BansheeEngine/Data/Raw/Engine/Shaders/PPEyeAdaptation.bsl

#include "$ENGINE$\PPBase.bslinc"

Parameters =
{
	Texture2D 	gHistogramTex;
};

Technique =
{
	Language = "HLSL11";
	
	Pass =
	{
		Fragment =
		{
			#define NUM_BUCKETS (THREADGROUP_SIZE_X * THREADGROUP_SIZE_Y)
		
			cbuffer Input
			{
				// [0]: x - histogram scale, y - histogram offset, z - histogram percent low, w - histogram percent high
				// [1]: x - min adaptation, y - max adaptation, z - adaptation speed up, w - adaptation speed down
				// [2]: x - exposure scale, y - frame time delta, zw - nothing
				float4 gEyeAdaptationParams[3];
			}		
			
			Texture2D gHistogramTex;
			
			/** 
			 * Returns luminance of the histogram bucket.
			 *
			 * @param pos	Position of the histogram bucket in range [0, 1].
			 * @return		Luminance of the bucket.
			 */
			float calcHistogramLuminance(float pos)
			{
				return exp2((pos - gEyeAdaptationParams[0].y) / gEyeAdaptationParams[0].x);
			}	
			
			/**
			 * Returns value of the histogram bucket.
			 *
			 * @param	histogram	Texture containing the histogram buckets in the first row.
			 * @param	bucketIdx	Index of the bucket. Caller must ensure it is in valid range.
			 * @return				Value of the needed histogram bucket.
			 */
			float getHistogramValue(Texture2D histogram, uint bucketIdx)
			{
				uint texelIdx = bucketIdx / 4;
				
				float4 packedValue = histogram.Load(int3(texelIdx, 0, 0));
				float4 mask = float4(
					(bucketIdx % 4) == 0,
					(bucketIdx % 4) == 1,
					(bucketIdx % 4) == 2,
					(bucketIdx % 4) == 3);

				return dot(packedValue, mask);	
			}

			/** 
			 * Calculates the sum of all values in the histogram.
			 *
			 * @param	histogram	Texture containing the histogram buckets in the first row.
			 * @return				Sum of all the values in the histogram.
			 */
			float calcHistogramSum(Texture2D histogram)
			{
				float sum = 0;

				for(uint i = 0; i < NUM_BUCKETS; i++)
					sum += getHistogramValue(histogram, i);

				return sum;
			}	

			/**
			 * Calculates the average luminance in the histogram, while ignoring the outlier values that may skew the result.
			 *
			 * @param	histogram	Texture containing the histogram buckets in the first row.
			 * @param	low			Sum below which to ignore values (removing lower end outliers), in range [0, histogramSum].
			 * @param	high		Sum above which to ignore values (removing higher end outliers), in range [0, histogramSum]. 
			 *                      Must be higher than @low.
			 * @return				Average luminance in the histogram.
			 */
			float calcHistogramAverageLuminance(Texture2D histogram, float low, float high)
			{
				float2 sumAndWeight = float2(0.0f, 0.0f);

				for(uint i = 0; i < NUM_BUCKETS; i++)
				{
					float value = getHistogramValue(histogram, i);

					// Ignore any values below the @low parameter, and then shift the valid range
					// by the amount we ignored. Eventually the low end of the range reaches zero
					// and values are no longer ignored.
					float offset = min(value, low);
					value = value - offset;
					low -= offset;
					high -= offset;

					// Ignore any values above the @high parameter, and then shift the valid range.
					value = min(value, high);
					high -= value;

					float histogramPos = i / (float)NUM_BUCKETS;
					float luminance = calcHistogramLuminance(histogramPos);
					
					sumAndWeight += float2(luminance, 1) * value;
				}
				
				return sumAndWeight.x / max(0.0001f, sumAndWeight.y);
			}
			
			/**
			 * Calculates the eye adaptation from the luminance in the provided histogram. Eye adaptation value will be 
			 * used for automatically scaling expsure based on scene brightness.
			 *
			 * @param	histogram	Texture containing the histogram buckets in the first row.
			 * @return				Ideal eye adaptation value for the provided luminance.
			 */
			float calcEyeAdaptation(Texture2D histogram)
			{
				float sum = calcHistogramSum(histogram);
				float lowRange = gEyeAdaptationParams[0].z * sum;
				float highRange = gEyeAdaptationParams[0].w * sum;
				
				float avgLuminance = calcHistogramAverageLuminance(histogram, lowRange, highRange);
				avgLuminance = clamp(avgLuminance, gEyeAdaptationParams[1].x, gEyeAdaptationParams[1].y);

				return avgLuminance;
			}
			
			/** 
			 * Smooths out eye adaptation changes over multiple frames so they aren't as jarring.
			 *
			 * @param	old			Eye adaptation value from the previous frame.
			 * @param	target		Ideal eye adaptation value for this frame.
			 * @param	frameDelta	Time difference between this and last frame, in seconds.
			 * @return				Smoothed eye adaptation.
			 */
			float smoothEyeAdaptation(float old, float target, float frameDelta)
			{
				float diff = target - old;

				float speedUp = gEyeAdaptationParams[1].z;
				float speedDown = gEyeAdaptationParams[1].w;

				float adaptionSpeed = (diff > 0) ? speedUp : speedDown;
				float scale = 1.0f - exp2(-frameDelta * adaptionSpeed);

				return clamp(old + diff * scale, gEyeAdaptationParams[1].x, gEyeAdaptationParams[1].y);
			}
			
			float4 main(VStoFS input) : SV_Target0
			{
				float exposureScale = gEyeAdaptationParams[2].x;
	
				float targetAdaptation = calcEyeAdaptation(gHistogramTex);
				float oldExposure = gHistogramTex.Load(int3(0, 1, 0)).x;
				float oldAdaptation = exposureScale / oldExposure; // Assuming same exposure scale as last frame
				float frameDelta = gEyeAdaptationParams[2].y;
				
				float smoothAdaptation = smoothEyeAdaptation(oldAdaptation, targetAdaptation, frameDelta);
				return exposureScale / smoothAdaptation; // Returns exposure
			}	
		};
	};
};

Technique =
{
	Language = "GLSL";
	
	Pass =
	{
		// TODO
	};
};