− + ⌂

factor map. A square at (x, y) is black when y divides x evenly — that is, x ÷ y leaves no remainder. Otherwise it's white. Right = +x, up = +y. The center (0, 0) is black, and the whole middle row (y = 0) stays white because you can't divide by zero.
Drag to pan · scroll / pinch to zoom · click to recenter · double-click to zoom in.