Paint something
The same brushes the generated painting uses, with your hand on them instead of a simulated one. Drag to paint. Each brush is a different way of putting paint down rather than a different setting — pour thins as you move faster and pools when you hesitate, fling lets the paint leave the stick partway and land as a droplet train, and the dry brush runs out as you drag it.
The canvas is the shape of One: Number 31, 1950, which is 269.5 × 530.8 cm.