Disney's two-legged BDX droids walk the way their animator, Jared Bishop, designed them. But "animation tools don't have physics built into them" (Moritz Bächer, Disney Research), and as a 2007 paper put it, there are "an infinite number of ways in which two characters might bump into each other." A hand can draw one stumble, not the answer to every push. So the team made the walk a target: in simulation, 8,192 copies were rewarded for "the close imitation of artist-specified kinematic reference motions, and also dynamic balancing" while being shoved at random, about two days on one graphics card. On the real robot, "it can determine that point for itself" (Ruben Grandia).

Our own walker shows the idea with a simple rule from 2007 (SIMBICON): each step, put the next foot where the body is falling. Switch the rule off and it falls within a few steps, push or no push. Switch it on and it catches what nobody drew: forty pushes, thirty-one catches, nine falls. Without the animator's walk, Disney's learner "rapidly shuffles the feet". As Morgan Pope put it, walking is one input; "the other important input is how it walks."

The walker, the pushes and the forty lanes are our own simulation; no footage or likeness of Disney's droids.

What would you push it with?

Sources: Ruben Grandia, Espen Knoop, Michael A. Hopkins, Georg Wiedebach, Jared Bishop, Steven Pickles, David Müller and Moritz Bächer, "Design and Control of a Bipedal Robotic Character", RSS 2024 (Disney Research, Walt Disney Imagineering R&D); IEEE Spectrum, 2023; KangKang Yin, Kevin Loken and Michiel van de Panne, "SIMBICON: Simple Biped Locomotion Control", SIGGRAPH 2007.
Sounds: our own, made in code (earshot, CC0).