Implement support for layers.

[bcampbell]

Aim is to use OpenRaster as our 'native' file format. There's a lot of ambiguity surrounding how to support animations in openraster. Seems to be an extension proposal but nothing too concrete. It'd be good to find if there are any existing apps that support animation in openraster files, and if there's any consensus building on how to do it...

[caiwan]

This is a good idea. Are muliple layers will be animated? Otherwise, I'd store the animation frames in separate layers; but somehow has to indicate that is is a multilayer animation, or not. The dirty way ia can come up, is store these informations in a separate file - like a human-readable/editable text. (json?) This could be usable even if having multipla layers, which are animated. Photoshop' approach for animation is storing the layer states on keyframes. The animation is done by switching the visibility of layers. I don't think we'll have to have it.

Soon I'll have more freetime, I'll take a closer look on the recent code.

[bcampbell]

I've got layers pretty clear in my head now I think... it's the next major thing I want to tackle, and it makes a lot of other stuff much easier. A partial brain dump of what I've got in mind:

• each layer can be animated or static (so you could have a static background layer, with animated layers on top)
• layers can all be different sizes and modes (paletted/rgb/rgba, non-square pixels, special palette restrictions etc...)
• openraster handles layers (it's a zip file with a png file for each layer + metadata in XML). Easy to extend to animation, but need to see if there is any standardised way.
• non-layered file formats saved with layers flattened into one
• paletted layers can either share a single global palette, or have a palette per frame.
• timing can be constant (eg "30fps") or set on a per-frame basis. (unsure if per-frame means different layers can have different key times or not...)