---
name: ferret-physics-fx-tuning
description: Use to draw physics results and keep them affordable in Ferret: glow of hot parts, water meshes, particle liquids, lamp lights (physics-fx); tiers, the Governor that gives up detail, sub-steps and step rates, and what to switch off first when the frame is over budget.
---

# Physics look and budget

## When to use
Making heat glow or water render; a frame over budget; choosing a tier.

## The model
Capabilities report numbers and own no look. `physics-fx` turns them into a picture: `glow(scene, mesh, () => part.T)`, `blackbody`, `incandescence` (nothing is visible below about 800 K), `waterMesh`, `fluidView`, `lampLight`. Cost: it grows with how often a model steps and how finely it sub-steps. The **tier table** sets budgets; the `Governor` watches the step time and gives up detail in the order of its rules, taking it back when there is room.

## Steps
1. Bind glow and lights to the state objects; give meshes a Standard or PBR material first.
2. Slow models do not need 60 Hz: heat 2–10 Hz, liquids 10–20 Hz, reactor coast 1 Hz; step from an accumulator and pass the true interval.
3. Raise `maxSubstep` (or `maxStep`) for coasts; keep fine sub-steps while something fast happens.
4. When over budget remove in order: fx particles, step rates of slow models, sub-steps, off-screen simulation. Never the rigid-body step first.
5. Read `physics.info`; add `DegradeRule`s if the defaults do not fit.

## Check afterwards
Step time in `physics.info` stays under budget on the target tier; behaviour (not looks) is unchanged after degrading.

## What usually goes wrong
- `mesh "x" has no material to glow`.
- Governor never acts: no rule applies, or the step is under budget.
- A rule has no effect: overridden at a narrower scope (the governor sets world scope).
- A room the player left keeps stepping: stop it, and advance by the elapsed time on return.

## Files

- `reference/fx.md` — Colour and glow of hot parts, a mesh that follows a water surface, a particle liquid drawn as instances, and a light that follows a lamp.
- `reference/budgets-and-tiers.md` — How much physics a device can afford: the tier table, the governor that gives up detail when the step runs long, custom degrade rules, and how to measure.
- `reference/api-physics-fx.md` — every export of `@ferret/physics-fx` with signature and doc comment
- `recipes/tuning.md` — Where the cost of the capabilities goes and which knob buys it back: sub-steps, step rates, off-screen sleep, and what to switch off first.