How to use pre-multiplied during image convolution to solve alpha bleed problem?

Question

i\'m trying to apply a box blur to an transparent image, and i\'m getting a \"dark halo\" around the edges.

Jerry Huxtable has a short mention of the problem, and a very

Answer 1

Following the advice in your link, you pre-multiply before blurring and un-pre-multiply after blurring. In your example, pre-multiplying actually does nothing, since there are no semi-transparent pixels. You did the blur, then you need you un-pre-multiply by doing (assuming normalized color values from 0 to 1):

RGB' = RGB/A  (if A is > 0)
A' = A

This will get you a non-pre-multiplied blurred final image.