UnrealEngineUWP/Engine/Shaders/Private/PostProcessHistogram.usf

// Copyright Epic Games, Inc. All Rights Reserved.

/*=============================================================================
	PostProcessHistogram.usf: PostProcessing histogram
=============================================================================*/

#include "Common.ush"
#include "ScreenPass.ush"
#include "PostProcessHistogramCommon.ush"

SCREEN_PASS_TEXTURE_VIEWPORT(Input)

Texture2D InputTexture;

// Output histogram texture (UAV)
RWTexture2D<float4> HistogramRWTexture;

// Number of thread groups in the dispatch
uint2 ThreadGroupCount;

// THREADGROUP_SIZEX*THREADGROUP_SIZEY histograms of the size HISTOGRAM_SIZE
groupshared float SharedHistogram[HISTOGRAM_SIZE][THREADGROUP_SIZEX][THREADGROUP_SIZEY];

[numthreads(THREADGROUP_SIZEX, THREADGROUP_SIZEY, 1)]
void MainCS(
	uint3 GroupId : SV_GroupID,
	uint3 DispatchThreadId : SV_DispatchThreadID,
    uint3 GroupThreadId : SV_GroupThreadID,
	uint GroupIndex: SV_GroupIndex)
{
	// todo: can be cleared more efficiently
	// clear all THREADGROUP_SIZEX*THREADGROUP_SIZEY histograms
	UNROLL for (uint i = 0; i < HISTOGRAM_SIZE; ++i)
	{
		SharedHistogram[i][GroupThreadId.x][GroupThreadId.y] = 0.0f;
	}
	
	GroupMemoryBarrierWithGroupSync();

	// Each thread group processes LoopX * LoopY texels of the input.
	uint2 TileSize = uint2(LOOP_SIZEX, LOOP_SIZEY);

	// Top left input texel for this group.
	uint2 LeftTop = Input_ViewportMin + DispatchThreadId.xy * TileSize;

	float2 InvViewSize = Input_ViewportSizeInverse.xy;

	// Accumulate all pixels into THREADGROUP_SIZEX*THREADGROUP_SIZEY histograms
	LOOP for (uint y = 0; y < LOOP_SIZEY; ++y)
	{
		LOOP for (uint x = 0; x < LOOP_SIZEX; ++x)
		{
			uint2 Tile = uint2(x, y);
			uint2 TexelPos = LeftTop + Tile;

			if(TexelPos.x < Input_ViewportMax.x && TexelPos.y < Input_ViewportMax.y)
			{
				float4 SceneColor = InputTexture.Load(int3(TexelPos, 0));

				SceneColor.xyz *= View.OneOverPreExposure;

				float LuminanceVal = max(dot(SceneColor.xyz, float3(1.0f, 1.0f, 1.0f)/3.0f), EyeAdaptation_LuminanceMin);

				float LogLuminance = ComputeHistogramPositionFromLuminance(LuminanceVal);
				float2 ScreenUV = (TexelPos.xy - Input_ViewportMin) * InvViewSize;
				float ScreenWeight = AdaptationWeightTexture(ScreenUV);

				// Map the normalized histogram position into texels.
				float fBucket = saturate(LogLuminance) * (HISTOGRAM_SIZE-1);// * 0.9999f;

				// Find two discrete buckets that straddle the continuous histogram position.
				uint Bucket0 = (uint)(fBucket);
				uint Bucket1 = Bucket0 + 1;

				Bucket0 = min(Bucket0, uint(HISTOGRAM_SIZE - 1));
				Bucket1 = min(Bucket1, uint(HISTOGRAM_SIZE - 1));

				// Weighted blend between the two buckets.
				float Weight1 = frac(fBucket);
				float Weight0 = 1.0f - Weight1;

				// When EyeAdaptation_BlackHistogramBucketInfluence=.0, we will ignore the last bucket. The main use
				// case is so the black background pixels in the editor have no effect. But if we have cases where
				// pixel values can actually be black, we want to set EyeAdaptation_LastHistogramBucketInfluence=1.0.
				// This value is controlled by the cvar "r.EyeAdaptation.BlackHistogramBucketInfluence"
				if (Bucket0 == 0)
				{
					Weight0 *= EyeAdaptation_BlackHistogramBucketInfluence;
				}

				// Accumulate the weight to the nearby history buckets.
				SharedHistogram[Bucket0][GroupThreadId.x][GroupThreadId.y] += Weight0 * ScreenWeight;
				SharedHistogram[Bucket1][GroupThreadId.x][GroupThreadId.y] += Weight1 * ScreenWeight;
			}
		}
	}

	GroupMemoryBarrierWithGroupSync();

	// Accumulate all histograms into one.
	if (GroupIndex < HISTOGRAM_SIZE / 4) 
	{
		float4 Sum = 0;

		LOOP for (uint y = 0; y < THREADGROUP_SIZEY; ++y)
		{
			LOOP for (uint x = 0; x < THREADGROUP_SIZEX; ++x)
			{
				Sum += float4(
					SharedHistogram[GroupIndex * 4 + 0][x][y],
					SharedHistogram[GroupIndex * 4 + 1][x][y],
					SharedHistogram[GroupIndex * 4 + 2][x][y],
					SharedHistogram[GroupIndex * 4 + 3][x][y]);
			}
		}

		float2 MaxExtent = Input_ViewportSize;
		float Area = MaxExtent.x * MaxExtent.y;
	
		// Fixed to include borders.
		float NormalizeFactor = 1.0f / Area;

		// Output texture with one histogram per line, x and y unwrapped into all the lines
		HistogramRWTexture[uint2(GroupIndex, GroupId.x + GroupId.y * ThreadGroupCount.x)] = Sum * NormalizeFactor;
	}
}