First off, I think jerryjin brought up a good point, that the name "Layers" has a high potential for being misunderstood.
I think it's a matter of "This is a 3D program term" vs. "Layers means something else in 2D programs" vs. "Spine breaks a lot of conventions anyway. Might as well pick something sensible."
Layers is a dialog having a list of layer names, each with options like visible, selectable, etc. Items (bones, attachments, etc) are placed in the default layer. An item can be in one layer at a time.
This allows you to organize your items and easily control options for groups of items.
Anyway.
Here are some of the things I would do with Layers:
- I guess one obvious purpose of layers would be to easily segregate which things to animate when you need to play multiple animations on one skeleton at the same time. I'd definitely use Layers for that. Just eyes? Just pupils? Items that affect facial expression?
- Some bones are inherently just for secondary motion and inertia: Feathery things, parts of hair or cloth, loose hats. I would put those bones in a layer and make them invisible and unselectable so there's less clutter when I'm trying to establish the primary motion/animation of the major body parts first.
- Likewise, some bones and meshes aren't essential to the main movement, but instead serve to warp images around to make them look more 3D or for squash and stretch (with FFD or skinning). I would put these in a layer too and only make them selectable when I get to that step. This is a bit more of a gray area in terms of wanting them selectable/working on them together with the main movement since sometimes warping is part of the main movement. This is where it would be beneficial if there was a quick way to access the layers options (ie, not have to scroll around).
- I would think this is borderline hackish, abusive or nuts, but if I really need a skeleton to contort into a pose that requires a different set of bones and images mid-animation, would also probably use Layers to hide and show that different set of bones and then turn and key attachments on and off.
If Layers were made of mutually exclusive sets of items, this would render the previously mentioned use cases unusable.
Thinking about mutual exclusivity reminds me of that feature some people were asking for before: tags.
