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James Gregory 2013-12-30 15:11:04 +11:00
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@ -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: Saving Screens and Other VGA Mysteries</title><!-- HEADER -->
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<!--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: Saving Screens and Other VGA Mysteries</title>
<meta name="chapter" content="29" />
<meta name="pages" content="547-550" />
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</head>
<body>
@ -47,10 +40,10 @@
<p>What&rsquo;s the solution? Frankly, the solution is to get VGA-specific. A TSR designed for the VGA can simply read out and save the state of the registers of interest, program those registers as needed, save the screen image, and restore the original settings. From a programmer&rsquo;s perspective, readable registers are certainly near the top of the list of things to like about the VGA! The remaining installed base of EGAs is steadily dwindling, and you may be able to ignore it as a market today, as you couldn&rsquo;t even a year or two ago.</p>
<p>If you are going to write a hi-res VGA version of the screen capture program, be sure to account for the increased size of the VGA&rsquo;s mode 12H bit map. The mode 12H (640x480) screen uses 37.5K per plane of display memory, so for mode 12H the displayed screen size equate in Listings 29.1 and 29.2 should be changed to:</p><!-- CODE SNIP //-->
<p>If you are going to write a hi-res VGA version of the screen capture program, be sure to account for the increased size of the VGA&rsquo;s mode 12H bit map. The mode 12H (640x480) screen uses 37.5K per plane of display memory, so for mode 12H the displayed screen size equate in Listings 29.1 and 29.2 should be changed to:</p>
<pre>
DISPLAYED_SCREEN_SIZEequ(640/8)*480
</pre><!-- END CODE SNIP //-->
</pre>
<p>Similarly, if you&rsquo;re capturing a graphics screen that starts at an offset other than 0 in the segment at A000H, you must change the memory offset used by the disk functions to match. You can, if you so desire, read the start offset of the display memory providing the information shown on the screen from the Start Address registers (CRT Controller registers 0CH and 0DH); these registers are readable even on an EGA.</p>
@ -66,7 +59,7 @@ DISPLAYED_SCREEN_SIZEequ(640/8)*480
</tr>
</table>
<h3><a id="Heading4"></a>16 Colors out of 64</h3>
<h3 id="Heading4">16 Colors out of 64</h3>
<p>How does one produce the 64 colors from which the 16 colors displayed by the EGA can be chosen? The answer is simple enough: There&rsquo;s a BIOS function that lets you select the mapping of the 16 possible pixel values to the 64 possible colors. Let&rsquo;s lay out a bit of background before proceeding, however.</p>
@ -78,9 +71,8 @@ DISPLAYED_SCREEN_SIZEequ(640/8)*480
<p>Each of the 16 palette registers stores the mapping of one of the 16 possible 4-bit pixel values from memory to one of 64 possible 6-bit pixel values to be sent to the monitor as video data, as shown in Figure 29.2. A 4-bit pixel value of 0 causes the 6-bit value stored in palette register 0 to be sent to the display as the color of that pixel, a pixel value of 1 causes the contents of palette register 1 to be sent to the display, and so on. Since there are only four input bits, it stands to reason that only 16 colors are available at any one time; since there are six output bits, however, those 16 colors can be mapped to any of 64 colors. The mapping for each of the 16 pixel values is controlled by the lower six bits of the corresponding palette register, as shown in Figure 29.3. Secondary red, green, and blue are less-intense versions of red, green, and blue, although their exact effects vary from monitor to monitor. The best way to figure out what the 64 colors look like on your monitor is to see them, and that&rsquo;s just what the program in Listing 29.3, which we&rsquo;ll discuss shortly, lets you do.</p>
<p><a id="Fig2"><!-- </A><A HREF="javascript:displayWindow('images/29-02.jpg',411,243 )"> --><img src="images/29-02.jpg" /><br />
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<BR><A HREF="javascript:displayWindow('images/29-02.jpg',411,243)"> --><b>Figure 29.2</b></a>&nbsp;&nbsp;<i>Color translation via the palette registers.</i></p>
<p><a id="Fig2"><img src="images/29-02.jpg" /><br />
<b>Figure 29.2</b></a>&nbsp;&nbsp;<i>Color translation via the palette registers.</i></p>
<p><br /></p>
@ -99,10 +91,6 @@ DISPLAYED_SCREEN_SIZEequ(640/8)*480
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Graphics Programming Black Book &copy; 2001 Michael Abrash
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