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James Gregory 2013-12-31 11:38:48 +11:00
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57-02.md
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@ -51,76 +51,78 @@ level of orientation-dependent asymmetry.
**LISTING 57.1 L57-1.C**
/* Texture-map-draw the scan line between two edges. 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. */
```c
/* Texture-map-draw the scan line between two edges. 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. */
void ScanOutLine(EdgeScan * LeftEdge, EdgeScan * RightEdge)
{
Fixedpoint SourceX;
Fixedpoint SourceY;
int DestX = LeftEdge->DestX;
int DestXMax = RightEdge->DestX;
Fixedpoint DestWidth;
Fixedpoint SourceStepX, SourceStepY;
void ScanOutLine(EdgeScan * LeftEdge, EdgeScan * RightEdge)
{
Fixedpoint SourceX;
Fixedpoint SourceY;
int DestX = LeftEdge->DestX;
int DestXMax = RightEdge->DestX;
Fixedpoint DestWidth;
Fixedpoint SourceStepX, SourceStepY;
/* Nothing to do if fully X clipped */
if ((DestXMax <= ClipMinX) || (DestX >= ClipMaxX)) {
return;
}
/* Nothing to do if fully X clipped */
if ((DestXMax <= ClipMinX) || (DestX >= ClipMaxX)) {
return;
}
if ((DestXMax - DestX) <= 0) {
return; /* nothing to draw */
}
SourceX = LeftEdge->SourceX;
SourceY = LeftEdge->SourceY;
if ((DestXMax - DestX) <= 0) {
return; /* nothing to draw */
}
SourceX = LeftEdge->SourceX;
SourceY = LeftEdge->SourceY;
/* Width of destination scan line, for scaling. Note: because this is an
integer-based scaling, it can have a total error of as much as nearly
one pixel. For more precise scaling, also maintain a fixed-point DestX
in each edge, and use it for scaling. If this is done, it will also
be necessary to nudge the source start coordinates to the right by an
amount corresponding to the distance from the the real (fixed-point)
DestX and the first pixel (at an integer X) to be drawn). */
DestWidth = INT-TO-FIXED(DestXMax - DestX);
/* Width of destination scan line, for scaling. Note: because this is an
integer-based scaling, it can have a total error of as much as nearly
one pixel. For more precise scaling, also maintain a fixed-point DestX
in each edge, and use it for scaling. If this is done, it will also
be necessary to nudge the source start coordinates to the right by an
amount corresponding to the distance from the the real (fixed-point)
DestX and the first pixel (at an integer X) to be drawn). */
DestWidth = INT-TO-FIXED(DestXMax - DestX);
/* Calculate source steps that correspond to each dest X step (across
the scan line) */
SourceStepX = FixedDiv(RightEdge->SourceX - SourceX, DestWidth);
SourceStepY = FixedDiv(RightEdge->SourceY - SourceY, DestWidth);
/* Calculate source steps that correspond to each dest X step (across
the scan line) */
SourceStepX = FixedDiv(RightEdge->SourceX - SourceX, DestWidth);
SourceStepY = FixedDiv(RightEdge->SourceY - SourceY, DestWidth);
/* 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.) */
SourceX += SourceStepX >> 1;
SourceY += SourceStepY >> 1;
/* 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.) */
SourceX += SourceStepX >> 1;
SourceY += SourceStepY >> 1;
/* Clip right edge if necssary */
if (DestXMax > ClipMaxX)
DestXMax = ClipMaxX;
/* Clip right edge if necssary */
if (DestXMax > ClipMaxX)
DestXMax = ClipMaxX;
/* Clip left edge if necssary */
if (DestX < ClipMinX) {
SourceX += FixedMul(SourceStepX, INT-TO-FIXED(ClipMinX - DestX));
SourceY += FixedMul(SourceStepY, INT-TO-FIXED(ClipMinX - DestX));
DestX = ClipMinX;
}
/* Scan across the destination scan line, updating the source image
position accordingly */
for (; DestX<DestXMax; DestX++) {
/* Get the currently mapped pixel out of the image and draw it to
the screen */
WritePixelX(DestX, DestY,
GET-IMAGE-PIXEL(TexMapBits, TexMapWidth,
ROUND-FIXED-TO-INT(SourceX), ROUND-FIXED-TO-INT(SourceY)) );
/* Point to the next source pixel */
SourceX += SourceStepX;
SourceY += SourceStepY;
}
}
/* Clip left edge if necssary */
if (DestX < ClipMinX) {
SourceX += FixedMul(SourceStepX, INT-TO-FIXED(ClipMinX - DestX));
SourceY += FixedMul(SourceStepY, INT-TO-FIXED(ClipMinX - DestX));
DestX = ClipMinX;
}
/* Scan across the destination scan line, updating the source image
position accordingly */
for (; DestX<DestXMax; DestX++) {
/* Get the currently mapped pixel out of the image and draw it to
the screen */
WritePixelX(DestX, DestY,
GET-IMAGE-PIXEL(TexMapBits, TexMapWidth,
ROUND-FIXED-TO-INT(SourceX), ROUND-FIXED-TO-INT(SourceY)) );
/* Point to the next source pixel */
SourceX += SourceStepX;
SourceY += SourceStepY;
}
}
```
#### Mapping Textures across Multiple Polygons {#Heading6}