distort() bulges, pinches or twists a round patch of the picture: a lens, a black hole, a whirlpool. Outside radius nothing moves, and the edge of the patch always meets the picture around it, so it can sit anywhere without a seam.
bulge goes from -1 (pinch) to 1 (bulge) and twist is how far the middle turns, in radians. The two mix: a bulge with a twist is a whirlpool. It reads the frame smoothly between pixels, which is what keeps a magnified patch from turning into steps.
Move the pointer to carry it, click to change mode, ↑ ↓ for its size and Space to switch it off.