// 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;
}