// MinecraftPT — Exposure_CS
// Computes the average scene luminance and smooths the exposure over time.
// Writes the result into a 1x1 R16F image (exposureTex) read by the final pass.

#include "/Lib/Settings.glsl"
#include "/Lib/Utilities.glsl"

const ivec3 workGroups = ivec3(8, 8, 1);
layout (local_size_x = 16, local_size_y = 16) in;

layout (r16f) uniform writeonly image2D img_exposureTex;

uniform sampler2D colortex12;

shared float luminanceSum[256];

void main(){
	ivec2 texel = ivec2(gl_GlobalInvocationID.xy);

	// Downsample luminance from the HDR scene
	float lum = 0.0;
	vec2 base = vec2(texel * 4);
	for (int i = 0; i < 4; i++){
	for (int j = 0; j < 4; j++){
		vec3 color = texelFetch(colortex12, ivec2(base + vec2(i, j)), 0).rgb;
		lum += luminance(color);
	}}
	lum /= 16.0;

	// Shared reduction
	uint local = gl_LocalInvocationID.x + gl_LocalInvocationID.y * 16u;
	luminanceSum[local] = lum;
	barrier();

	if (local == 0u){
		float avg = 0.0;
		for (uint i = 0u; i < 256u; i++){
			avg += luminanceSum[i];
		}
		avg /= 256.0;

		// Target exposure (inverse of average luminance)
		float targetExposure = 1.0 / max(avg, 0.001);

		// Read previous exposure for temporal smoothing (single thread)
		float prevExposure = imageLoad(img_exposureTex, ivec2(0, 0)).r;
		if (prevExposure <= 0.0) prevExposure = 1.0;

		float adapted = mix(prevExposure, targetExposure, SMOOTH_EXPOSURE);
		adapted = clamp(adapted, 0.01, 100.0);

		imageStore(img_exposureTex, ivec2(0, 0), vec4(adapted));
	}
}