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I posted a solution on Github https://github.com/lmccart/gswp5.js-code/issues in November 2015 and have not received a response. I'm posting it here in case you have trouble getting the code to work. My solution works. Don't know if it is an error in the code provided, or if p5.js is not working as intended. I put a push() after line 14 and a pop() after line 22.