ComputeShader

//=============================================================================
// Performs a separable Guassian blur with a blur radius up to 5 pixels.
//=============================================================================

cbuffer cbSettings : register(b0)
{
    // We cannot have an array entry in a constant buffer that gets mapped onto
    // root constants, so list each element.

    int gBlurRadius;

    // Support up to 11 blur weights.
    float w0;
    float w1;
    float w2;
    float w3;
    float w4;
    float w5;
    float w6;
    float w7;
    float w8;
    float w9;
    float w10;
};

static const int gMaxBlurRadius = 5;


Texture2D gInput            : register(t0);
RWTexture2D<float4> gOutput : register(u0);

#define N 256
#define CacheSize (N + 2*gMaxBlurRadius)
groupshared float4 gCache[CacheSize];

[numthreads(N, 1, 1)]
void HorzBlurCS(int3 groupThreadID : SV_GroupThreadID,
                int3 dispatchThreadID : SV_DispatchThreadID)
{
    // Put in an array for each indexing.
    float weights[11] = { w0, w1, w2, w3, w4, w5, w6, w7, w8, w9, w10 };

    //
    // Fill local thread storage to reduce bandwidth.  To blur
    // N pixels, we will need to load N + 2*BlurRadius pixels
    // due to the blur radius.
    //

    // This thread group runs N threads.  To get the extra 2*BlurRadius pixels,
    // have 2*BlurRadius threads sample an extra pixel.
    if(groupThreadID.x < gBlurRadius)
    {
        // Clamp out of bound samples that occur at image borders.
        int x = max(dispatchThreadID.x - gBlurRadius, 0);
        gCache[groupThreadID.x] = gInput[int2(x, dispatchThreadID.y)];
    }
    if(groupThreadID.x >= N-gBlurRadius)
    {
        // Clamp out of bound samples that occur at image borders.
        int x = min(dispatchThreadID.x + gBlurRadius, gInput.Length.x-1);
        gCache[groupThreadID.x+2*gBlurRadius] = gInput[int2(x, dispatchThreadID.y)];
    }

    // Clamp out of bound samples that occur at image borders.
    gCache[groupThreadID.x+gBlurRadius] = gInput[min(dispatchThreadID.xy, gInput.Length.xy-1)];

    // Wait for all threads to finish.
    GroupMemoryBarrierWithGroupSync();

    //
    // Now blur each pixel.
    //

    float4 blurColor = float4(0, 0, 0, 0);

    for(int i = -gBlurRadius; i <= gBlurRadius; ++i)
    {
        int k = groupThreadID.x + gBlurRadius + i;

        blurColor += weights[i+gBlurRadius]*gCache[k];
    }

    gOutput[dispatchThreadID.xy] = blurColor;
}

[numthreads(1, N, 1)]
void VertBlurCS(int3 groupThreadID : SV_GroupThreadID,
                int3 dispatchThreadID : SV_DispatchThreadID)
{
    // Put in an array for each indexing.
    float weights[11] = { w0, w1, w2, w3, w4, w5, w6, w7, w8, w9, w10 };

    //
    // Fill local thread storage to reduce bandwidth.  To blur
    // N pixels, we will need to load N + 2*BlurRadius pixels
    // due to the blur radius.
    //

    // This thread group runs N threads.  To get the extra 2*BlurRadius pixels,
    // have 2*BlurRadius threads sample an extra pixel.
    if(groupThreadID.y < gBlurRadius)
    {
        // Clamp out of bound samples that occur at image borders.
        int y = max(dispatchThreadID.y - gBlurRadius, 0);
        gCache[groupThreadID.y] = gInput[int2(dispatchThreadID.x, y)];
    }
    if(groupThreadID.y >= N-gBlurRadius)
    {
        // Clamp out of bound samples that occur at image borders.
        int y = min(dispatchThreadID.y + gBlurRadius, gInput.Length.y-1);
        gCache[groupThreadID.y+2*gBlurRadius] = gInput[int2(dispatchThreadID.x, y)];
    }

    // Clamp out of bound samples that occur at image borders.
    gCache[groupThreadID.y+gBlurRadius] = gInput[min(dispatchThreadID.xy, gInput.Length.xy-1)];


    // Wait for all threads to finish.
    GroupMemoryBarrierWithGroupSync();

    //
    // Now blur each pixel.
    //

    float4 blurColor = float4(0, 0, 0, 0);

    for(int i = -gBlurRadius; i <= gBlurRadius; ++i)
    {
        int k = groupThreadID.y + gBlurRadius + i;

        blurColor += weights[i+gBlurRadius]*gCache[k];
    }

    gOutput[dispatchThreadID.xy] = blurColor;
}

Texture2D<float4> myTexture;
SamplerState linearClampSampler;

// Tips: 下面代码是错误的，computer shader 中无法自动计算mipmap level，必须手动指定
// float4 color = myTexture.Sample(linearClampSampler, uv);
float4 color = myTexture.SampleLevel(linearClampSampler, uv, 0);