Pretty much as the title says. Me and Davio were messing with canvases pasting effects into themselves to get positive feedback so that the effect feeds last frames result into itself each frame. I got the idea to try to hack mass particle simulation out of the technique. Wrote a few shaders and here it is. The fps will NOT change even if the entire screen is covered.
Draw with mouse button. Z,X,mousewheel up/down/click, left click, and right click all do stuff.
X+mousewheel adjusts the size of the "brush" you get when you press X
Mousewheel click draws "terrain" (blue)