Handling alpha channel in WPF pixel shader effect

Question

Is there something unusual about how the alpha component is handled in a pixel shader? I have a WPF application for which my artist is giving me grayscale images to use as backgrounds, and the application colorizes those images according to the current state. So I wrote a pixel shader (using the WPF Pixel Shader Effects Library infrastructure) to use as an effect on an Image element. The shader takes a color as a parameter, which it converts to HSL so it can manipulate brightness. Then for each grey pixel, it computes a color whose brightness is interpolated between the color parameter and white in proportion to the brightness of the source pixel.

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

    // ...Do messy computation involving src brightness and color parameter...

    float4 dst;
    dst.r = ...
    dst.g = ...
    dst.b = ...
    dst.a = src.a;
    return dst;
}

This works just fine on the pixels where alpha = 1. But where alpha = 0, the resultant pixels come out white, rather than having the window's background show through. So I made a tiny change:

float4 main(float2 uv : TEXCOORD) : COLOR
{
    float4 src = tex2D(implicitInputSampler, uv);
    if (src.a == 0) 
        return src;
    ...

and now the transparent parts really are transparent. Why? Why didn't the dst.a = src.a statement in the first version accomplish that? Unfortunately, even this is only a partial fix, because it looks to me like the pixels with 0 < alpha < 1 are coming out white.

Does anyone know what I'm not understanding about alpha?

Answer 1

After some more web searching, I discovered the piece I was missing.

According to an article on MSDN: "WPF uses pre-multiplied alpha everywhere internally for a number of performance reasons, so that's also the way we interpret the color values in the custom pixel shader."

So the fix turns out to be to throw in a multiplication by alpha:

float4 main(float2 uv : TEXCOORD) : COLOR
{
    ...
    dst.rgb *= src.a;
    return dst;
}

And now my output looks as I expect it to.