UnrealEngineUWP/Engine/Shaders/Private/PostProcessHistogramCommon.ush

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

/*=============================================================================
	PostProcessHistogramCommon.usf: PostProcessing histogram shared functions and structures.
=============================================================================*/

float EyeAdaptation_ExposureLowPercent;
float EyeAdaptation_ExposureHighPercent;
float EyeAdaptation_MinAverageLuminance;
float EyeAdaptation_MaxAverageLuminance;
float EyeAdaptation_ExposureCompensationSettings;
float EyeAdaptation_ExposureCompensationCurve;
float EyeAdaptation_DeltaWorldTime;
float EyeAdaptation_ExposureSpeedUp;
float EyeAdaptation_ExposureSpeedDown;
float EyeAdaptation_HistogramScale;
float EyeAdaptation_HistogramBias;
float EyeAdaptation_LuminanceMin;
float EyeAdaptation_BlackHistogramBucketInfluence;
float EyeAdaptation_GreyMult;
float EyeAdaptation_ExponentialUpM;
float EyeAdaptation_ExponentialDownM;
float EyeAdaptation_StartDistance;
float EyeAdaptation_LuminanceMax;
float EyeAdaptation_ForceTarget;
int EyeAdaptation_VisualizeDebugType;
Texture2D EyeAdaptation_MeterMaskTexture;
SamplerState EyeAdaptation_MeterMaskSampler;

// inverse of ComputeLuminanceFromHistogramPosition
// is executed more often than ComputeLuminanceFromHistogramPosition()
// @param Luminance
// @return HistogramPosition 0..1
float ComputeHistogramPositionFromLuminance(float Luminance)
{
	return log2(Luminance) * EyeAdaptation_HistogramScale + EyeAdaptation_HistogramBias;
}

// inverse of ComputeHistogramPositionFromLuminance()
// is not as often executed as ComputeHistogramPositionFromLuminance()
// @param HistogramPosition 0..1
// @return Luminance
float ComputeLuminanceFromHistogramPosition(float HistogramPosition)
{
	return exp2((HistogramPosition - EyeAdaptation_HistogramBias) / EyeAdaptation_HistogramScale);
}


float ComputeLogLuminanceFromHistogramPosition(float HistogramPosition)
{
	return ((HistogramPosition - EyeAdaptation_HistogramBias) / EyeAdaptation_HistogramScale);
}


#define HISTOGRAM_SIZE 64
#define HISTOGRAM_TEXEL_SIZE (HISTOGRAM_SIZE / 4)

float4 ComputeARGBStripeMaskInt(uint x)
{
	return float4(
		(x % 4) == 0,
		(x % 4) == 1,
		(x % 4) == 2,
		(x % 4) == 3);
}

float GetHistogramBucket(Texture2D HistogramTexture, uint BucketIndex)
{
	uint Texel = BucketIndex / 4;
	
	float4 HistogramColor = HistogramTexture.Load(int3(Texel, 0, 0));

	uint channel = BucketIndex % 4;
	float UnweightedValue = HistogramColor.r;
	UnweightedValue = (channel == 1) ? HistogramColor.g : UnweightedValue;
	UnweightedValue = (channel == 2) ? HistogramColor.b : UnweightedValue;
	UnweightedValue = (channel == 3) ? HistogramColor.a : UnweightedValue;

	return UnweightedValue;
}

float ComputeHistogramSum(Texture2D HistogramTexture)
{
	float Sum = 0;

	for(uint i = 0; i < HISTOGRAM_SIZE; ++i)
	{
		Sum += GetHistogramBucket(HistogramTexture, i);
	}

	return Sum;
}

// @param MinFractionSum e.g. ComputeHistogramSum() * 0.5f for 50% percentil
// @param MaxFractionSum e.g. ComputeHistogramSum() * 0.9f for 90% percentil
float ComputeAverageLuminanceWithoutOutlier(Texture2D HistogramTexture, float MinFractionSum, float MaxFractionSum)
{
	float2 SumWithoutOutliers = 0;
	
	UNROLL for(uint i = 0; i < HISTOGRAM_SIZE; ++i)
	{
		float LocalValue = GetHistogramBucket(HistogramTexture, i);

		// remove outlier at lower end
		float Sub = min(LocalValue, MinFractionSum);
		LocalValue = LocalValue - Sub;
		MinFractionSum -= Sub;
		MaxFractionSum -= Sub;
		
		// remove outlier at upper end
		LocalValue = min(LocalValue, MaxFractionSum);
		MaxFractionSum -= LocalValue;

		float LogLuminanceAtBucket = ComputeLogLuminanceFromHistogramPosition(float(i) / (float)(HISTOGRAM_SIZE-1));

		SumWithoutOutliers += float2(LogLuminanceAtBucket, 1) * LocalValue;
	}

	float AvgLogLuminance = SumWithoutOutliers.x / max(0.0001f, SumWithoutOutliers.y);

	return exp2(AvgLogLuminance);
}

float ComputeEyeAdaptationExposure(Texture2D HistogramTexture)
{
	const float HistogramSum = ComputeHistogramSum(HistogramTexture);
	const float AverageSceneLuminance = ComputeAverageLuminanceWithoutOutlier(HistogramTexture, HistogramSum * EyeAdaptation_ExposureLowPercent, HistogramSum * EyeAdaptation_ExposureHighPercent);
	const float LumAve = AverageSceneLuminance;

	const float ClampedLumAve = LumAve;
		
	// No longer clamping here. We are letting the target exposure be outside the min/max range, and then letting the exposure
	// gradually transion.
	return ClampedLumAve;
}

float AdaptationWeightTexture(float2 UV)
{
	return Texture2DSampleLevel(EyeAdaptation_MeterMaskTexture, EyeAdaptation_MeterMaskSampler, UV, 0).x;
}

float ExponentialAdaption(float Current, float Target, float FrameTime, float AdaptionSpeed, float M)
{
	const float Factor = 1.0f - exp2(-FrameTime * AdaptionSpeed);
	const float Value = Current + (Target - Current) * Factor * M;
	return Value;
}

float LinearAdaption(float Current, float Target, float FrameTime, float AdaptionSpeed)
{
	const float Offset = FrameTime * AdaptionSpeed;

	const float Value = (Current < Target) ? min(Target, Current + Offset) : max(Target, Current - Offset);

	return Value;
}

float ComputeEyeAdaptation(float OldExposure, float TargetExposure, float FrameTime)
{
	const float LogTargetExposure = log2(TargetExposure);
	const float LogOldExposure = log2(OldExposure);

	const float LogDiff = LogTargetExposure - LogOldExposure;

	const float AdaptionSpeed = (LogDiff > 0) ? EyeAdaptation_ExposureSpeedUp : EyeAdaptation_ExposureSpeedDown;
	const float M = (LogDiff > 0) ? EyeAdaptation_ExponentialUpM : EyeAdaptation_ExponentialDownM;

	const float AbsLogDiff = abs(LogDiff);

	// blended exposure
	const float LogAdaptedExposure_Exponential = ExponentialAdaption(LogOldExposure, LogTargetExposure, FrameTime, AdaptionSpeed, M);
	const float LogAdaptedExposure_Linear = LinearAdaption(LogOldExposure, LogTargetExposure, FrameTime, AdaptionSpeed);

	const float LogAdaptedExposure = AbsLogDiff > EyeAdaptation_StartDistance ? LogAdaptedExposure_Linear : LogAdaptedExposure_Exponential;

	// Note: no clamping here. The target exposure should always be clamped so if we are below the min or above the max,
	// instead of clamping, we will gradually transition to the target exposure. If were to clamp, the then we would have a harsh transition
	// when going from postFX volumes with different min/max luminance values.
	const float AdaptedExposure = exp2(LogAdaptedExposure);

	// for manual mode or camera cuts, just lerp to the target
	const float AdjustedExposure = lerp(AdaptedExposure,TargetExposure,EyeAdaptation_ForceTarget);
	
	return AdjustedExposure;
}