On: Premultiplied alpha vs. not-premultiplied alpha blending and compositing

[im3w1l]

I think corysama had the right of it, you need to do all the operations in linear colors, i.e.

original image -> convert to linear color space -> multiply alpha -> blur -> composite -> possibly convert back to sRGB if needed

The wrong way I meant was

original image -> multiply alpha -> gamma-correct-blur -> composite

where gamma-correct-blur = convert to linear color space -> blur filter -> convert to sRGB