A physics sandbox that runs in your terminal. Rigid bodies, n-body gravity, cloth, and fluid, projected in 3D onto braille cells with no GPU.

What it is

Inertia is a real-time 3D physics playground rendered entirely in text. There is no window, no GPU, and no web view: the whole scene is projected by hand onto ratatui's braille canvas, so it runs anywhere a terminal does, including over SSH, inside a container, or on a headless box.

It bundles four different simulations behind one camera and one UI:

• Rigid bodies (via rapier3d): spawn boxes and spheres, stack them, launch them, watch them bounce and settle.
• N-body gravity: stars in mutual attraction, with a brute-force or Barnes-Hut solver and fading orbit trails. Ships with the three-body figure-eight choreography.
• Cloth: a custom Verlet spring-mass mesh you can pin, drag, drape over rigid bodies, and blow around with wind.
• Fluid: an SPH particle fluid you can pour, splash, and drop rigid bodies into.

Run up to nine independent sandboxes side by side and switch between them live.

Install

From source:

git clone https://github.com/aclfe/inertia
cd inertia
cargo run --release

Or install the binary (published as inertia-tui, the command is inertia):

cargo install inertia-tui
inertia

A terminal with truecolor and mouse support gives the best experience. Pick a demo from the start menu, or start blank and build a scene yourself.

How it works

The scene is always 3D. Each frame transforms every body from world space through the camera and a perspective projection down to normalized device coordinates, corrects for the 2:1 height-to-width ratio of terminal cells, and maps the result onto the braille canvas. With no per-pixel depth buffer available, bodies are depth-sorted and drawn back to front (painter's algorithm).

Physics runs on a fixed 1/60s timestep decoupled from rendering, driven by an accumulator with a wall-clock catch-up budget: if a step gets too heavy to keep up (a large fluid pool on slow hardware), the simulation eases into slow motion instead of stalling input and rendering.

License

Dual-licensed under either of

• MIT license (LICENSE-MIT)
• Apache License 2.0 (LICENSE-APACHE)

at your option.