Convolution filter outputs wrong color values

Question

I've just started to learn DirectCompute and for learning purposes, I want to make a simple convolution filter that blurs a texture. I've written the following compute shader to blur the texture:

Texture2D<float4> inputTex;
RWTexture2D<float4> outputTex;

[numthreads(32, 32, 1)]
void main( uint3 DTid : SV_DispatchThreadID )
{
    float4 totalPixVal = float4( 0.0f, 0.0f, 0.0f, 0.0f );
    for ( int y = -3; y < 3; ++y )
        for ( int x = -3; x < 3; ++x )
            totalPixVal += inputTex.Load( uint3(DTid.x + x, DTid.y + y, DTid.z) );

    totalPixVal = float4( totalPixVal.xyz / 49.0f, 1.0f );

    float4 inPix       = inputTex.Load( DTid );
    outputTex[DTid.xy] = totalPixVal;
}

Which I call with the following parameters:

DeviceContext->Dispatch( inputTexture->Width / 32, inputTexture->Height / 32, 1 );

This shader takes the average color value of the current pixel and its 48 neighbors, which causes a blurring effect.

Now the blurring part works fine, but the color that the shader outputs does not seem to be correct. To test the shader I'm using a simple image with a pure white shape on a black background. The output I expect is a blurred pure white shape on a black background, but the white shape isn't white anymore, it's grey. Now I've gone over my code several times and I cannot find where the problem is. Each texture has the same size of 512x512 pixels.

This is my input image:

Input image

And this is the output I get:

Output image

Any help would be greatly appreciated.

Thanks :D

Paul R Paul R · Accepted Answer · 2014-07-25T17:32:37

You're only summing 36 pixels, not 49. Change:

for ( int y = -3; y < 3; ++y )
    for ( int x = -3; x < 3; ++x )

to:

for ( int y = -3; y <= 3; ++y )
    for ( int x = -3; x <= 3; ++x )

Convolution filter outputs wrong color values

1 Answers