HLSL: Redvision

/*
redvision by Bryce Kerley
makes it black and white except for areas with twice as much red
as both other channels combined
*/
sampler s0 : register(s0);
float4 p0 : register(c0);

#define width (p0[0])
#define height (p0[1])
#define counter (p0[2])
#define clock (p0[3])
#define threshold (0.0)

#define PI acos(-1)

float4 main(float2 tex : TEXCOORD0) : COLOR
{
    float4 c0;
    //the thresholds are in the float4() function
    if(dot(tex2D(s0,tex),float4(.5,-1,-1,0)) >threshold) {
         c0 = tex2D(s0,tex);
    }
    else {
    //black and white code yanked from the MPC grayscale filter
        c0 = dot(tex2D(s0, tex), float4(0.299, 0.587, 0.114, 0));
    }
    return c0;
}

This shader was made to use with Media Player Classic.