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James Gregory
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49-01.html
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49-01.html
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@ -39,13 +39,13 @@
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<H2><A NAME="Heading1"></A><FONT COLOR="#000077">Chapter 49<BR>Mode X 256-Color Animation
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</FONT></H2>
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<H3><A NAME="Heading2"></A><FONT COLOR="#000077">How to Make the VGA Really Get up and Dance</FONT></H3>
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<P>Okay—no amusing stories or informative anecdotes to kick off this chapter; lotta ground to cover, gotta hurry—you’re impatient, I can smell it. I won’t talk about the time a friend made the mistake of loudly saying “$100 bill” during an animated discussion while walking among the bums on Market Street in San Francisco one night, thereby graphically illustrating that context is everything. I can’t spare a word about how my daughter thinks my 11-year-old floppy-disk-based CP/M machine is more powerful than my 386 with its 100-MB hard disk because the CP/M machine’s word processor loads and runs twice as fast as the 386’s Windows-based word processor, demonstrating that progress is not the neat exponential curve we’d like to think it is, and that features and performance are often conflicting notions. And, lord knows, I can’t take the time to discuss the habits of small white dogs, notwithstanding that such dogs seem to be relevant to just about every aspect of computing, as Jeff Duntemann’s writings make manifest. No lighthearted fluff for us; we have real work to do, for today we animate with 256 colors in Mode X.
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<P>Okay—no amusing stories or informative anecdotes to kick off this chapter; lotta ground to cover, gotta hurry—you’re impatient, I can smell it. I won’t talk about the time a friend made the mistake of loudly saying “$100 bill” during an animated discussion while walking among the bums on Market Street in San Francisco one night, thereby graphically illustrating that context is everything. I can’t spare a word about how my daughter thinks my 11-year-old floppy-disk-based CP/M machine is more powerful than my 386 with its 100-MB hard disk because the CP/M machine’s word processor loads and runs twice as fast as the 386’s Windows-based word processor, demonstrating that progress is not the neat exponential curve we’d like to think it is, and that features and performance are often conflicting notions. And, lord knows, I can’t take the time to discuss the habits of small white dogs, notwithstanding that such dogs seem to be relevant to just about every aspect of computing, as Jeff Duntemann’s writings make manifest. No lighthearted fluff for us; we have real work to do, for today we animate with 256 colors in Mode X.
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</P>
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<H3><A NAME="Heading3"></A><FONT COLOR="#000077">Masked Copying</FONT></H3>
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<P>Over the past two chapters, we’ve put together most of the tools needed to implement animation in the VGA’s undocumented 320×240 256-color Mode X. We now have mode set code, solid and 4×4 pattern fills, system memory-to-display memory block copies, and display memory-to-display memory block copies. The final piece of the puzzle is the ability to copy a nonrectangular image to display memory. I call this <I>masked copying</I>.</P>
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<P>Over the past two chapters, we’ve put together most of the tools needed to implement animation in the VGA’s undocumented 320x240 256-color Mode X. We now have mode set code, solid and 4x4 pattern fills, system memory-to-display memory block copies, and display memory-to-display memory block copies. The final piece of the puzzle is the ability to copy a nonrectangular image to display memory. I call this <I>masked copying</I>.</P>
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<P>Masked copying is sort of like drawing through a stencil, in that only certain pixels within the destination rectangle are drawn. The objective is to fit the image seamlessly into the background, without the rectangular fringe that results when nonrectangular images are drawn by block copying their bounding rectangle. This is accomplished by using a second rectangular bitmap, separate from the image but corresponding to it on a pixel-by-pixel basis, to control which destination pixels are set from the source and which are left unchanged. With a masked copy, only those pixels properly belonging to an image are drawn, and the image fits perfectly into the background, with no rectangular border. In fact, masked copying even makes it possible to have transparent areas within images.</P>
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<P>Note that another way to achieve this effect is to implement copying code that supports a transparent color; that is, a color that doesn’t get copied but rather leaves the destination unchanged. Transparent copying makes for more compact images, because no separate mask is needed, and is generally faster in a software-only implementation. However, Mode X supports masked copying but not transparent copying in hardware, so we’ll use masked copying in this chapter.</P>
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<P>The system memory to display memory masked copy routine in Listing 49.1 implements masked copying in a straightforward fashion. In the main drawing loop, the corresponding mask byte is consulted as each image pixel is encountered, and the image pixel is copied only if the mask byte is nonzero. As with most of the system-to-display code I’ve presented, Listing 49.1 is not heavily optimized, because it’s inherently slow; there’s a better way to go when performance matters, and that’s to use the VGA’s hardware.</P>
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<P>Note that another way to achieve this effect is to implement copying code that supports a transparent color; that is, a color that doesn’t get copied but rather leaves the destination unchanged. Transparent copying makes for more compact images, because no separate mask is needed, and is generally faster in a software-only implementation. However, Mode X supports masked copying but not transparent copying in hardware, so we’ll use masked copying in this chapter.</P>
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<P>The system memory to display memory masked copy routine in Listing 49.1 implements masked copying in a straightforward fashion. In the main drawing loop, the corresponding mask byte is consulted as each image pixel is encountered, and the image pixel is copied only if the mask byte is nonzero. As with most of the system-to-display code I’ve presented, Listing 49.1 is not heavily optimized, because it’s inherently slow; there’s a better way to go when performance matters, and that’s to use the VGA’s hardware.</P>
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<P><B>LISTING 49.1 L49-1.ASM</B></P>
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<!-- CODE //-->
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<PRE>
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@ -87,7 +87,7 @@ SourceBitmapWidth dw ? ;# of pixels across source bitmap (also must
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DestBitmapWidth dw ? ;# of pixels across dest bitmap (must be multiple of 4)
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MaskPtr dw ? ;pointer in DS to start of bitmap in which mask
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; resides (byte-per-pixel format, just like the source
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; image; 0-bytes mean don’t copy corresponding source
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; image; 0-bytes mean don’t copy corresponding source
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; pixel, 1-bytes mean do copy)
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parms ends
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@ -99,62 +99,62 @@ STACK_FRAME_SIZE equ 6
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.code
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public _CopySystemToScreenMaskedX
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_CopySystemToScreenMaskedX proc near
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push bp ;preserve caller’s stack frame
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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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sub sp,STACK_FRAME_SIZE ;allocate space for local vars
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push si ;preserve caller’s register variables
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push si ;preserve caller’s register variables
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push di
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mov ax,SCREEN_SEG ;point ES to display memory
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mov es,ax
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mov ax,[bp+SourceBitmapWidth]
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mul [bp+SourceStartY] ;top source rect scan line
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add ax,[bp+SourceStartX]
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mov ax,[bp+SourceBitmapWidth]
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mul [bp+SourceStartY] ;top source rect scan line
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add ax,[bp+SourceStartX]
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mov bx,ax
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add ax,[bp+SourcePtr] ;offset of first source rect pixel
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add ax,[bp+SourcePtr] ;offset of first source rect pixel
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mov si,ax ; in DS
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add bx,[bp+MaskPtr] ;offset of first mask pixel in DS
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add bx,[bp+MaskPtr] ;offset of first mask pixel in DS
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mov ax,[bp+DestBitmapWidth]
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mov ax,[bp+DestBitmapWidth]
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shr ax,1 ;convert to width in addresses
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shr ax,1
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mov [bp+DestBitmapWidth],ax ;remember address width
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mul [bp+DestStartY] ;top dest rect scan line
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mov di,[bp+DestStartX]
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mov [bp+DestBitmapWidth],ax ;remember address width
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mul [bp+DestStartY] ;top dest rect scan line
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mov di,[bp+DestStartX]
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mov cx,di
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shr di,1 ;X/4 = offset of first dest rect pixel in
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shr di,1 ; scan line
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add di,ax ;offset of first dest rect pixel in page
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add di,[bp+DestPageBase] ;offset of first dest rect pixel
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add di,[bp+DestPageBase] ;offset of first dest rect pixel
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; in display memory
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and cl,011b ;CL = first dest pixel’ ;s plane
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and cl,011b ;CL = first dest pixel’s plane
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mov al,11h ;upper nibble comes into play when plane wraps
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; from 3 back to 0
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shl al,cl ;set the bit for the first dest pixel’s plane
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mov [bp+LeftMask],al ; in each nibble to 1
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shl al,cl ;set the bit for the first dest pixel’s plane
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mov [bp+LeftMask],al ; in each nibble to 1
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mov ax,[bp+SourceEndX] ;calculate # of pixels across
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sub ax,[bp+SourceStartX] ; rect
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mov ax,[bp+SourceEndX] ;calculate # of pixels across
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sub ax,[bp+SourceStartX] ; rect
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jle CopyDone ;skip if 0 or negative width
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mov [bp+RectWidth],ax
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sub word ptr [bp+SourceBitmapWidth],ax
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mov [bp+RectWidth],ax
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sub word ptr [bp+SourceBitmapWidth],ax
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;distance from end of one source scan line
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to start of next
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mov ax,[bp+SourceEndY]
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sub ax,[bp+SourceStartY] ;height of rectangle
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mov ax,[bp+SourceEndY]
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sub ax,[bp+SourceStartY] ;height of rectangle
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jle CopyDone ;skip if 0 or negative height
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mov [bp+RectHeight],ax
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mov [bp+RectHeight],ax
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mov dx,SC_INDEX ;point to SC Index register
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mov al,MAP_MASK
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out dx,al ;point SC Index reg to the Map Mask
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inc dx ;point DX to SC Data reg
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CopyRowsLoop:
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mov al,[bp+LeftMask]
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mov cx,[bp+RectWidth]
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mov al,[bp+LeftMask]
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mov cx,[bp+RectWidth]
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push di ;remember the start offset in the dest
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CopyScanLineLoop:
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cmp byte ptr [bx],0 ;is this pixel mask-enabled?
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jz MaskOff ;no, so don’t draw it
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jz MaskOff ;no, so don’t draw it
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;yes, draw the pixel
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out dx,al ;set the plane for this pixel
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mov ah,[si] ;get the pixel from the source
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@ -162,24 +162,24 @@ CopyScanLineLoop:
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MaskOff:
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inc bx ;advance the mask pointer
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inc si ;advance the source pointer
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rol al,1 ;set mask for next pixel’s plane
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rol al,1 ;set mask for next pixel’s plane
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adc di,0 ;advance destination address only when
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;wrapping from plane 3 to plane 0
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loop CopyScanLineLoop
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pop di ;retrieve the dest start offset
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add di,[bp+DestBitmapWidth];point to the start of the
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add di,[bp+DestBitmapWidth];point to the start of the
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; next scan line of the dest
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add si,[bp+SourceBitmapWidth] ;point to the start of the
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add si,[bp+SourceBitmapWidth] ;point to the start of the
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;next scan line of the source
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add bx,[bp+SourceBitmapWidth] ;point to the start of the
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add bx,[bp+SourceBitmapWidth] ;point to the start of the
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;next scan line of the mask
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dec word ptr [bp+RectHeight] ;count down scan lines
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dec word ptr [bp+RectHeight] ;count down scan lines
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jnz CopyRowsLoop
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CopyDone:
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pop di ;restore caller’s register variables
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pop di ;restore caller’s register variables
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pop si
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mov sp,bp ;discard storage for local variables
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pop bp ;restore caller’s stack frame
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pop bp ;restore caller’s stack frame
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ret
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_CopySystemToScreenMaskedX endp
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end
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