Ellipse overlapped by another ellipse

are there any posibilities to code this in Codea? Imagine that you have ellipse whitch are not overlayed by anything else so you can see any part of this ellipse(pic.1 http://db.tt/JFRQwHd4). And the problem is when something overlayed it (pic.2) and you can see only for examle 75% of previus ellipse how to. calculate this %. Sorry for my english.Thanks

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