Great work so far. It's awesome to see how much momentum and interest this project is gathering.
Question and a comment.
If a sprite part is supposed to dissappear mid animation (or appear for only part of it - to put it another way) should this be accomplished with the alpha channel flipping 0%->100% mid animation? The only other way I could see this is Z-order but you can't take for granted that whatever is rendering won't have tearing or a slowdown in painting that would show what's underneath.
Regarding the file format and directory. I think the lack of support for packed spritesheets is problematic for some devices. Specifically the xbox360 - but every system benefits from lowering the file IO and having one large sheet to pick sprites from. I wrote a winforms application to extract the files and regenerate a spritesheet along with the mappings. I'd rather see support for the x/y within the format, at least as optional fields. I can see how the proposed format could be coherced into that using the other XY (size) values and the same image file. On that note, I don't see any attributes specified as optional unless I'm not reading this correctly.
