Chapter fourteen. Rewriting History

MANY PROGRAMS HAVE AN UNDO FEATURE, and some programs provide unlimited undo as long as a document remains open. Photoshop turns undo into a resource by providing the History palette, which keeps a list of recent undo steps. Not only can you return the document to any of the undo steps (called history states), but you can also use any history state as a source of content that you can paint into any other history state, essentially letting you mix versions freely.