// Sobel Edge Detector based from ATI Video shader 
  float sampleDist = 1.0f/512.0f;  // distance one pixel in u/v
  
  sampler2D image : register(s0);
  
  static const float2 offsets[8] = 
  {
          -1, -1,
           0, -1,
           1, -1,
           
          -1,  0,
          // 0,  0, // no center pixel
           1,  0,
           
          -1,  1,
           0,  1,
           1,  1,
  };
  
  float4 main(
          float4 Pos : POSITION,
          float2 texCoord: TEXCOORD0) : COLOR 
  {
  
      int i =0;
      float4 c = .5;
      float2 texCoords;
      float4 texSamples[8];
      float4 vertGradient;
      float4 horzGradient;
                  float4 color;
                  
                  
                  color = tex2D( image, texCoord );
                  
      for(i =0; i < 8; i++)
      {
          texCoords = texCoord + sampleDist * offsets[i];    // add sample offsets stored in c10-c17 (inclusive)
          // take sample
          texSamples[i] =  tex2D( image, texCoords); 
          
          //  convert to b&w
          texSamples[i] = dot(texSamples[i], .333333f);
      }
      // VERTICAL Gradient
      vertGradient = -(texSamples[0] + texSamples[5] + 2*texSamples[3]);
      vertGradient += (texSamples[2] + texSamples[7] + 2*texSamples[4]);
      
      // Horizontal Gradient
      horzGradient = -(texSamples[0] + texSamples[2] + 2*texSamples[1]);
      horzGradient += (texSamples[5] + texSamples[7] + 2*texSamples[6]);
  
      // we could approximate by adding the abs value..but we have the horse power
      c = 1 - sqrt( horzGradient*horzGradient + vertGradient*vertGradient );
                          
      if ( c.r > 0.4f )
              color = c;
      return color;
  }