Hey, nice file format! Will require a rewrite of my structure for sure, but it's nice haha :D
I'd like something to be added: a parameter width and a parameter height on the elements. This would be the pre-calculated dimensions of each key, with all the sprites inside it, with their scale and rotations in mind. This would help enormously if we're using a cached blitting technique in our renderers, as we won't have to determine those dimensions in-game, which would be a waste of resources.
Also, I'm not sure, but I'm assuming the folders is all the images used in the spriter right? How do they work exactly? Also, I don't think the image width and height is needed, we can retrieve this from the code easily anyway.
One last suggestion. The frame w and h properties are a bit confusing. Right now, I'd name them scaleX and scaleY (or sx and sy for short...), because it's what they represent, the scale, instead of width and height.
Thanks!
Abel.