maszyna/shaders/shadow_fit_depth.comp

// the cascades of the sun shadows fitted to the depth of the main view (opengl46_shadow_fit), first pass: how many pixels of
// the resolved depth of the view aren't the sky, and the nearest and the farthest of their distances from the camera.
// the depth is reversed with the far plane at infinity: the sky, which writes no depth, stays at 0.
// shadow_fit_bounds.comp splits the distances between the cascades

#include <common>

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

layout(binding = 0) uniform sampler2D scene_depth;

layout(std430, binding = SHADOW_FIT_SSBO_BINDING) buffer shadow_fit_ssbo
{
	uint measurement;
	uint samples;
	// positive floats, as bits
	uint depthmin;
	uint depthmax;
	float splits[4];
	uint counts[4];
	uint bounds[MAX_CASCADES * 8U];
};

// the inverse of the projection of the view
layout(location = 0) uniform mat4 inv_projection;

shared uint shared_samples;
shared uint shared_min;
shared uint shared_max;

void main()
{
	if (gl_LocalInvocationIndex == 0U)
	{
		shared_samples = 0U;
		// infinity
		shared_min = 0x7F800000U;
		shared_max = 0U;
	}
	barrier();

	ivec2 pixel = ivec2(gl_GlobalInvocationID.xy);
	ivec2 size = textureSize(scene_depth, 0);
	if (all(lessThan(pixel, size)))
	{
		float depth = texelFetch(scene_depth, pixel, 0).r;
		if (depth > 0.0)
		{
			vec2 ndc = (vec2(pixel) + 0.5) / vec2(size) * 2.0 - 1.0;
			vec4 position = inv_projection * vec4(ndc, depth, 1.0);
			uint distance = floatBitsToUint(length(position.xyz / position.w));
			atomicAdd(shared_samples, 1U);
			atomicMin(shared_min, distance);
			atomicMax(shared_max, distance);
		}
	}
	barrier();

	if ((gl_LocalInvocationIndex == 0U) && (shared_samples > 0U))
	{
		atomicAdd(samples, shared_samples);
		atomicMin(depthmin, shared_min);
		atomicMax(depthmax, shared_max);
	}
}