What i say in the title is what i'm doing manually right now for 30 levels!
I'm building different color tshirt previews, every level represents a color and there's a level on top with the draw to be printed.
If I can do it automatically I'll have lot of time saved!
Anybody can help?
Use convert from the ImageMagick tools:
convert multilayer.xcf output.png
The result will be a separate PNG for each layer:
output-0.png
output-1.png
output-2.png
[…]
And: don't use JPG if you continue to print it on T-Shirts… JPG is lossy and doesn't support alpha-channels, which is important for most T-Shirt prints.