If you need to load screen snapshot to DrawingCanvas, and then save DrawingCanvas image to disk, I think you need something like this:
Note, that you need another empty sprite (ExportSprite) to load CanvasSnapshot, which you then paste onto the DrawingCanvas. ExportSprite and CanvasSnapshot should be the same size (big enough to fit the screenshot) and placed at the same position.
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Also note, that DrawingCanvas actions "Save snapshot", "Load snapshot" etc. have nothing to do with the System "Take snapshot" action. They just happen to use the same word "snapshot".
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If you need to save the entire layout, which is larger than the screen size, you will need to repeat steps in event #2 multiple times. Each time scrolling to a new position, taking new snapshot, loading into ExportSprite and pasting onto the DrawingCanvas. Refer to that demo from R0J0hound I posted earlier.