How to do smooth Alpha chanel keying with Silverlight 3 Pixel Shaders?

Question

How to do smooth Alpha channel keying with Silverlight 3 Pixel Shaders?

I want some HLSL filter (like this Shazzam HLSL example)

             sampler2D  implicitInputSampler : register(S0);


             float4 main(float2 uv : TEXCOORD) : COLOR
             {
               float4 color = tex2D( implicitInputSampler, uv );

             if( color.r + color.g + color.b < 1.9 ) {
             color.rgba = 0;
                 }

             return color;
             } 

to key not just the color I’m trying to key but for example if dark red consists of red and blue and I’m keying all blue i want to get transparent red. (Probably this picture can explain what do I want)
_{(source: narod.ru)}

Answer 1

Sounds like you just want to subtract a color rather then key it.

float4 subtract = ... ; // color you want to remove
float4 color = ... ;

color.r -= subtract.r;
... // for b and g

if ( color.r < 0 )
    color.r = 0;
... // for b and g

After this, you can then use a color chooser to pick which color "subtract" would be and remove it. Hope thats what your trying to do.