maszyna/shaders/hiz_build.comp

// one level of the depth pyramid of the occlusion test of the instances (opengl46_hiz): each texel holds the farthest
// depth of the texels of the level below it which it covers. the depth is reversed (1 near, 0 far), so the farthest is
// the smallest. the first level is made from the resolved depth of the scene, the others from the level below.
// a level halves the one below, rounding down; when that one has an odd size, the last row and column of the new level
// take its last row and column as well, so that nothing is left out

#include <common>

layout(local_size_x = HIZ_GROUP_SIZE, local_size_y = HIZ_GROUP_SIZE) in;

layout(binding = 0) uniform sampler2D source;
layout(r32f, binding = 0) writeonly uniform image2D target;

// xy: size of the source level, zw: size of the target level
layout(location = 0) uniform ivec4 sizes;
// x: level of the source read
layout(location = 1) uniform ivec4 params;

void main()
{
	ivec2 texel = ivec2(gl_GlobalInvocationID.xy);
	if (any(greaterThanEqual(texel, sizes.zw)))
		return;

	ivec2 first = min(2 * texel, sizes.xy - 1);
	ivec2 last = min(2 * texel + 1, sizes.xy - 1);
	if (texel.x == sizes.z - 1)
		last.x = sizes.x - 1;
	if (texel.y == sizes.w - 1)
		last.y = sizes.y - 1;

	float farthest = 1.0;
	for (int y = first.y; y <= last.y; ++y)
	{
		for (int x = first.x; x <= last.x; ++x)
			farthest = min(farthest, texelFetch(source, ivec2(x, y), params.x).r);
	}
	imageStore(target, texel, vec4(farthest));
}