grain() sprinkles the picture with grain, new every frame unless told to hold still. amount is how strong it is, size how big one grain is and animated whether it moves.
size is in game pixels, not screen pixels: a grain of 1 is as big as one pixel of the art, so the noise belongs to the same picture instead of looking finer than everything else on screen.
↑ ↓ for the amount, ← → for the size, F to freeze it and Space to switch it off.