CullOBB.c

/*
 * Used for culling tiles in a map renderer I wrote.
 * All tiles used geometry in the [-1, 1] range, and their mvp matrix would get precalculated.
 * So it was simply a matter of seeing if their projected OBB overlapped with the screen's clip space.
 * Returns true if the cube is visible. (false positives for near misses due to conservative w-value)
 */
static inline bool MapTileFrustumCull(const mat4 mvp){
	// Clip space center of the cube is the last column of the matrix.
	vec4 c = {mvp.m[12], mvp.m[13], mvp.m[14], mvp.m[15]};
  	// Clip space extents are the component wise absolute values of the first three columns.
	float ex = fabsf(mvp.m[ 0]) + fabsf(mvp.m[ 4]) + fabsf(mvp.m[ 8]);
	float ey = fabsf(mvp.m[ 1]) + fabsf(mvp.m[ 5]) + fabsf(mvp.m[ 9]);
	float ez = fabsf(mvp.m[ 2]) + fabsf(mvp.m[ 6]) + fabsf(mvp.m[10]);
	
	// Calculate a conservative w-value for the center.
	float w = fmaxf(0, c.w + fabsf(mvp.m[ 3]) + fabsf(mvp.m[ 7]) + fabsf(mvp.m[11]));
  	// Compare the center coord against the screen's clip space bounds using the w-value.
	return ((fabsf(c.x) - ex < w) && (fabsf(c.y) - ey < w) && (fabsf(c.z) - ez < w));
}