LISTING 57.2 L57-2.ASM
; Draws all pixels in the specified scan line, with the pixel colors
; taken from the specified texture map. Uses approach of pre-stepping
; 1/2 pixel into the source image and rounding to the nearest source
; pixel at each step, so that texture maps will appear reasonably similar
; at all angles. This routine is specific to 320-pixel-wide planar
; (non-chain4) 256-color modes, such as mode X, which is a planar
; (non-chain4) 256-color mode with a resolution of 320x240.
; C near-callable as:
; void ScanOutLine(EdgeScan * LeftEdge, EdgeScan * RightEdge);
; Tested with TASM 3.0.
SC-INDEX equ 03c4h ;Sequence Controller Index
MAP-MASK equ 02h ;index in SC of Map Mask register
SCREEN-SEG equ 0a000h ;segment of display memory in mode X
SCREEN-WIDTH equ 80 ;width of screen in bytes from one scan line
; to the next
.model small
.data
extrn -TexMapBits:word, -TexMapWidth:word, -DestY:word
extrn -CurrentPageBase:word, -ClipMinX:word
extrn -ClipMinY:word, -ClipMaxX:word, -ClipMaxY:word
; Describes the current location and stepping, in both the source and
; the destination, of an edge. Mirrors structure in DRAWTEXP.C.
EdgeScan struc
Direction dw ? ;through edge list; 1 for a right edge (forward
; through vertex list), -1 for a left edge (backward
; through vertex list)
RemainingScans dw ? ;height left to scan out in dest
CurrentEnd dw ? ;vertex # of end of current edge
SourceX dd ? ;X location in source for this edge
SourceY dd ? ;Y location in source for this edge
SourceStepX dd ? ;X step in source for Y step in dest of 1
SourceStepY dd ? ;Y step in source for Y step in dest of 1
;variables used for all-integer Bresenham’s-type
; X stepping through the dest, needed for precise
; pixel placement to avoid gaps
DestX dw ? ;current X location in dest for this edge
DestXIntStep dw ? ;whole part of dest X step per scan-line Y step
DestXDirection dw ? ;-1 or 1 to indicate which way X steps (left/right)
DestXErrTerm dw ? ;current error term for dest X stepping
DestXAdjUp dw ? ;amount to add to error term per scan line move
DestXAdjDown dw ? ;amount to subtract from error term when the
; error term turns over
EdgeScan ends
Parms struc
dw 2 dup(?) ;return address & pushed BP
LeftEdge dw ? ;pointer to EdgeScan structure for left edge
RightEdge dw ? ;pointer to EdgeScan structure for right edge
Parms ends
;Offsets from BP in stack frame of local variables.
lSourceX equ -4 ;current X coordinate in source image
lSourceY equ -8 ;current Y coordinate in source image
lSourceStepX equ -12 ;X step in source image for X dest step of 1
lSourceStepY equ -16 ;Y step in source image for X dest step of 1
lXAdvanceByOne equ -18 ;used to step source pointer 1 pixel
; incrementally in X
lXBaseAdvance equ -20 ;use to step source pointer minimum number of
; pixels incrementally in X
lYAdvanceByOne equ -22 ;used to step source pointer 1 pixel
; incrementally in Y
lYBaseAdvance equ -24 ;use to step source pointer minimum number of
; pixels incrementally in Y
LOCAL-SIZE equ 24 ;total size of local variables
.code
extrn -FixedMul:near, -FixedDiv:near
align 2
ToScanDone:
jmp ScanDone
public -ScanOutLine
align 2
-ScanOutLine proc near
push bp ;preserve caller’s stack frame
mov bp,sp ;point to our stack frame
sub sp,LOCAL-SIZE ;allocate space for local variables
push si ;preserve caller’s register variables
push di
; Nothing to do if destination is fully X clipped.
mov di,[bp].RightEdge
mov si,[di].DestX
cmp si,[-ClipMinX]
jle ToScanDone ;right edge is to left of clip rect, so done
mov bx,[bp].LeftEdge
mov dx,[bx].DestX
cmp dx,[-ClipMaxX]
jge ToScanDone ;left edge is to right of clip rect, so done
sub si,dx ;destination fill width
jle ToScanDone ;null or negative full width, so done
mov ax,word ptr [bx].SourceX ;initial source X coordinate
mov word ptr [bp].lSourceX,ax
mov ax,word ptr [bx].SourceX+2
mov word ptr [bp].lSourceX+2,ax
mov ax,word ptr [bx].SourceY ;initial source Y coordinate
mov word ptr [bp].lSourceY,ax
mov ax,word ptr [bx].SourceY+2
mov word ptr [bp].lSourceY+2,ax
; Calculate source steps that correspond to each 1-pixel destination X step
; (across the destination scan line).
push si ;push dest X width, in fixedpoint form
sub ax,ax
push ax ;push 0 as fractional part of dest X width
mov ax,word ptr [di].SourceX
sub ax,word ptr [bp].lSourceX ;low word of source X width
mov dx,word ptr [di].SourceX+2
sbb dx,word ptr [bp].lSourceX+2 ;high word of source X width
push dx ;push source X width, in fixedpoint form
push ax
call -FixedDiv ;scale source X width to dest X width
add sp,8 ;clear parameters from stack
mov word ptr [bp].lSourceStepX,ax ;remember source X step for
mov word ptr [bp].lSourceStepX+2,dx ; 1-pixel destination X step
mov cx,1 ;assume source X advances non-negative
and dx,dx ;which way does source X advance?
jns SourceXNonNeg ;non-negative
neg cx ;negative
cmp ax,0 ;is the whole step exactly an integer?
jz SourceXNonNeg ;yes
inc dx ;no, truncate to integer in the direction of
; 0, because otherwise we’ll end up with a
; whole step of 1-too-large magnitude
SourceXNonNeg:
mov [bp].lXAdvanceByOne,cx ;amount to add to source pointer to
; move by one in X
mov [bp].lXBaseAdvance,dx ;minimum amount to add to source
; pointer to advance in X each time
; the dest advances one in X
push si ;push dest Y height, in fixedpoint form
sub ax,ax
push ax ;push 0 as fractional part of dest Y height
mov ax,word ptr [di].SourceY
sub ax,word ptr [bp].lSourceY ;low word of source Y height
mov dx,word ptr [di].SourceY+2
sbb dx,word ptr [bp].lSourceY+2 ;high word of source Y height
push dx ;push source Y height, in fixedpoint form
push ax
call -FixedDiv ;scale source Y height to dest X width
add sp,8 ;clear parameters from stack
mov word ptr [bp].lSourceStepY,ax ;remember source Y step for
mov word ptr [bp].lSourceStepY+2,dx ; 1-pixel destination X step
mov cx,[-TexMapWidth] ;assume source Y advances non-negative
and dx,dx ;which way does source Y advance?
jns SourceYNonNeg ;non-negative
neg cx ;negative
cmp ax,0 ;is the whole step exactly an integer?
jz SourceYNonNeg ;yes
inc dx ;no, truncate to integer in the direction of
; 0, because otherwise we’ll end up with a
; whole step of 1-too-large magnitude
SourceYNonNeg:
mov [bp].lYAdvanceByOne,cx ;amount to add to source pointer to
; move by one in Y
mov ax,[-TexMapWidth] ;minimum distance skipped in source
imul dx ; image bitmap when Y steps (ignoring
mov [bp].lYBaseAdvance,ax ; carry from the fractional part)
; Advance 1/2 step in the stepping direction, to space scanned pixels evenly
; between the left and right edges. (There’s a slight inaccuracy in dividing
; negative numbers by 2 by shifting rather than dividing, but the inaccuracy
; is in the least significant bit, and we’ll just live with it.)
mov ax,word ptr [bp].lSourceStepX
mov dx,word ptr [bp].lSourceStepX+2
sar dx,1
rcr ax,1
add word ptr [bp].lSourceX,ax
adc word ptr [bp].lSourceX+2,dx
mov ax,word ptr [bp].lSourceStepY
mov dx,word ptr [bp].lSourceStepY+2
sar dx,1
rcr ax,1
add word ptr [bp].lSourceY,ax
adc word ptr [bp].lSourceY+2,dx
; Clip right edge if necessary.
mov si,[di].DestX
cmp si,[-ClipMaxX]
jl RightEdgeClipped
mov si,[-ClipMaxX]
RightEdgeClipped:
; Clip left edge if necssary
mov bx,[bp].LeftEdge
mov di,[bx].DestX
cmp di,[-ClipMinX]
jge LeftEdgeClipped
; Left clipping is necessary; advance the source accordingly
neg di
add di,[-ClipMinX] ;ClipMinX - DestX
;first, advance the source in X
push di ;push ClipMinX - DestX, in fixedpoint form
sub ax,ax
push ax ;push 0 as fractional part of ClipMinX-DestX
push word ptr [bp].lSourceStepX+2
push word ptr [bp].lSourceStepX
call -FixedMul ;total source X stepping in clipped area
add sp,8 ;clear parameters from stack
add word ptr [bp].lSourceX,ax;step the source X past clipping
adc word ptr [bp].lSourceX+2,dx
;now advance the source in Y
push di ;push ClipMinX - DestX, in fixedpoint form
sub ax,ax
push ax ;push 0 as fractional part of ClipMinX-DestX
push word ptr [bp].lSourceStepY+2
push word ptr [bp].lSourceStepY
call -FixedMul ;total source Y stepping in clipped area
add sp,8 ;clear parameters from stack
add word ptr [bp].lSourceY,ax;step the source Y past clipping
adc word ptr [bp].lSourceY+2,dx
mov di,[-ClipMinX] ;start X coordinate in dest after clipping
LeftEdgeClipped:
; Calculate actual clipped destination drawing width.
sub si,di
; Scan across the destination scan line, updating the source image position
; accordingly.
; Point to the initial source image pixel, adding 0.5 to both X and Y so that
; we can truncate to integers from now on but effectively get rounding.
add word ptr [bp].lSourceY,8000h;add 0.5
mov ax,word ptr [bp].lSourceY+2
adc ax,0
mul [-TexMapWidth] ;initial scan line in source image
add word ptr [bp].lSourceX,8000h;add 0.5
mov bx,word ptr [bp].lSourceX+2 ;offset into source scan line
adc bx,ax ;initial source offset in source image
add bx,[-TexMapBits] ;DS:BX points to the initial image pixel
; Point to initial destination pixel.
mov ax,SCREEN-SEG
mov es,ax
mov ax,SCREEN-WIDTH
mul [-DestY] ;offset of initial dest scan line
mov cx,di ;initial destination X
shr di,1
shr di,1 ;X/4 = offset of pixel in scan line
add di,ax ;offset of pixel in page
add di,[-CurrentPageBase] ;offset of pixel in display memory
;ES:DI now points to the first destination pixel
and cl,011b ;CL = pixel’s plane
mov al,MAP-MASK
mov dx,SC-INDEX
out dx,al ;point the SC Index register to the Map Mask
mov al,11h ;one plane bit in each nibble, so we’ll get carry
; automatically when going from plane 3 to plane 0
shl al,cl ;set the bit for the first pixel’s plane to 1
; If source X step is negative, change over to working with non-negative
; values.
cmp word ptr [bp].lXAdvanceByOne,0
jge SXStepSet
neg word ptr [bp].lSourceStepX
not word ptr [bp].lSourceX
SXStepSet:
; If source Y step is negative, change over to working with non-negative
; values.
cmp word ptr [bp].lYAdvanceByOne,0
jge SYStepSet
neg word ptr [bp].lSourceStepY
not word ptr [bp].lSourceY
SYStepSet:
; At this point:
; AL = initial pixel’s plane mask
; BX = pointer to initial image pixel
; SI = # of pixels to fill
; DI = pointer to initial destination pixel
mov dx,SC-INDEX+1 ;point to SC Data; Index points to Map Mask
TexScanLoop:
; Set the Map Mask for this pixel’s plane, then draw the pixel.
out dx,al
mov ah,[bx] ;get image pixel
mov es:[di],ah ;set image pixel
; Point to the next source pixel.
add bx,[bp].lXBaseAdvance ;advance the minimum # of pixels in X
mov cx,word ptr [bp].lSourceStepX
add word ptr [bp].lSourceX,cx;step the source X fractional part
jnc NoExtraXAdvance ;didn’t turn over; no extra advance
add bx,[bp].lXAdvanceByOne ;did turn over; advance X one extra
NoExtraXAdvance:
add bx,[bp].lYBaseAdvance;advance the minimum # of pixels in Y
mov cx,word ptr [bp].lSourceStepY
add word ptr [bp].lSourceY,cx;step the source Y fractional part
jnc NoExtraYAdvance;didn’t turn over; no extra advance
add bx,[bp].lYAdvanceByOne;did turn over; advance Y one extra
NoExtraYAdvance:
; Point to the next destination pixel, by cycling to the next plane, and
; advancing to the next address if the plane wraps from 3 to 0.
rol al,1
adc di,0
; Continue if there are any more dest pixels to draw.
dec si
jnz TexScanLoop
ScanDone:
pop di ;restore caller’s register variables
pop si
mov sp,bp ;deallocate local variables
pop bp ;restore caller’s stack frame
ret
-ScanOutLine endp
end
Graphics Programming Black Book © 2001 Michael Abrash