The latest version of this document can be found at www.broad.ology.org.uk/amiga/sketchblock/layerswindow.html
This window allows control of layers and will also incude brush selection and other features.
SketchBlock has a unique and powerful system for undoing and redoing previous image editing actions. At the top level each action can be undone step by step much as in any other application. This is achieved via these controls. Clicking the undo button (left) will undo one level of previously performed actions. (Actions might be painting, moving a layer, resizing the project etc). When action are performed by the user via the GUI each action typically constitutes one 'level'.
However script programmers can group actions into more meaningful groups so that all the actions performed by the script are undone at the same time. See the FixUndoLevel function for more information.
The twist with the SketchBlock system is that the undoable actions are grouped into sepaerate stacks for each object. Objects being layers or the project itself. And these actions can be undone using the gadgets below, out of sequence with the main undo stack.
At first glance this might seem confusing but it is in fact very powerful. Consider you have a two layer project. You paint something on the first layer, then do some more painting on the second. Having done that you realise that the first painting actions need to be undone and redrawn. With a traditional undo feature you would have to undo all the actions up to the earlier point losing the subsequent painting on the second layer. But with SketchBlock you can go the first layer and undo the action independantly. This allows for much greater flexibility in editing an image.
The latest version of this document can be found at www.broad.ology.org.uk/amiga/sketchblock/layerswindow.html