Desktop: hit C to clear, S to save an image or R to restart.
Touchscreen: tap with two fingers on top (clear), bottom (save) or top-left (restart).
Read full post for some images and extra modes.
To change drawing mode on desktop, use number keys 1,2 or 3.
On touchscreen, to cycle through modes tap with two fingers in the center of the screen.
Here’s what each mode does:
1: Creates curves only towards the closest wall to initial tap;
2: Curves are created towards two of the closest walls;
3: Allows drawing lines directly (adds to 2);
If you stop drawing for a few seconds the animation restarts and the mode is reset to default (2). Have fun!