building on other sketches licensed under CC-BY-SA
Unless otherwise specified by their author(s), works built on Sketchpad are distributed under the Creative Commons Attribution-ShareAlike 3.0 Unported license. This means that Sketchpad sketches can be cloned and modified by others, both on Sketchpad and elsewhere. This also means that sketches can build on other CC-BY works, such as those built using other wonderful Processing websites, such as SketchPatch and OpenProcessing.
I built the sketch below based on a work published on openprocessing (rainbowdrops by gift was licensed under Creative Commons Attribution-Share Alike 3.0 license. Original work License). One interesting note here is that the original sketch was built for the Java implementation of Processing, so the first step was porting it to processing.js. This particular sketch was easy enough to do manually, but for more complex sketches, Florian Jenett's processing.js exporter would be quite helpful.
I liked the idea of pre-populating the canvas with some drawing, if only to give a sense of what the sketch might look like after some mouse interaction. I also chose to vary the opacity of the circles to give it a bit more depth. As with all exported sketches, you can observe the process that went into this modification, re-rendering sketch at each step along the way. Check it out!