I did some messing around with it... source-over doesn't seem to work right with Firefox for me. Used source-atop instead. http://beej.us/x/scratcher/ http://

I did some messing around with it... source-over doesn't seem to work right with Firefox for me. Used source-atop instead. http://beej.us/x/scratcher/ http://beej.us/x/scratcher/ (In that demo, the B&W image is pre-made--I don't do your cool in-canvas edge detection, but the drawing stuff should still apply.)