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Okayno amusing stories or informative anecdotes to kick off this chapter; lotta ground to cover, gotta hurryyoure impatient, I can smell it. I wont 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 cant 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 machines word processor loads and runs twice as fast as the 386s Windows-based word processor, demonstrating that progress is not the neat exponential curve wed like to think it is, and that features and performance are often conflicting notions. And, lord knows, I cant 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 Duntemanns writings make manifest. No lighthearted fluff for us; we have real work to do, for today we animate with 256 colors in Mode X.
Over the past two chapters, weve put together most of the tools needed to implement animation in the VGAs 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 masked copying.
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.
Note that another way to achieve this effect is to implement copying code that supports a transparent color; that is, a color that doesnt 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 well use masked copying in this chapter.
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 Ive presented, Listing 49.1 is not heavily optimized, because its inherently slow; theres a better way to go when performance matters, and thats to use the VGAs hardware.
LISTING 49.1 L49-1.ASM
; Mode X (320x240, 256 colors) system memory-to-display memory masked copy ; routine. Not particularly fast; images for which performance is critical ; should be stored in off-screen memory and copied to screen via latches. Works ; on all VGAs. Copies up to but not including column at SourceEndX and row at ; SourceEndY. No clipping is performed. Mask and source image are both byte- ; per-pixel, and must be of same widths and reside at same coordinates in their ; respective bitmaps. Assembly code tested with TASM C near-callable as: ; ; void CopySystemToScreenMaskedX(int SourceStartX, ; int SourceStartY, int SourceEndX, int SourceEndY, ; int DestStartX, int DestStartY, char * SourcePtr, ; unsigned int DestPageBase, int SourceBitmapWidth, ; int DestBitmapWidth, char * MaskPtr); SC_INDEX equ 03c4h ;Sequence Controller Index register port MAP_MASK equ 02h ;index in SC of Map Mask register SCREEN_SEG equ 0a000h ;segment of display memory in mode X parms struc dw 2 dup (?) ;pushed BP and return address SourceStartX dw ? ;X coordinate of upper left corner of source ; (source is in system memory) SourceStartY dw ? ;Y coordinate of upper left corner of source SourceEndX dw ? ;X coordinate of lower right corner of source ; (the column at EndX is not copied) SourceEndY dw ? ;Y coordinate of lower right corner of source ; (the row at EndY is not copied) DestStartX dw ? ;X coordinate of upper left corner of dest ; (destination is in display memory) DestStartY dw ? ;Y coordinate of upper left corner of dest SourcePtr dw ? ;pointer in DS to start of bitmap which source resides DestPageBase dw ? ;base offset in display memory of page in ; which dest resides SourceBitmapWidth dw ? ;# of pixels across source bitmap (also must ; be width across the mask) DestBitmapWidth dw ? ;# of pixels across dest bitmap (must be multiple of 4) MaskPtr dw ? ;pointer in DS to start of bitmap in which mask ; resides (byte-per-pixel format, just like the source ; image; 0-bytes mean dont copy corresponding source ; pixel, 1-bytes mean do copy) parms ends RectWidth equ -2 ;local storage for width of rectangle RectHeight equ -4 ;local storage for height of rectangle LeftMask equ -6 ;local storage for left rect edge plane mask STACK_FRAME_SIZE equ 6 .model small .code public _CopySystemToScreenMaskedX _CopySystemToScreenMaskedX proc near push bp ;preserve callers stack frame mov bp,sp ;point to local stack frame sub sp,STACK_FRAME_SIZE ;allocate space for local vars push si ;preserve callers register variables push di mov ax,SCREEN_SEG ;point ES to display memory mov es,ax mov ax,[bp+SourceBitmapWidth] mul [bp+SourceStartY] ;top source rect scan line add ax,[bp+SourceStartX] mov bx,ax add ax,[bp+SourcePtr] ;offset of first source rect pixel mov si,ax ; in DS add bx,[bp+MaskPtr] ;offset of first mask pixel in DS mov ax,[bp+DestBitmapWidth] shr ax,1 ;convert to width in addresses shr ax,1 mov [bp+DestBitmapWidth],ax ;remember address width mul [bp+DestStartY] ;top dest rect scan line mov di,[bp+DestStartX] mov cx,di shr di,1 ;X/4 = offset of first dest rect pixel in shr di,1 ; scan line add di,ax ;offset of first dest rect pixel in page add di,[bp+DestPageBase] ;offset of first dest rect pixel ; in display memory and cl,011b ;CL = first dest pixel ;s plane mov al,11h ;upper nibble comes into play when plane wraps ; from 3 back to 0 shl al,cl ;set the bit for the first dest pixels plane mov [bp+LeftMask],al ; in each nibble to 1 mov ax,[bp+SourceEndX] ;calculate # of pixels across sub ax,[bp+SourceStartX] ; rect jle CopyDone ;skip if 0 or negative width mov [bp+RectWidth],ax sub word ptr [bp+SourceBitmapWidth],ax ;distance from end of one source scan line to start of next mov ax,[bp+SourceEndY] sub ax,[bp+SourceStartY] ;height of rectangle jle CopyDone ;skip if 0 or negative height mov [bp+RectHeight],ax mov dx,SC_INDEX ;point to SC Index register mov al,MAP_MASK out dx,al ;point SC Index reg to the Map Mask inc dx ;point DX to SC Data reg CopyRowsLoop: mov al,[bp+LeftMask] mov cx,[bp+RectWidth] push di ;remember the start offset in the dest CopyScanLineLoop: cmp byte ptr [bx],0 ;is this pixel mask-enabled? jz MaskOff ;no, so dont draw it ;yes, draw the pixel out dx,al ;set the plane for this pixel mov ah,[si] ;get the pixel from the source mov es:[di],ah ;copy the pixel to the screen MaskOff: inc bx ;advance the mask pointer inc si ;advance the source pointer rol al,1 ;set mask for next pixels plane adc di,0 ;advance destination address only when ;wrapping from plane 3 to plane 0 loop CopyScanLineLoop pop di ;retrieve the dest start offset add di,[bp+DestBitmapWidth];point to the start of the ; next scan line of the dest add si,[bp+SourceBitmapWidth] ;point to the start of the ;next scan line of the source add bx,[bp+SourceBitmapWidth] ;point to the start of the ;next scan line of the mask dec word ptr [bp+RectHeight] ;count down scan lines jnz CopyRowsLoop CopyDone: pop di ;restore callers register variables pop si mov sp,bp ;discard storage for local variables pop bp ;restore callers stack frame ret _CopySystemToScreenMaskedX endp end
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