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/* Polygon clip detection code
*
* The polygon clipping logic is based on the Cohen-Sutherland algorithm, but
* only the off-screen detection logic is used to determine which polygon edges
* are off-screen.
*
* In tri_clip, the following edges are checked as follows:
*
* |\
* | \
* | \
* | \
* |--------
*
* In quad_clip, the following edges are checked as follows:
*
* |---------|
* | \ / |
* | \ / |
* | / \ |
* | / \ |
* |---------|
*
* The inner portion of the quad is checked, otherwise the quad will be
* culled out if the camera faces right into it, where all four edges
* are off-screen at once.
*
*/
#include "clip.h"
#define CLIP_LEFT 1
#define CLIP_RIGHT 2
#define CLIP_TOP 4
#define CLIP_BOTTOM 8
int test_clip(RECT *clip, short x, short y) {
// Tests which corners of the screen a point lies outside of
int result = 0;
if ( x < clip->x ) {
result |= CLIP_LEFT;
}
if ( x >= (clip->x+(clip->w-1)) ) {
result |= CLIP_RIGHT;
}
if ( y < clip->y ) {
result |= CLIP_TOP;
}
if ( y >= (clip->y+(clip->h-1)) ) {
result |= CLIP_BOTTOM;
}
return result;
}
int tri_clip(RECT *clip, DVECTOR *v0, DVECTOR *v1, DVECTOR *v2) {
// Returns non-zero if a triangle is outside the screen boundaries
short c[3];
c[0] = test_clip(clip, v0->vx, v0->vy);
c[1] = test_clip(clip, v1->vx, v1->vy);
c[2] = test_clip(clip, v2->vx, v2->vy);
if ( ( c[0] & c[1] ) == 0 )
return 0;
if ( ( c[1] & c[2] ) == 0 )
return 0;
if ( ( c[2] & c[0] ) == 0 )
return 0;
return 1;
}
int quad_clip(RECT *clip, DVECTOR *v0, DVECTOR *v1, DVECTOR *v2, DVECTOR *v3) {
// Returns non-zero if a quad is outside the screen boundaries
short c[4];
c[0] = test_clip(clip, v0->vx, v0->vy);
c[1] = test_clip(clip, v1->vx, v1->vy);
c[2] = test_clip(clip, v2->vx, v2->vy);
c[3] = test_clip(clip, v3->vx, v3->vy);
if ( ( c[0] & c[1] ) == 0 )
return 0;
if ( ( c[1] & c[2] ) == 0 )
return 0;
if ( ( c[2] & c[3] ) == 0 )
return 0;
if ( ( c[3] & c[0] ) == 0 )
return 0;
if ( ( c[0] & c[2] ) == 0 )
return 0;
if ( ( c[1] & c[3] ) == 0 )
return 0;
return 1;
}
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