I'm interested in why Acorn ties the concept of a layer so tightly to a bmp or shape blob rather then segregating the two. It seems that having a library table that the layer table links to would allow for better reuse of images.

Library table <- pixel data

Layer table <- heirarchy/duplication of library items

Attributes table <- attributes of layers

Also, are the concept of Pages accounted for by the ordering of parentless layers, or does Acorn not support the notion of pages? Would that be its own table or a modification on the Layer table.