How to clear the insides of a path? It's not a rectangle so I can use clearRect().

You can use the fill() command when defining your path. In conjunction with Canvas.globalCompositeOperation you can achieve an effect similar to clearing a path.

I'm not 100% sure what's going on with your drawing routine, so I can't give you the exact composite operations, but the HTML5 Canvas Cheat Sheet (which should be available by searching the nihilogic.dk site) should give you enough of an idea on how composite operations work.

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