You could do it in-place with a memory cost of one pixel row (for 3x3, or two for 5x5) plus one (or 2) pixels. That might make be worthwhile depending on where

You could do it in-place with a memory cost of one pixel row (for 3x3, or two for 5x5) plus one (or 2) pixels. That might make be worthwhile depending on where you're actually doing it. It might be more efficient - it's worth implementing both to see.