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James Gregory 2013-12-30 15:11:04 +11:00
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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: Yet Another VGA Write Mode</title><!-- HEADER -->
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<!--ISBN=1576101746//-->
<!--TITLE=Michael Abrash's Graphics Programming Black Book Special Edition//-->
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<!--PUBLISHER=The Coriolis Group, Inc.//-->
<title>Michael Abrash's Graphics Programming Black Book Special Edition: Yet Another VGA Write Mode</title>
<meta name="chapter" content="27" />
<meta name="pages" content="508-515" />
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<h4 align="left"><a id="Heading6"></a>Drawing Color-Patterned Lines Using Write Mode 2</h4>
<h4 align="left" id="Heading6">Drawing Color-Patterned Lines Using Write Mode 2</h4>
<p>A more serviceable use of write mode 2 is shown in the program presented in Listing 27.2. The program draws multicolored horizontal, vertical, and diagonal lines, basing the color patterns on passed color tables. Write mode 2 is ideal because in this application color can vary from one pixel to the next, and in write mode 2 all that&rsquo;s required to set pixel color is a change of the lower nibble of the byte written by the CPU. Set/reset could be used to achieve the same result, but an index/data pair of <b>OUT</b>s would be required to set the Set/Reset register to each new color. Similarly, the Map Mask register could be used in write mode 0 to set pixel color, but in this case not only would an index/data pair of <b>OUT</b>s be required but there would also be no guarantee that data already in display memory wouldn&rsquo;t interfere with the color of the pixel being drawn, since the Map Mask register allows only selected planes to be drawn to.</p>
<p>Listing 27.2 is hardly a comprehensive line drawing program. It draws only a few special line cases, and although it is reasonably fast, it is far from the fastest possible code to handle those cases, because it goes through a dot-plot routine and because it draws horizontal lines a pixel rather than a byte at a time. Write mode 2 would, however, serve just as well in a full-blown line drawing routine. For any type of patterned line drawing on the VGA, the basic approach remains the same: Use the bit mask to select the pixel (or pixels) to be altered and use the CPU byte in write mode 2 to select the color in which to draw.</p>
<p><b>LISTING 27.2 L27-2.ASM</b></p><!-- CODE //-->
<p><b>LISTING 27.2 L27-2.ASM</b></p>
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; Program to illustrate one use of write mode 2 of the VGA and EGA by
; drawing lines in color patterns.
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<p><br /></p>
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<div align="center">
Graphics Programming Black Book &copy; 2001 Michael Abrash
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