o3de/Gems/Atom/Feature/Common/Assets/Shaders/PostProcessing/BloomDownsampleCS.azsl

/*
 * Copyright (c) Contributors to the Open 3D Engine Project.
 * For complete copyright and license terms please see the LICENSE at the root of this distribution.
 *
 * SPDX-License-Identifier: Apache-2.0 OR MIT
 *
 */

#include <Atom/Features/SrgSemantics.azsli>
#include <Atom/Features/ColorManagement/TransformColor.azsli>

ShaderResourceGroup PassSrg : SRG_PerPass
{
    Texture2D<float4>       m_sourceTexture;
    RWTexture2D<float4>     m_targetTexture;  

    RWTexture2D<float4>     m_targetMipLevel0;
    RWTexture2D<float4>     m_targetMipLevel1;
    RWTexture2D<float4>     m_targetMipLevel2;
    RWTexture2D<float4>     m_targetMipLevel3;
    RWTexture2D<float4>     m_targetMipLevel4;

    Sampler LinearSampler
    {
        AddressU = Clamp;
        AddressV = Clamp;
        MinFilter = Linear;
        MagFilter = Linear;
        MipFilter = Linear;
    };

    // 1.0 / source image size
    float2 m_sourceImageTexelSize;

    // Elements:
    // x -> threshold
    // y -> (knee - 1) * threshold
    // z -> 2 * knee * threshold
    // w -> 1 / (4 * threshold * knee + EPSILON(1e-5))
    float4 m_thresholdConstants;

    groupshared float3 smColor1[256];
    groupshared float3 smColor2[64];

    float3 Threshold(float3 color)
    {
        float luminance = CalculateLuminance(color, ColorSpaceId::ACEScg);
        float softMin = luminance + m_thresholdConstants.y;
        softMin = clamp(softMin, 0.0, m_thresholdConstants.z);
        softMin = softMin * softMin * m_thresholdConstants.w;

        float weight = max(softMin, luminance - m_thresholdConstants.x);
        weight /= max(luminance, 1e-5);
        return weight * color;
    }

}

// This function is used to reorder texels in the shared memory in a way of:
// -+---+---+---+---+-         
//  | 0 | 1 | 2 | 3 |           -+---+---+---+---+---+---+---+---+-
// -+---+---+---+---+-   ===>    | 0 | 1 | 4 | 5 | 2 | 3 | 6 | 7 |
//  | 4 | 5 | 6 | 7 |           -+---+---+---+---+---+---+---+---+-
// -+---+---+---+---+-        
// to place elements in each 2x2 block to 4 consecutvie slots
uint FlatID(uint2 xy, uint gridSize)
{
    uint rowOffset = gridSize * (xy.y & 0xFFFFFFFE);
    uint rowPos = (xy.y & 1) * 2 + xy.x + (xy.x &0xFFFFFFFE);
    return rowOffset + rowPos;
}

[numthreads(16, 16, 1)]
void MainCS(uint3 dID : SV_DispatchThreadID, uint3 gtID : SV_GroupThreadID)
{        
    // Group thread ID for downsampling level 2
    uint2 gtID2 = gtID.xy >> 1;
    uint2 gtID3 = gtID.xy >> 2;
    uint2 gtID4 = gtID.xy >> 3;

    // Uv of sampled point in the source image, offseted to the center of a 2x2 tap
    float2 uv = (dID.xy * 2 + 1) * PassSrg::m_sourceImageTexelSize;

    //------------------ Mip 0 (1 / 2 downscale, 16x16 tap for each thread group) -----------------
    // Apply threshold to extract bright pixels
    float3 colorDownsample1 = 
        PassSrg::Threshold(PassSrg::m_sourceTexture.SampleLevel(PassSrg::LinearSampler, uv, 0).rgb);

    // Cache 16x16 tap in each thread group for following downsampling steps
    PassSrg::smColor1[FlatID(gtID.xy, 16)] = colorDownsample1;    

    // Wait shared memory ready
    PassSrg::m_targetMipLevel0[dID.xy] = float4(colorDownsample1, 1.0);
    GroupMemoryBarrierWithGroupSync();
    //---------------------------------------------------------------------------------------------

    uint parity = dID.x | dID.y;
    //------------------ Mip 1 (1 / 4 downscale, 8x8 tap for each thread group) -------------------
    float3 colorDownsample2;
    if((parity & 1) == 0)
    {
        uint index = FlatID(gtID.xy, 16);
        colorDownsample2 = (colorDownsample1 + PassSrg::smColor1[index + 1] + 
                            PassSrg::smColor1[index + 2] + PassSrg::smColor1[index + 3]) * 0.25;
        PassSrg::smColor2[FlatID(gtID2, 8)] = colorDownsample2;
        PassSrg::m_targetMipLevel1[dID.xy >> 1] = float4(colorDownsample2, 1.0);
    }
    GroupMemoryBarrierWithGroupSync();
    //---------------------------------------------------------------------------------------------

    //------------------ Mip 2 (1 / 8 downscale, 4x4 tap for each thread group) -------------------
    float3 colorDownsample3;
    if((parity & 3) == 0)
    {
        uint index = FlatID(gtID2, 8);
        colorDownsample3 = (colorDownsample2 + PassSrg::smColor2[index + 1] + 
                            PassSrg::smColor2[index + 2] + PassSrg::smColor2[index + 3]) * 0.25;
        PassSrg::smColor1[FlatID(gtID3, 4)] = colorDownsample3;
        PassSrg::m_targetMipLevel2[dID.xy >> 2] = float4(colorDownsample3, 1.0);
    }
    GroupMemoryBarrierWithGroupSync();
    //---------------------------------------------------------------------------------------------

    //------------------ Mip 3 (1 / 16 downscale, 2x2 tap for each thread group) ------------------
    float3 colorDownsample4;
    if((parity & 7) == 0)
    {
        uint index = FlatID(gtID3, 4);
        colorDownsample4 = (colorDownsample3 + PassSrg::smColor1[index + 1] + 
                            PassSrg::smColor1[index + 2] + PassSrg::smColor1[index + 3]) * 0.25;
        PassSrg::smColor2[FlatID(gtID4, 2)] = colorDownsample4;
        PassSrg::m_targetMipLevel3[dID.xy >> 3] = float4(colorDownsample4, 1.0);
    }
    GroupMemoryBarrierWithGroupSync();
    //---------------------------------------------------------------------------------------------

    //------------------ Mip 4 (1 / 32 downscale, 1x1 tap for each thread group) ------------------
    if((gtID.x | gtID.y) == 0)
    {
        float3 colorDownsample5 = (colorDownsample4 + PassSrg::smColor2[1] + 
                                   PassSrg::smColor2[2] + PassSrg::smColor2[3]) * 0.25;
        PassSrg::m_targetMipLevel4[dID.xy >> 4] = float4(colorDownsample5, 1.0);
    }
    //---------------------------------------------------------------------------------------------
}