While doing homework for the coming #glTF workflow in #SecondLife I struggled with #channelpacking textures for manipulating how light reflects off of materials in a game engine. It’s easy in Photoshop, harder in GIMP.
There used to be a plug-in for extracting different kinds of “maps” from images, but it was easy to screw up. The manual method is tedious; you can’t just plop a texture into an RGB channel, you have to make each greyscale.
This open source tool might be helpful, and the aiaicapta.in is cited in the SL wiki as authoritative. The same site also has a good explanation of the pending PBR workflow, but for backwards compatibility, it’s good to know both.