0
148
ViewsBlend mode with alpha
I am trying to replicate blend modes. Currently I have recreated a screen blend mode, but it doesn't seem to work really well with transparent images.
I have two textures
Image 1 (Foreground)
Image 2 (Background)
And the result I get is this
The Compute Shader code
#pragma kernel Blend
RWTexture2D<float4> Result;
Texture2D<float4> TopLayer;
Texture2D<float4> BottomLayer;
[numthreads(8, 8, 1)]
void Blend(uint3 id : SV_DispatchThreadID)
{
if (TopLayer[id.xy].w == 0 && BottomLayer[id.xy].w > 0)
{
Result[id.xy] = BottomLayer[id.xy];
}
else if (BottomLayer[id.xy].w == 0 && TopLayer[id.xy].w > 0)
{
Result[id.xy] = TopLayer[id.xy];
}
else if (BottomLayer[id.xy].w == 0 && TopLayer[id.xy].w == 0)
{
Result[id.xy] = float4(0, 0, 0, 0);
}
else
{
float alpha_final = BottomLayer[id.xy].w + TopLayer[id.xy].w - BottomLayer[id.xy].w * TopLayer[id.xy].w;
float4 tex = 1 - (1 - TopLayer[id.xy]) * (1 - BottomLayer[id.xy]);
Result[id.xy] = float4(tex.x, tex.y, tex.z, alpha_final);
}
}
How do I blend images with different alphas properly?
1 answers
Answer question0
Blending with alpha channels requires weighting with the alpha value.
I hadn’t before thought of how to properly weight in the screen blend case, which is a multiplicative composition. I think it makes sense to do it like a weighted geometric mean: https://en.m.wikipedia.org/wiki/Weighted_geometric_mean): raise the multiplicands to the power of the corresponding weight (the alpha value), then raise the result to the power of the inverse of the sum of weights. Because we want to replicate the standard case when both alphas are 1:
1 - (1-a) (1-b)
which doesn’t have a square root in it, we will divide the sum of alpha values by 2:
1 - [ (1-a)wa (1-b)wb ] 2/(wa+wb)
The output alpha channel itself could be computed in the same way, or could be a simple maximum of the two alpha values. I don’t know what would look better.