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Remove colour attributes from body and strip most of the font tags out

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James Gregory 2013-12-30 13:57:02 +11:00
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<!--CHAPTER=27//-->
<!--PAGES=508-515//-->
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<!--UNASSIGNED2//--></HEAD><BODY LINK=#0000FF ALINK=#000099 VLINK=#0000FF BGCOLOR=#FFFFFF>
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</P>
<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP"><IMG SRC="images/i.jpg"><TD WIDTH="95%"><SMALL><I>For performance, it&rsquo;s best to store 16-color bitmaps in pre-separated four-plane format in system memory, and copy one plane at a time to the screen. Ideally, such bitmaps should be copied one scan line at a time, with all four planes completed for one scan line before moving on to the next. I say this because when entire images are copied one plane at a time, nasty transient color effects can occur as one plane becomes visibly changed before other planes have been modified.</I></SMALL>
</TABLE>
<H4 ALIGN="LEFT"><A NAME="Heading6"></A><FONT COLOR="#000077">Drawing Color-Patterned Lines Using Write Mode 2</FONT></H4>
<H4 ALIGN="LEFT"><A NAME="Heading6"></A>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>
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<font face="Verdana,sans-serif" size="1">Graphics Programming Black Book &copy; 2001 Michael Abrash</font>
Graphics Programming Black Book &copy; 2001 Michael Abrash
</div>
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