it's using line() to draw the dots. and opengl might optimise that out of existence if the line is zero-length (but i haven't tested this...)

in particle.pde display():
line (px,py,x,y);

try changing that to line(px, py, x + 1, y);


two invocations of INVERT filter will cancel each other out. but everything that happens between them will be affected. i think he just uses it to make the colour maths easier.

but, i think, filter works on the pixels. and you don't get access to the pixels when using opengl (not easily) as the pixels are in the memory on the card.

there is opengl code that'll do the same, just the not processing version of invert()

from http://www.allegro.cc/forums/thread/590930/664329#target

(which seems to be drawing a textured quad with an 'invert' blend function (ONE_MINUS_SRC_COLOR) over the entire screen)