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1991
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1991-02-15
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428 lines
_GRAPHICS PROGRAMMING COLUMN_
by Michael Abrash
[LISTING ONE]
/* Draws all pixels in the list of horizontal lines passed in, in
mode 13h, the VGA's 320x200 256-color mode. Uses memset to fill
each line, which is much faster than using DrawPixel but requires
that a large data model (compact, large, or huge) be in use when
running in real mode or 286 protected mode.
All C code tested with Turbo C++ */
#include <string.h>
#include <dos.h>
#include "polygon.h"
#define SCREEN_WIDTH 320
#define SCREEN_SEGMENT 0xA000
void DrawHorizontalLineList(struct HLineList * HLineListPtr,
int Color)
{
struct HLine *HLinePtr;
int Length, Width;
unsigned char far *ScreenPtr;
/* Point to the start of the first scan line on which to draw */
ScreenPtr = MK_FP(SCREEN_SEGMENT,
HLineListPtr->YStart * SCREEN_WIDTH);
/* Point to the XStart/XEnd descriptor for the first (top)
horizontal line */
HLinePtr = HLineListPtr->HLinePtr;
/* Draw each horizontal line in turn, starting with the top one and
advancing one line each time */
Length = HLineListPtr->Length;
while (Length-- > 0) {
/* Draw the whole horizontal line if it has a positive width */
if ((Width = HLinePtr->XEnd - HLinePtr->XStart + 1) > 0)
memset(ScreenPtr + HLinePtr->XStart, Color, Width);
HLinePtr++; /* point to next scan line X info */
ScreenPtr += SCREEN_WIDTH; /* point to next scan line start */
}
}
[LISTING TWO]
/* 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 and
precision */
#include <math.h>
#include "polygon.h"
void ScanEdge(int X1, int Y1, int X2, int Y2, int SetXStart,
int SkipFirst, struct HLine **EdgePointPtr)
{
int Y, DeltaX, Height, Width, AdvanceAmt, ErrorTerm, i;
int ErrorTermAdvance, XMajorAdvanceAmt;
struct HLine *WorkingEdgePointPtr;
WorkingEdgePointPtr = *EdgePointPtr; /* avoid double dereference */
AdvanceAmt = ((DeltaX = X2 - X1) > 0) ? 1 : -1;
/* direction in which X moves (Y2 is
always > Y1, so Y always counts up) */
if ((Height = Y2 - Y1) <= 0) /* Y length of the edge */
return; /* guard against 0-length and horizontal edges */
/* Figure out whether the edge is vertical, diagonal, X-major
(mostly horizontal), or Y-major (mostly vertical) and handle
appropriately */
if ((Width = abs(DeltaX)) == 0) {
/* The edge is vertical; special-case by just storing the same
X coordinate for every scan line */
/* Scan the edge for each scan line in turn */
for (i = Height - SkipFirst; i-- > 0; WorkingEdgePointPtr++) {
/* Store the X coordinate in the appropriate edge list */
if (SetXStart == 1)
WorkingEdgePointPtr->XStart = X1;
else
WorkingEdgePointPtr->XEnd = X1;
}
} else if (Width == Height) {
/* The edge is diagonal; special-case by advancing the X
coordinate 1 pixel for each scan line */
if (SkipFirst) /* skip the first point if so indicated */
X1 += AdvanceAmt; /* move 1 pixel to the left or right */
/* Scan the edge for each scan line in turn */
for (i = Height - SkipFirst; i-- > 0; WorkingEdgePointPtr++) {
/* Store the X coordinate in the appropriate edge list */
if (SetXStart == 1)
WorkingEdgePointPtr->XStart = X1;
else
WorkingEdgePointPtr->XEnd = X1;
X1 += AdvanceAmt; /* move 1 pixel to the left or right */
}
} else if (Height > Width) {
/* Edge is closer to vertical than horizontal (Y-major) */
if (DeltaX >= 0)
ErrorTerm = 0; /* initial error term going left->right */
else
ErrorTerm = -Height + 1; /* going right->left */
if (SkipFirst) { /* skip the first point if so indicated */
/* Determine whether it's time for the X coord to advance */
if ((ErrorTerm += Width) > 0) {
X1 += AdvanceAmt; /* move 1 pixel to the left or right */
ErrorTerm -= Height; /* advance ErrorTerm to next point */
}
}
/* Scan the edge for each scan line in turn */
for (i = Height - SkipFirst; i-- > 0; WorkingEdgePointPtr++) {
/* Store the X coordinate in the appropriate edge list */
if (SetXStart == 1)
WorkingEdgePointPtr->XStart = X1;
else
WorkingEdgePointPtr->XEnd = X1;
/* Determine whether it's time for the X coord to advance */
if ((ErrorTerm += Width) > 0) {
X1 += AdvanceAmt; /* move 1 pixel to the left or right */
ErrorTerm -= Height; /* advance ErrorTerm to correspond */
}
}
} else {
/* Edge is closer to horizontal than vertical (X-major) */
/* Minimum distance to advance X each time */
XMajorAdvanceAmt = (Width / Height) * AdvanceAmt;
/* Error term advance for deciding when to advance X 1 extra */
ErrorTermAdvance = Width % Height;
if (DeltaX >= 0)
ErrorTerm = 0; /* initial error term going left->right */
else
ErrorTerm = -Height + 1; /* going right->left */
if (SkipFirst) { /* skip the first point if so indicated */
X1 += XMajorAdvanceAmt; /* move X minimum distance */
/* Determine whether it's time for X to advance one extra */
if ((ErrorTerm += ErrorTermAdvance) > 0) {
X1 += AdvanceAmt; /* move X one more */
ErrorTerm -= Height; /* advance ErrorTerm to correspond */
}
}
/* Scan the edge for each scan line in turn */
for (i = Height - SkipFirst; i-- > 0; WorkingEdgePointPtr++) {
/* Store the X coordinate in the appropriate edge list */
if (SetXStart == 1)
WorkingEdgePointPtr->XStart = X1;
else
WorkingEdgePointPtr->XEnd = X1;
X1 += XMajorAdvanceAmt; /* move X minimum distance */
/* Determine whether it's time for X to advance one extra */
if ((ErrorTerm += ErrorTermAdvance) > 0) {
X1 += AdvanceAmt; /* move X one more */
ErrorTerm -= Height; /* advance ErrorTerm to correspond */
}
}
}
*EdgePointPtr = WorkingEdgePointPtr; /* advance caller's ptr */
}
[LISTING THREE]
; Draws all pixels in the list of horizontal lines passed in, in
; mode 13h, the VGA's 320x200 256-color mode. Uses REP STOS to fill
; each line.
; C near-callable as:
; void DrawHorizontalLineList(struct HLineList * HLineListPtr,
; int Color);
; All assembly code tested with TASM 2.0 and MASM 5.0
SCREEN_WIDTH equ 320
SCREEN_SEGMENT equ 0a000h
HLine struc
XStart dw ? ;X coordinate of leftmost pixel in line
XEnd dw ? ;X coordinate of rightmost pixel in line
HLine ends
HLineList struc
Lngth dw ? ;# of horizontal lines
YStart dw ? ;Y coordinate of topmost line
HLinePtr dw ? ;pointer to list of horz lines
HLineList ends
Parms struc
dw 2 dup(?) ;return address & pushed BP
HLineListPtr dw ? ;pointer to HLineList structure
Color dw ? ;color with which to fill
Parms ends
.model small
.code
public _DrawHorizontalLineList
align 2
_DrawHorizontalLineList proc
push bp ;preserve caller's stack frame
mov bp,sp ;point to our stack frame
push si ;preserve caller's register variables
push di
cld ;make string instructions inc pointers
mov ax,SCREEN_SEGMENT
mov es,ax ;point ES to display memory for REP STOS
mov si,[bp+HLineListPtr] ;point to the line list
mov ax,SCREEN_WIDTH ;point to the start of the first scan
mul [si+YStart] ; line in which to draw
mov dx,ax ;ES:DX points to first scan line to
; draw
mov bx,[si+HLinePtr] ;point to the XStart/XEnd descriptor
; for the first (top) horizontal line
mov si,[si+Lngth] ;# of scan lines to draw
and si,si ;are there any lines to draw?
jz FillDone ;no, so we're done
mov al,byte ptr [bp+Color] ;color with which to fill
mov ah,al ;duplicate color for STOSW
FillLoop:
mov di,[bx+XStart] ;left edge of fill on this line
mov cx,[bx+XEnd] ;right edge of fill
sub cx,di
js LineFillDone ;skip if negative width
inc cx ;width of fill on this line
add di,dx ;offset of left edge of fill
test di,1 ;does fill start at an odd address?
jz MainFill ;no
stosb ;yes, draw the odd leading byte to
; word-align the rest of the fill
dec cx ;count off the odd leading byte
jz LineFillDone ;done if that was the only byte
MainFill:
shr cx,1 ;# of words in fill
rep stosw ;fill as many words as possible
adc cx,cx ;1 if there's an odd trailing byte to
; do, 0 otherwise
rep stosb ;fill any odd trailing byte
LineFillDone:
add bx,size HLine ;point to the next line descriptor
add dx,SCREEN_WIDTH ;point to the next scan line
dec si ;count off lines to fill
jnz FillLoop
FillDone:
pop di ;restore caller's register variables
pop si
pop bp ;restore caller's stack frame
ret
_DrawHorizontalLineList endp
end
[LISTING FOUR]
; 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
align 2
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:
push bp ;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
pop bp ;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