**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