The Witcher 3 - average log luminance calculation 1.0

cbuffer cbPerFrame : register (b0)
{
 	float4 cb0_v0;      // xy - downscaled buffer size, zw - start/end params
}

RWStructuredBuffer<uint> g_buffer : register (u0);
RWTexture2D<float> g_avgLuminance : register (u1);

groupshared uint shared_data[256];
// There is one thread group has 64 threads and shared storage (shared_data)
[numthreads(64, 1, 1)]
void TestCS(uint3 GTid : SV_GroupThreadID)
{
    // GroupThreadID: Number of currently processed thread within a single thread group.
    // In this case values of threadID range [0-63]
    const uint threadID = GTid.x;

    // The first step is to set whole shared data with data from previous stage.
    // Because each thread group has 64 threads, each one can fill 4 elements in one thread
	// using a simple offset.
    [unroll] for (uint idx=0; idx < 4; idx++)
    {
        const uint offset = threadID + idx*64;

        shared_data[ offset ] = g_buffer[offset];
    }

    // We set a barrier here, which means we block execution of all threads in a group until all group
    // shared accesses have been completed and all threads in the group have reached this call.
    GroupMemoryBarrierWithGroupSync();

    // Perform calculations only with the thread with '0' index.
    [branch] if (threadID == 0)
    {
        // Total number of pixels in downscaled buffer
        uint fTotalPixels = cb0_v0.x * cb0_v0.y;

        // Start number of pixels we want to incorporate in average luminance calculation
        int pixelsToConsiderStart = fTotalPixels * cb0_v0.z;
        int pixelsToConsiderEnd =   fTotalPixels * cb0_v0.w;

        int pixelsMinusOne = fTotalPixels - 1;

        pixelsToConsiderStart = clamp( pixelsToConsiderStart, 0, pixelsMinusOne );
        pixelsToConsiderEnd =   clamp( pixelsToConsiderEnd, pixelsToConsiderStart, pixelsMinusOne );

        // Number of already processed pixels
        int numProcessedPixels = 0;

        // Luma cell [0-255]
        int lumaValue = 0;

        // Whether to continue execution of loop
        bool bExitLoop = false;

        // The purpose of the first loop is to omit "pixelsToConsiderStart" pixels.
        // We keep number of omitted pixels from previous cells and lumaValue to use in the next loop.
        //
        // For example:
        // * pixelsToConsiderStart = 33000
        // * In the first pass of loop, shared_data[0] has 37000 pixels, which exits the loop
        // The output from the loop are:
        // numProcessedPixels = 0 (33000 pixels should have been ommitted, but 4000 pixels must be taken into consideration)
        // lumaValue = 0 (because 4000 pixels left in shared_data[0], we will start calculating total luminance from
		// exactly that point)
        [loop]
        while (!bExitLoop)
        {
            // Get number of pixels with specific luma value.
            uint numPixels = shared_data[lumaValue];

            // Check how many pixels we would have with lumaValue
            int tempSum = numProcessedPixels + numPixels;

            // If more than pixelsToConsiderStart, exit the loop.
            // Therefore, we will start calculating luminance from lumaValue.
            // Simply speaking, pixelsToConsiderStart is number of "darken" pixels to omit before starting calculation.
            [flatten]
            if (tempSum > pixelsToConsiderStart)
            {
                bExitLoop = true;
            }
            else
            {
                numProcessedPixels = tempSum;
                lumaValue++;
            }
        }


        float finalAvgLuminance = 0.0f;

        // Number of omitted pixels in the first loop
        uint numProcessedPixelStart = numProcessedPixels;

        // The purpose of this loop is to calculate contribution of pixels and average luminance.
        // We start from point calculated in the previous loop, keeping number of omitted pixels and starting lumaValue positon.
        // We decode luma value from [0-255] range, multiply it by number of pixels which have this specific luma, and sum it up until
        // we process pixelsToConsiderEnd pixels.
        // After that, we divide total contribution by number of analyzed pixels.
        bExitLoop = false;
        [loop]
        while (!bExitLoop)
        {
            // Get number of pixels with specific luma value.
            uint numPixels = shared_data[lumaValue];

            // Add to all processed pixels
            numProcessedPixels += numPixels;

            // Currently processed luma, distributed in [0-255] range (uint)
            uint encodedLumaUint = lumaValue;

            // Number of pixels with currently processed luma
            float numberOfPixelsWithCurrentLuma = numPixels;

            // Currently processed, encoded [0-255] luma (float)
            float encodedLumaFloat = encodedLumaUint;


            // Reconstruct encodedLumaFloat by inversing encoding process from the first (distribution) pass,
            // which was:
            //
            // float luma = dot(hdrPixelColor, LUMA_RGB);
            // float outLuma;
            //
            // outLuma = luma + 1.0;        // because log(0) is undef and log(1) = 0
            // outLuma = log( outLuma )     // logarithmically distribute
            // outLuma = outLuma * 128      // scale by 128, which means log(1) * 128 = 0, log(2,71828) * 128 = 128,
											// log(7,38905) * 128 = 256

            // we start by adding half (we don't want to have zero)
            float fDecodedLuma = encodedLumaFloat + 0.5;

            // and decode luminance
            fDecodedLuma /= 128.0;              // Divide by 128
            fDecodedLuma = exp(fDecodedLuma);   // exp(x) which cancels log(x)
            fDecodedLuma -= 1.0;                // Subtract 1.0

            // Calculate contribution of this luma
            float fCurrentLumaContribution = numberOfPixelsWithCurrentLuma * fDecodedLuma;

            // (Temporary) contribution from all previous passes and current one.
            float tempTotalContribution = fCurrentLumaContribution + finalAvgLuminance;


            [flatten]
            if (numProcessedPixels > pixelsToConsiderEnd )
            {
                // to exit the loop
                bExitLoop = true;

                 // We already processed all pixels we wanted, so perform final division here.
                // Number of all processed pixels from user-selected start
                int diff = numProcessedPixels - numProcessedPixelStart;

                // Calculate final average luminance
                finalAvgLuminance = tempTotalContribution /  float(diff);
            }
            else
            {
                // Pass current contribution further and increase lumaValue
                finalAvgLuminance = tempTotalContribution;
                lumaValue++;
            }
        }

        // Save average luminance
        g_avgLuminance[uint2(0,0)] = finalAvgLuminance;
    }
}