Generator fails if there is no Background color set

[sk33z3r]

If the traits.py file has no background set, python tries to get the first image from a folder named layer00 which doesn't jive with the way we have things explained in the docs.

Either we have to be specific about the need to keep background color in traits, even if you don't want it; or we add logic that says "if Background Color" doesn't exist, start at layer01.

The preferable solution is the latter, so people that don't care about background and have one set in images already won't get confused.

[sk33z3r]

I just raise a more specific error as a temporary solution, and added a note to the comments of traits.example.py

[sk33z3r]

Thought of a better solution for this, tested and pushed. Now you can remove the background layer from the dict and it will adjust layers accordingly.

This does require the Background Color layer to be the first layer in the dict, still, but it's explained in the docs this should be the case any way.