# Move only the shadow, and the ball flies.

- Series: [How It's Clipped](https://miu.fm/howitsclipped.md)
- Published: 2026-10-08
- Page: https://miu.fm/howitsclipped/move-only-the-shadow-and-the-ball-flies

## Story

Move only the shadow, and the ball flies.

In 1986 Pixar made Luxo Jr., two desk lamps whose heads are the lights. The shadow method of the day (Lance Williams' depth map, 1978) drew jagged, stair-stepped edges. Bill Reeves, David Salesin and Robert Cook flipped two steps: ask "is this point in shadow?" many times around a spot, then average the answers. Fifty-five percent yes gives fifty-five percent shadow, and the edges go soft. To save time the floor was left invisible to the lights, because it never shadows anything. Their own paper admits the shadows weren't correct: the lamps had motion blur and their shadows didn't. It calls the method adequate.

A decade later, vision scientists showed why that was enough. With no shadow and no bounced light, people can't tell whether a box touches the floor. Move only a ball's shadow and people see the ball rise. A shadow doesn't need to be right. It needs to be there.

The ball, the boxes and the lamps here are our own recreations, not footage from the film.

Which movie trick should we take apart next?

Sources: Reeves, Salesin and Cook, "Rendering Antialiased Shadows with Depth Maps", SIGGRAPH 1987 (Pixar); Kersten, Mamassian and Knill, Perception 1997; Madison et al., Perception and Psychophysics 2001; Ostrovsky, Cavanagh and Sinha, Perception 2005.
Sounds: Apple iMovie sound effects.

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