121 lines
5.4 KiB
HTML
121 lines
5.4 KiB
HTML
<!DOCTYPE html>
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta name="vsisbn" content="1576101746" />
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<meta name="vstitle" content="Michael Abrash's Graphics Programming Black Book, Special Edition" />
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<meta name="vsauthor" content="Michael Abrash" />
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<meta name="vspublisher" content="The Coriolis Group" />
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<meta name="vspubdate" content="07/01/97" />
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<meta name="vscategory" content="Web and Software Development: Game Development,Web and Software Development: Graphics and Multimedia Development" />
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<title>Michael Abrash's Graphics Programming Black Book Special Edition: Mode X 256-Color Animation</title>
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<meta name="chapter" content="49" />
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<meta name="pages" content="929-930" />
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</head>
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<body>
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<center>
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<table border="1">
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<tr>
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<a href="49-04.html">Previous</a>
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<a href="index.html">Table of Contents</a>
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<a href="50-01.html">Next</a>
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</center>
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<p>Here’s something worth noting: The animation is extremely smooth on a 20 MHz 386. It is somewhat more jerky on an 8 MHz 286, because only 30 frames a second can be processed. If animation looks jerky on your PC, try reducing the number of kites.</p>
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<p>The kites draw perfectly into the background, with no interference or fringe, thanks to masked copying. In fact, the kites also cross with no interference (the last-drawn kite is always in front), although that’s not readily apparent because they all look the same anyway and are moving fast. Listing 49.5 isn’t inherently limited to kites; create your own images and initialize the object list to display a mix of those images and see the full power of Mode X animation.</p>
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<p>The external functions called by Listing 49.5 can be found in Listings 49.1, 49.2, 49.3, and 49.6, and in the listings for the previous two chapters.</p>
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<p><b>LISTING 49.6 L49-6.ASM</b></p>
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<pre>
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; Shows the page at the specified offset in the bitmap. Page is displayed when
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; this routine returns.
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; C near-callable as: void ShowPage(unsigned int StartOffset);
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INPUT_STATUS_1 equ 03dah ;Input Status 1 register
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CRTC_INDEX equ 03d4h ;CRT Controller Index reg
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START_ADDRESS_HIGH equ 0ch ;bitmap start address high byte
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START_ADDRESS_LOW equ 0dh ;bitmap start address low byte
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ShowPageParms struc
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dw 2 dup (?) ;pushed BP and return address
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StartOffset dw ? ;offset in bitmap of page to display
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ShowPageParms ends
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.model small
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.code
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public _ShowPage
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_ShowPage proc near
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push bp ;preserve caller’s stack frame
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mov bp,sp ;point to local stack frame
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; Wait for display enable to be active (status is active low), to be
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; sure both halves of the start address will take in the same frame.
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mov bl,START_ADDRESS_LOW ;preload for fastest
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mov bh,byte ptr StartOffset[bp] ; flipping once display
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mov cl,START_ADDRESS_HIGH ; enable is detected
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mov ch,byte ptr StartOffset+1[bp]
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mov dx,INPUT_STATUS_1
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WaitDE:
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in al,dx
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test al,01h
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jnz WaitDE ;display enable is active low (0 = active)
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; Set the start offset in display memory of the page to display.
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mov dx,CRTC_INDEX
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mov ax,bx
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out dx,ax ;start address low
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mov ax,cx
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out dx,ax ;start address high
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; Now wait for vertical sync, so the other page will be invisible when
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; we start drawing to it.
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mov dx,INPUT_STATUS_1
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WaitVS:
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in al,dx
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test al,08h
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jz WaitVS ;vertical sync is active high (1 = active)
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pop bp ;restore caller’s stack frame
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ret
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_ShowPage endp
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end
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</pre>
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<h3 id="Heading8">Works Fast, Looks Great</h3>
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<p>We now end our exploration of Mode X, although we’ll use it again shortly for 3-D animation. Mode X admittedly has its complexities; that’s why I’ve provided a broad and flexible primitive set. Still, so what if it <i>is</i> complex? Take a look at Listing 49.5 in action. That sort of colorful, high-performance animation is worth jumping through a few hoops for; drawing 20, or even 10, fair-sized objects at a rate of 60 Hz, with no flicker, interference, or fringe, is no mean accomplishment, even on a 386.</p>
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<p>There’s much more we could do with animation in general and with Mode X in particular, but it’s time to move on to new challenges. In closing, I’d like to point out that all of the VGA’s hardware features, including the built-in AND, OR, and XOR functions, are available in Mode X, just as they are in the standard VGA modes. If you understand the VGA’s hardware in mode 12H, try applying that knowledge to Mode X; you might be surprised at what you find you can do.</p>
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<center>
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<table border="1">
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<tr>
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<td>
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<a href="49-04.html">Previous</a>
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</td>
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<td>
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<a href="index.html">Table of Contents</a>
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</td>
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<td>
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<a href="50-01.html">Next</a>
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</td>
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</tr>
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</table>
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</center>
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<hr width="90%" size="1" noshade="noshade" />
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<div align="center">
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Graphics Programming Black Book © 2001 Michael Abrash
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</div>
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</body>
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</html>
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