LISTING 39.4 L39-4.ASM
; Scan converts an edge from (X1,Y1) to (X2,Y2), not including the
; point at (X2,Y2). If SkipFirst == 1, the point at (X1,Y1) isn’t
; drawn; if SkipFirst == 0, it is. For each scan line, the pixel
; closest to the scanned edge without being to the left of the scanned
; edge is chosen. Uses an all-integer approach for speed & precision.
; C near-callable as:
; void ScanEdge(int X1, int Y1, int X2, int Y2, int SetXStart,
; int SkipFirst, struct HLine **EdgePointPtr);
; Edges must not go bottom to top; that is, Y1 must be <= Y2.
; Updates the pointer pointed to by EdgePointPtr to point to the next
; free entry in the array of HLine structures.
HLine struc
XStart dw ? ;X coordinate of leftmost pixel in scan line
XEnd dw ? ;X coordinate of rightmost pixel in scan line
HLine ends
Parms struc
dw 2 dup(?) ;return address & pushed BP
X1 dw ? ;X start coord of edge
Y1 dw ? ;Y start coord of edge
X2 dw ? ;X end coord of edge
Y2 dw ? ;Y end coord of edge
SetXStart dw ? ;1 to set the XStart field of each
; HLine struc, 0 to set XEnd
SkipFirst dw ? ;1 to skip scanning the first point
; of the edge, 0 to scan first point
EdgePointPtr dw ? ;pointer to a pointer to the array of
; HLine structures in which to store
; the scanned X coordinates
Parms ends
;Offsets from BP in stack frame of local variables.
AdvanceAmt equ -2
Height equ -4
LOCAL_SIZE equ 4 ;total size of local variables
.model small
.code
public _ScanEdge
align 2
_ScanEdge proc
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
mov di,[bp+EdgePointPtr]
mov di,[di] ;point to the HLine array
cmp [bp+SetXStart],1 ;set the XStart field of each HLine
; struc?
jz HLinePtrSet ;yes, DI points to the first XStart
add di,XEnd ;no, point to the XEnd field of the
; first HLine struc
HLinePtrSet:
mov bx,[bp+Y2]
sub bx,[bp+Y1] ;edge height
jle ToScanEdgeExit ;guard against 0-length & horz edges
mov [bp+Height],bx ;Height = Y2 - Y1
sub cx,cx ;assume ErrorTerm starts at 0 (true if
; we’re moving right as we draw)
mov dx,1 ;assume AdvanceAmt = 1 (move right)
mov ax,[bp+X2]
sub ax,[bp+X1] ;DeltaX = X2 - X1
jz IsVertical ;it’s a vertical edge--special case it
jns SetAdvanceAmt ;DeltaX >= 0
mov cx,1 ;DeltaX < 0 (move left as we draw)
sub cx,bx ;ErrorTerm = -Height + 1
neg dx ;AdvanceAmt = -1 (move left)
neg ax ;Width = abs(DeltaX)
SetAdvanceAmt:
mov [bp+AdvanceAmt],dx
; Figure out whether the edge is diagonal, X-major (more horizontal),
; or Y-major (more vertical) and handle appropriately.
cmp ax,bx ;if Width==Height, it’s a diagonal edge
jz IsDiagonal ;it’s a diagonal edge--special case
jb YMajor ;it’s a Y-major (more vertical) edge
;it’s an X-major (more horz) edge
sub dx,dx ;prepare DX:AX (Width) for division
div bx ;Width/Height
;DX = error term advance per scan line
mov si,ax ;SI = minimum # of pixels to advance X
; on each scan line
test [bp+AdvanceAmt],8000h ;move left or right?
jz XMajorAdvanceAmtSet ;right, already set
neg si ;left, negate the distance to advance
; on each scan line
XMajorAdvanceAmtSet: ;
mov ax,[bp+X1] ;starting X coordinate
cmp [bp+SkipFirst],1 ;skip the first point?
jz XMajorSkipEntry ;yes
XMajorLoop:
mov [di],ax ;store the current X value
add di,size HLine ;point to the next HLine struc
XMajorSkipEntry:
add ax,si ;set X for the next scan line
add cx,dx ;advance error term
jle XMajorNoAdvance ;not time for X coord to advance one
; extra
add ax,[bp+AdvanceAmt] ;advance X coord one extra
sub cx,[bp+Height] ;adjust error term back
XMajorNoAdvance:
dec bx ;count off this scan line
jnz XMajorLoop
jmp ScanEdgeDone
align 2
ToScanEdgeExit:
jmp ScanEdgeExit
align2
IsVertical:
mov ax,[bp+X1] ;starting (and only) X coordinate
sub bx,[bp+SkipFirst] ;loop count = Height - SkipFirst
jz ScanEdgeExit ;no scan lines left after skipping 1st
VerticalLoop:
mov [di],ax ;store the current X value
add di,size HLine ;point to the next HLine struc
dec bx ;count off this scan line
jnz VerticalLoop
jmp ScanEdgeDone
align 2
IsDiagonal:
mov ax,[bp+X1] ;starting X coordinate
cmp [bp+SkipFirst],1 ;skip the first point?
jz DiagonalSkipEntry ;yes
DiagonalLoop:
mov [di],ax ;store the current X value
add di,size HLine ;point to the next HLine struc
DiagonalSkipEntry:
add ax,dx ;advance the X coordinate
dec bx ;count off this scan line
jnz DiagonalLoop
jmp ScanEdgeDone
align 2
YMajor:
pushbp ;preserve stack frame pointer
mov si,[bp+X1] ;starting X coordinate
cmp [bp+SkipFirst],1 ;skip the first point?
mov bp,bx ;put Height in BP for error term calcs
jz YMajorSkipEntry ;yes, skip the first point
YMajorLoop:
mov [di],si ;store the current X value
add di,size HLine ;point to the next HLine struc
YMajorSkipEntry:
add cx,ax ;advance the error term
jle YMajorNoAdvance ;not time for X coord to advance
add si,dx ;advance the X coordinate
sub cx,bp ;adjust error term back
YMajorNoAdvance:
dec bx ;count off this scan line
jnz YMajorLoop
popbp ;restore stack frame pointer
ScanEdgeDone:
cmp [bp+SetXStart],1 ;were we working with XStart field?
jz UpdateHLinePtr ;yes, DI points to the next XStart
sub di,XEnd ;no, point back to the XStart field
UpdateHLinePtr:
mov bx,[bp+EdgePointPtr] ;point to pointer to HLine array
mov [bx],di ;update caller’s HLine array pointer
ScanEdgeExit:
pop di ;restore caller’s register variables
pop si
mov sp,bp ;deallocate local variables
pop bp ;restore caller’s stack frame
ret
_ScanEdge endp
end
Graphics Programming Black Book © 2001 Michael Abrash