Sandpile
Any square holding four grains or more gives one away to each of its four neighbours, and that is the whole rule. Drop a million grains on one square, watch the toppling spread outwards, then zoom in on the straight edges and the triangles nobody put there.
Board, pace and image size
A pile of n grains settles into a disc of radius about the square root of n over six and a half, so the board has to be wider than that or the edges eat the picture. Grains that fall off the edge are gone, which is what stops the board filling up.
A round topples every unstable square at once. The order never matters, which is the abelian part of the name, so a round is a legal shortcut for toppling them one at a time and lands on the same picture.
Written from the grid rather than off the screen, so every square lands on a whole number of pixels and no edge is blurred.