Bilinear Sobel Filtering on The GPU

The Sobel filter traditionally requires sampling 8 pixels surrounding the center pixel. Leveraging the filter’s linearity, we can reduce this to 4 texture fetches by sampling between pixels.

Source Code

struct Sobel
{
   float4 Ix;
   float4 Iy;
};

Sobel GetSobel(sampler SampleImage, float2 Tex, float2 PixelSize)
{
   Sobel Output;
   float4 A = tex2D(SampleImage, Tex + (float2(-0.5, 0.5) * PixelSize));
   float4 B = tex2D(SampleImage, Tex + (float2(0.5, 0.5) * PixelSize));
   float4 C = tex2D(SampleImage, Tex + (float2(-0.5, -0.5) * PixelSize));
   float4 D = tex2D(SampleImage, Tex + (float2(0.5, -0.5) * PixelSize));
   Output.Ix = (B + D) - (A + C);
   Output.Iy = (A + B) - (C + D);
   return Output;
}