A definite great read….

I write this shader (for ex., for difference):

// Performs “over”-type blending (colors must be premultiplied by alpha)
float4 blend(float4 overlying, float4 underlying)
{
float3 blended = overlying.rgb + ((1-overlying.a)*underlying.rgb);
float alpha = underlying.a + (1-underlying.a)*overlying.a;
return float4(blended, alpha);
}

// Performs advanced “over”-type blending
// (newcolor is the result of advanced blending (for ex, overlying.rgb * underlying.rgb))
float4 blend(float3 newcolor, float4 overlying, float4 underlying)
{
// Here the problem! (I think…)
float alpha = overlying.a * underlying.a;
float3 blended = newcolor*alpha + ((1-alpha)*overlying.rgb);

return blend(float4(blended, overlying.a), underlying);
}

// Input images
sampler2D underlyingSampler : register(s0);
sampler2D overlyingSampler : register(s1);
// Opacity of source image
float opacity : register(c0);

float4 main(float2 uv : TEXCOORD) : COLOR
{
float4 overlying = tex2D(overlyingSampler, uv);
float4 underlying = tex2D(underlyingSampler, uv);
overlying.a *= opacity;
overlying.rgb *= overlying.a;
underlying.rgb *= underlying.a;

float4 result = blend(abs(overlying.rgb – underlying.rgb), overlying, underlying);
return float4(result.rgb/result.a, result.a);
}

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Thanks for all.

I will like to know how I change in the fragment shader the HUE of a texture . and not modify the saturation and brithness

Thanks for all