Physics Simulations,
Rendered As Video

A coefficient of restitution greater than 1.0 is not physical. It means a collision returns more kinetic energy than it received, which no real material does. The setting allows it anyway, and the reason is worth stating plainly: a physically accurate run is usually a boring one.

With restitution below 1.0 every simulation eventually settles. Energy leaves the system on each contact and the bodies come to rest. That is correct and it is also the exact shape of clip that loses viewers, because the most interesting part happens early and the tail is decay.

Setting restitution just above 1.0 inverts that curve. The run gains energy, collisions get faster, and the interesting part is the end rather than the beginning. If you need physical accuracy, keep the value under 1.0 and use the speed build-up controls instead, which escalate the run without breaking the collision model.

Discrete-time physics advances in steps. Between two steps a body moves in a straight line, and if that line is longer than the thickness of the surface it should have hit, the collision is simply missed. The body passes through a solid wall. This is tunnelling, and it is the most common way a simulation run visibly breaks.

The Maximum Speed Limit control exists for this. Clamping velocity so no body travels further per step than the thinnest collider is thick makes tunnelling impossible rather than unlikely. It is worth setting even when a run seems stable, because the failure only shows up once speed build-up has been running for a while, which is usually well into a take you thought was finished.

The related setting is Minimum Ball Size. Small fast bodies tunnel more readily than large fast ones simply because they clear a wall in fewer steps, so raising the floor on radius is a second lever on the same problem.

The solver executes in a web worker, separate from the thread that draws the interface. This is why adjusting a control during a heavy run does not stall, and why scenes with several thousand bodies remain usable rather than turning the page into a slideshow.

Recording adds a second consideration. Capturing at 4K and 60fps while simulating thousands of bodies is demanding on any machine, so larger exports can be handed off to cloud rendering instead. The run is reproduced from its configuration on a remote machine and the finished file comes back, which leaves your hardware free.