Hi, sorry to wake up this old thread, but I have the same question,
We are creating a side scroller action/rpg game and we want our spine characters to be rendered according to the Y axis (bottom most apears in front).
We are currently using SpriteBatch and it works great but we need to manage the ordering ourselves. We read about decals and I understand it allows you to use hardware z buffer ordering which we could benefit from.
I've also noticed Spine is casting the batch I'm passing it to a PolygonSpriteBatch, and I don't understand how it differs from the regular SpriteBatch, also there doesn't seem to be an equivalent PolygonDecalBatch.
We'll also be glad to stick with our current solution or open to other solutions to solve our character ordering problem.. we are just new to libgdx and not really sure what's the best approach… (migrated from Corona SDK and not looking back 🙂)