// luminance histogram of the image for the automatic exposure: every second pixel in both directions, binned by the log2
// of its luminance. bin 0 takes the black pixels, which the average leaves out. exposure_average.comp reads and clears it

#include <common>

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

layout(binding = 0) uniform sampler2D hdr_image;

layout(std430, binding = EXPOSURE_SSBO_BINDING) buffer exposure_ssbo
{
	uint histogram[EXPOSURE_HISTOGRAM_BINS];
	float adapted_luminance;
	float exposure;
	float average_luminance;
	uint valid;
};

// x: log2 of the lowest luminance binned, y: number of bins per unit of log2 luminance
layout(location = 0) uniform vec2 log_range;

shared uint shared_histogram[EXPOSURE_HISTOGRAM_BINS];

void main()
{
	uint index = gl_LocalInvocationIndex;
	shared_histogram[index] = 0U;
	barrier();

	ivec2 pixel = ivec2(gl_GlobalInvocationID.xy) * 2;
	ivec2 size = textureSize(hdr_image, 0);
	if (pixel.x < size.x && pixel.y < size.y)
	{
		vec3 color = texelFetch(hdr_image, pixel, 0).rgb;
		float luminance = dot(color, vec3(0.2126, 0.7152, 0.0722));
		uint bin = 0U;
		// also false for a NaN
		if (luminance > 1e-6)
		{
			float position = clamp((log2(luminance) - log_range.x) * log_range.y, 0.0, float(EXPOSURE_HISTOGRAM_BINS - 2U));
			bin = 1U + uint(position);
		}
		atomicAdd(shared_histogram[bin], 1U);
	}
	barrier();

	if (shared_histogram[index] != 0U)
		atomicAdd(histogram[index], shared_histogram[index]);
}