"></a>The Split Screen and Horizontal Panning: An Example</h4>
<h4 align="left" id="Heading9">The Split Screen and Horizontal Panning: An Example</h4>
<p>Listing 30.2 illustrates the interaction of horizontal smooth panning with the split screen, as well as the suppression of pel panning in the split screen. Listing 30.2 creates a virtual screen 1024 pixels across by setting the Offset register (CRTC register 13H) to 64, sets the normal screen to scan video data beginning far enough up in display memory to leave room for the split screen starting at offset zero, turns on the split screen, and fills in the normal screen and split screen with distinctive patterns. Next, Listing 30.2 pans the normal screen horizontally without setting bit 5 of the AC Mode Control register to 1. As you&rsquo;d expect, the split screen jerks about quite horribly. After a key press, Listing 30.2 sets bit 5 of the Mode Control register and pans the normal screen again. This time, the split screen doesn&rsquo;t budge an inch&mdash;<i>if</i> the code is running on a VGA.</p>
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<div align="center">
Graphics Programming Black Book &copy; 2001 Michael Abrash
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