Misc/abrash-black-book
Watch
1
0
Fork
You've already forked abrash-black-book
 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

Strip comments and inline ids onto h*

Browse source
This commit is contained in:
James Gregory 2013-12-30 15:11:04 +11:00
commit 0f5f617adb
362 changed files with 2519 additions and 6694 deletions
Download patch file Download diff file Expand all files Collapse all files

24
55-04.html
View file

@ -10,16 +10,9 @@
<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: Color Modeling in 256-Color Mode</title><!-- HEADER -->
<!-- Empty Reference Subhead -->
<!--ISBN=1576101746//-->
<!--TITLE=Michael Abrash's Graphics Programming Black Book Special Edition//-->
<!--AUTHOR=Michael Abrash//-->
<!--PUBLISHER=The Coriolis Group, Inc.//-->
<title>Michael Abrash's Graphics Programming Black Book Special Edition: Color Modeling in 256-Color Mode</title>
<meta name="chapter" content="55" />
<meta name="pages" content="1040-1044" />
<!--UNASSIGNED1//-->
<!--UNASSIGNED2//-->
</head>
<body>
@ -39,15 +32,14 @@
<p>This is where it gets a little complicated. In write mode 3 (which incidentally is not available on the EGA), each byte value that the CPU writes to the VGA does not get written to display memory. Instead, it turns into the bit mask. (Actually, it&rsquo;s ANDed with the Bit Mask register, and the result becomes the bit mask, but we&rsquo;ll leave the Bit Mask register set to 0xFF, so the CPU value will become the bit mask.) The bit mask selects, on a bit-by-bit basis, between the data in the latches for each plane (the previously loaded background color, in this case) and the foreground color. Where does the foreground color come from, if not from the CPU? From the Set/Reset register, as shown in Figure 55.3. Thus, each byte written by the CPU (font data, presumably) selects foreground or background color for each of eight pixels, all done with a single write to display memory.</p>
<p><a id="Fig3"><!-- </A><A HREF="javascript:displayWindow('images/55-03.jpg',407,375 )"> --><img src="images/55-03.jpg" /><br />
<!-- </A>
<BR><A HREF="javascript:displayWindow('images/55-03.jpg',407,375)"> --><b>Figure 55.3</b></a>&nbsp;&nbsp;<i>The data path in write mode 3.</i></p>
<p><a id="Fig3"><img src="images/55-03.jpg" /><br />
<b>Figure 55.3</b></a>&nbsp;&nbsp;<i>The data path in write mode 3.</i></p>
<p>I know this sounds pretty esoteric, but think of it this way: The latches hold the background color in a form suitable for writing eight background pixels (one full byte) at a pop. Write mode 3 allows each CPU byte to punch holes in the background color provided by the latches, holes through which the foreground color from the Set/Reset register can flow. The result is that a single write draws exactly the combination of foreground and background pixels described by each font byte written by the CPU. It may help to look at Listing 55.4, which shows The BitMan&rsquo;s technique in action. And yes, this technique is absolutely worth the trouble; it&rsquo;s about three times faster than the fill-then-draw approach described above, and about twice as fast as transparent text. So far as I know, there is no faster way to draw text on a VGA.</p>
<p>It&rsquo;s important to note that the BitMan&rsquo;s technique only works on full bytes of display memory. There&rsquo;s no way to clip to finer precision; the background color will inevitably flood all of the eight destination pixels that aren&rsquo;t selected as foreground pixels. This makes The BitMan&rsquo;s technique most suitable for monospaced fonts with characters that are multiples of eight pixels in width, and for drawing to byte-aligned addresses; the technique can be used in other situations, but is considerably more difficult to apply.</p>
<p><b>LISTING 55.4 L55-4.ASM</b></p><!-- CODE //-->
<p><b>LISTING 55.4 L55-4.ASM</b></p>
<pre>
; Demonstrates drawing solid text on the VGA, using the BitMan&rsquo;s write mode
; 3-based, one-pass technique.
@ -218,7 +210,7 @@ DrawCharLoop: ;draw all lines of the character
ret
DrawTextString endp
end start
</pre><!-- END CODE //-->
</pre>
<p><br /></p>
@ -237,10 +229,6 @@ DrawTextString endp
<div align="center">
Graphics Programming Black Book &copy; 2001 Michael Abrash
</div><!-- all of the reference materials (books) have the footer and subfoot reveresed -->
<!-- reference_subfoot = footer -->
<!-- reference_footer = subfoot -->
<!-- BEGIN SUB FOOTER -->
<!-- END FOOTER -->
</div>
</body>
</html>