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Graphics Programming Black Book (Special Edition)
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BlackBook.bin
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disk1
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source
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chapter69
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l69-1.c
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1997-06-18
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112 lines
// Quake's recursive subdivision triangle rasterizer; draws all
// pixels in a triangle other than the vertices by splitting an
// edge to form a new vertex, drawing the vertex, and recursively
// processing each of the two new triangles formed by using the
// new vertex. Results are less accurate than from a precise
// affine or perspective texture mapper, and drawing boundaries
// are not identical to those of a precise polygon drawer, although
// they are consistent between adjacent polygons drawn with this
// technique.
//
// Invented and implemented by John Carmack of id Software.
void D_PolysetRecursiveTriangle (int *lp1, int *lp2, int *lp3)
{
int *temp;
int d;
int new[6];
int z;
short *zbuf;
// try to find an edge that's more than one pixel long in x or y
d = lp2[0] - lp1[0];
if (d < -1 || d > 1)
goto split;
d = lp2[1] - lp1[1];
if (d < -1 || d > 1)
goto split;
d = lp3[0] - lp2[0];
if (d < -1 || d > 1)
goto split2;
d = lp3[1] - lp2[1];
if (d < -1 || d > 1)
goto split2;
d = lp1[0] - lp3[0];
if (d < -1 || d > 1)
goto split3;
d = lp1[1] - lp3[1];
if (d < -1 || d > 1)
{
split3:
// shuffle points so first edge is edge to split
temp = lp1;
lp1 = lp3;
lp3 = lp2;
lp2 = temp;
goto split;
}
return; // no pixels left to fill in triangle
split2:
// shuffle points so first edge is edge to split
temp = lp1;
lp1 = lp2;
lp2 = lp3;
lp3 = temp;
split:
// split first edge screen x, screen y, texture s, texture t, and z
// to form a new vertex. Lighting (index 4) is ignored; the
// difference between interpolating lighting and using the same
// shading for the entire triangle is unnoticeable for small
// triangles, so we just use the lighting for the first vertex of
// the original triangle (which was used during set-up to set
// d_colormap, used below to look up lit texels)
new[0] = (lp1[0] + lp2[0]) >> 1; // split screen x
new[1] = (lp1[1] + lp2[1]) >> 1; // split screen y
new[2] = (lp1[2] + lp2[2]) >> 1; // split texture s
new[3] = (lp1[3] + lp2[3]) >> 1; // split texture t
new[5] = (lp1[5] + lp2[5]) >> 1; // split z
// draw the point if splitting a leading edge
if (lp2[1] > lp1[1])
goto nodraw;
if ((lp2[1] == lp1[1]) && (lp2[0] < lp1[0]))
goto nodraw;
z = new[5]>>16;
// point to the pixel's z-buffer entry, looking up the scanline start
// address based on screen y and adding in the screen x coordinate
zbuf = zspantable[new[1]] + new[0];
// draw the split vertex if it's not obscured by something nearer, as
// indicated by the z-buffer
if (z >= *zbuf)
{
int pix;
// set the z-buffer to the new pixel's distance
*zbuf = z;
// get the texel from the model's skin bitmap, according to
// the s and t texture coordinates, and translate it through
// the lighting look-up table set according to the first
// vertex for the original (top-level) triangle. Both s and
// t are in 16.16 format
pix = d_pcolormap[skintable[new[3]>>16][new[2]>>16]];
// draw the pixel, looking up the scanline start address
// based on screen y and adding in the screen x coordinate
d_viewbuffer[d_scantable[new[1]] + new[0]] = pix;
}
nodraw:
// recursively draw the two new triangles we created by adding the
// split vertex
D_PolysetRecursiveTriangle (lp3, lp1, new);
D_PolysetRecursiveTriangle (lp3, new, lp2);
}