Can Gimp scripts/plugins add new layer modes?

Question

The title pretty much says it. What I really want is a layer mode that takes the alpha channel of the one below it and in all other respects behaves the same. The general question seems worth asking.

I'm skimming the docs, and it seems like layer modes are a fixed enum, but I wan't to be sure there isn't something I'm overlooking. I'll also take any alternative suggestions.

Thanks.

Answer 1

No - it is not possible to add new layer modes but for including your own modes inside GIMP source code.

However, layers are a bit more generic now, since they can be written as a GEGL operation - I'd have to check the source, but all that is needed is probably to write the proper GEGL operation (which is easy to derive from the other layer modes), and add the new operation to the enums. The big drawback of this approach as compared to plug-ins is that you can't share the layer mode with other GIMP users, and even worse: the XCF files you create with your custom mode will only be "readable" in your modified copy of GIMP.

An workaround is to write a plug-in that creates a new layer from two underlying layers, combining them as you like. You'd have to invoke it manually each time you updated each layer. You'd have to use Python-fu, instead of script-fu,a s the later does not give one access to pixel values.

For the simple case you describe, though, it seems like a sequence of "alpha-to-selection", "selection-to-channel", "copy", "add-layer-mask", "paste" can do what you want without a need to copy pixels around in a high level language.