// Copyright 1998-2014 Epic Games, Inc. All Rights Reserved.

/*=============================================================================
	PostProcessEyeAdaptation.usf: PostProcessing eye adaptation
=============================================================================*/

#include "Common.usf"
#include "PostProcessCommon.usf"
#include "PostProcessHistogramCommon.usf"

// @param FrameTime in seconds
// @return smoothed exposure
float ComputeEyeAdaptation(float OldExposure, float TargetExposure, float FrameTime)
{
	float Diff = TargetExposure - OldExposure;

	const float EyeAdaptionSpeedUp = EyeAdaptationParams[1].z;
	const float EyeAdaptionSpeedDown = EyeAdaptationParams[1].w;

    float AdaptionSpeed = (Diff > 0) ? EyeAdaptionSpeedUp : EyeAdaptionSpeedDown;

    float Factor = 1.0f - exp2(-FrameTime * AdaptionSpeed);

	return clamp(OldExposure + Diff * Factor, EyeAdaptationParams[0].z, EyeAdaptationParams[0].w);
}

void MainPS(float4 UVAndScreenPos : TEXCOORD0, out float OutColor : SV_Target0)
{
	float2 UV = UVAndScreenPos.xy;

	float ExposureOffsetMultipler = EyeAdaptationParams[1].x;
	
	float TargetExposure = ComputeEyeAdaptationExposure(InputNew0);
	float OldExposureScale = InputNew0.Load(int3(0, 1, 0)).x;
	float OldExposure = ExposureOffsetMultipler / OldExposureScale;
	float FrameTime = EyeAdaptationParams[1].y;

	// eye adaptation changes over time
	float SmoothedExposure = ComputeEyeAdaptation(OldExposure, TargetExposure, FrameTime);

	float SmoothedExposureScale = 1.0f / max(0.0001f, SmoothedExposure);
		
	OutColor = SmoothedExposureScale * ExposureOffsetMultipler;
}