I'm currently implementing support for Spine in my toy engine.
Is it possible to draw a complex character, like SpineBoy, using a single drawcall for each frame? Or is it expected that it will take multiple drawcalls to draw a frame of animation for such a character?
Extending this question to the more general case, can any animation produced by Spine be drawn in a single drawcall, or are there some animations that required the use multiple drawcalls?
As long as all the attachments of a skeleton are on the same texture atlas page, you can easily render a Spine skeleton via batching the triangles of individual attachments, and binding the texture atlas page texture for the draw call. You could even pack the atlases of multiple skeletons into a single texture atlas page and draw multiple different skeletons with a single draw call.
We have a batching implementation of our cocos2dx runtime which might be informative: spine-runtimes/SkeletonBatch.cpp at 3.7
