ImageMagick removes colors when alpha is 0 (png)

Question

I am converting many 3D textures with imagemagick for a video game. My source files are png, my target files are png, too. But I notice that whenever the alpha channel drops to 0.0 my color information are gone (and I need them). I just want to scale all channels as they are. I guess there is a small switch that fixes that problem, but the deadline is near and I cannot find anything about that.

Simple command to reproduce this: convert source-with-alpha.png -scale 2014 target.png (I also tried -resize and it also didn't work).

Doing just convert source-with-alpha.png target.png works fine though (but has no scaledown).

Thank you for your help.

Answer 1

I guess ImageMagick is trying to optimise something but not sure what/why. Maybe the idea is that if something is transparent you can't see it, so we might as well make it black so it compresses well.

Anyway, try separating the channels so they are all just treated as independent channels, then resizing and recombining:

convert input.png -channel RGBA -separate -resize XxY -combine result.png