[rjmunro]

Not just the color, if you resize the window all calculations are totally thrown off. Try it with his live demo page: https://www.onsclom.net/dvd-logo

To avoid that, I'd use percentage positioning and have all widths and heights from 0-100. This would mean it would move faster horizontally than vertically in a wide window, but that's probably OK.

[account42]

Making the simulated window square would not be OK because that makes the period after which the simulation repeats / number of possible states much much smaller. A large part popularity of the DVD logo comes from waiting for it to hit a corner which in the original requires two modulo to line up exactly - whith your modulo that condition is either impossible or happens all the time because the logo only bounces between two corners.

[rjmunro]

Only if it's vertical speed and horizontal speed are the same. If you make it travel slightly faster vertically, it's going to look more equal when stretched to 16:9, and it's not going to follow identical paths.