Why the same seed produces the same video on any machine

The seed is the variation

Every simulation needs random numbers: where the ball is born, which way it goes, how the pegs are offset. Simbounce does not draw those numbers from the clock. It derives them from a single integer, the seed, through a generator that always produces the same sequence from the same value.

That is why the seed is on screen and editable. Changing the seed changes the variation; repeating the seed repeats the video. There is no third thing happening underneath.

The recipe is everything the video needs

A recipe is the trio of scene, parameters and seed, plus the theme and sound you chose. It is what a project saves and what the share link carries — not a video file, but the instruction to produce it.

It is also what makes the gallery on this site work: every video published on the channel comes with the recipe that produced it, and opening the recipe in the editor rebuilds that exact video, collision by collision.

Why it comes out the same on your computer and mine

The same seed is not enough: the physics has to give the same result on any processor. Simbounce uses a physics engine with cross-platform determinism and a fixed time step that never depends on the machine's clock. Chrome, Firefox and Safari reach the same state after the same number of steps, and that is verified on every code change against reference vectors — the same numbers shown at the bottom of the home page.

The preview and the export call exactly the same functions. The only difference is who drives the clock: the screen in the preview, the encoder in the export. That is why the frame you saw is the frame in the file.

When a video can change

One thing can change the result of an old recipe: a new engine version. When the physical behavior changes, the engine version goes up — it is shown in the site footer and in every saved project — and the editor warns when it opens a project made with another version. This is rare and deliberate; it never happens by accident.

Two things do not change the result: resolution and frame rate. The physics runs in world units, and resolution only decides the drawing scale. Exporting at another size is the same video with different sharpness.

What this makes possible

Trying hundreds of seeds without rendering any of them, because simulating without drawing is far cheaper than drawing — that is the editor's seed search. Re-exporting an old video at a higher resolution. Sending a link instead of a file. And publishing a video knowing anyone can reproduce it, which turns every video on the channel into a demonstration of the product.