I do a lot of traditional drawing, so one of the common tasks I perform in GIMP is cleaning up the background after scanning a character I've sketched on paper (erasing stray pencil flecks and so on). I've frequently used the Erase (with alpha channel) to perform this task, because it means I end up with a transparency that I can paint some manner of digital backsplash underneath.

My general process for doing this was: 1 - Duplicate the layer
2 - Perform a full Color to Alpha transformation on the lower layer (white as background) 3 - Use the Eraser on the upper layer, with a soft-edged brush, to clean up the background. Any mistakes during the process I can clean up using the Alt (anti-erase) modifier.

This yields a smoother foreground-background fade around the edges than simply the Eraser on a single layer. But recently I discovered the faster way of doing this task: The "Color erase" painting mode, which performs a Color-to-Alpha transition as part of any other drawing operation. So I don't need to worry about managing multiple layers anymore. However I do need a function to perform the inverse, so I can compensate for mistakes...

...and it dawns on me that the "Behind" paint mode is exactly what I'm looking for here. All I need to do now is set up a keyboard shortcut to toggle between the two modes and I'll be good to go.

Anyway, it should be noted in GIMP's online manual (http://docs.gimp.org/2.6/en/gimp-tools-paint.html#gimp-paint-mode-examples ) that the "Behind" and "Color Erase" blending modes are inverse functions of each other, with "color erase" transforming solid color to alpha and "behind" transforming alpha to solid color. (Should I file this on the bugtracker?)

-- Stratadrake strata_ranger@hotmail.com
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Numbers may not lie, but neither do they tell the whole truth.