A riso printer makes a picture from two inks and a cheerful disregard for accuracy. There is no grey ink. There is paper, there is ink, and between them a dot: big where the picture is dark, small where it's light. Your eye does the mixing from across the room.

So a frame is split into two plates. The main ink takes the tone, shadows up to mid greys. The spot ink takes the warm light, strongest in the middle tones, and where it steps in the main ink steps back, which is why riso skin glows rather than muddies. Each plate goes through a screen of round dots, one tilted 15 degrees and the other 75, so the two grids don't fight. The spot plate sits 1.4 pixels up and to the right and drifts a little each beat, because a real plate never sits still either. Neither ink may go fully solid, and the ink's mottle and the paper's cloud come from a random field placed by a seed, so a frame prints the same way every time.

Detail sets the size of the dot. Then the catch: a dot under two pixels can't be drawn; it fights the pixel grid and comes out as moiré, the shimmer you get photographing a screen. So as dots shrink they give way to the flat tone the screen would make from across the room. That tone is not the coverage you asked for: a round dot carries less than its share of ink in the highlights and more in the shadows, so the screen's real curve is integrated once and kept in a table. A small preview looks like its large clip.

Two inks, a screen, a nudge, and the paper showing through. The trick is that your eye wants to be lied to from a distance.
