Images are specified as a collection of tiles.
Header information specifies whether tiles can be assumed not to overlap (for performance). If overlap, then alpha compositing. Specify background color for areas not covered by any tile.
Tiles have arbitrary origin, dimensions, scanning direction.
Tiles could be composed hierarchically of smaller tiles. This allows a "putPixel" feature: a 1x1 tile at a given location. Also drawRectangle, using the background color.
Compression algorithms could work on each tile separately.
Overlapping tiles, with each tile the size of the whole image, is equivalent to layers.
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