Fuel, heat and smoke

#The combustion model

The state pass runs once per simulation step, for every voxel. After the three fields are advected:

1. Inject. Emitters and bursts add fuel and smoke, and raise the temperature to at least their own temperature. Emitters ignite what they inject.
2. Burn. Fuel burns at params.burnRate, but only where the gas is hot enough. The ignition ramp runs from T = 0.3 to 0.55.
3. Release. Each unit of burnt fuel adds heat (×1.25), soot (×0.55 × params.smoke) and volume. The volume becomes the combustion expansion in the pressure solve (× params.expansion).
4. Cool and fade. Temperature loses 0.3·T + 0.28·T³ per second, scaled by params.cooling. Smoke and fuel dissipate slowly, and everything fades near the side and top walls, so nothing piles up at the edge of the box.

The fields are clamped to temperature 0–1.7, fuel 0–12 and smoke 0–6. Hot gas rises with 9.5 × params.buoyancy × T, and smoke weighs it down a little. Obstacle cells are always empty.

#The dials

These are the seven sliders in the demo. All are multipliers with a default of 1.

fire.setParams({ cooling: 1.4, buoyancy: 1.2 });   // merge
fire.params.smoke = 0.4;                           // params is live: mutate it directly

#Flame look

The raymarcher turns the fields into colour. None of these affect the simulation.

Hot soot is drawn thin. Smoke only shows at full density once it has cooled, so you don't get dark smoke inside the flame.

#The blackbody ramp

Flame colour comes from a lookup texture that integrates Planck's law from 380 to 780 nm against the CIE 1931 observer (the Wyman, Sloan and Shirley multi-lobe fit) and converts to linear sRGB. It covers 500 K to 4000 K. The fire light uses the same ramp, so the light always matches the flame. You can use it yourself:

import { blackbodyRGB, kelvinToColor } from 'naturegl-fire';

blackbodyRGB(1800);                            // [r, g, b, Y]: normalised linear sRGB + relative luminance
kelvinToColor(2400, lampLight.color);          // write straight into a THREE.Color

#Smoke lighting

The light-volume pass stores three things per voxel. They are the transmittance toward the key light, the shadowed and distance-attenuated fire light, and the transmittance straight up. The upward term self-shadows the ambient light, so the inside of thick smoke darkens instead of glowing flat.

The key light is a direction plus a colour, set with setLighting(). The demo points it at its moon. See In your scene.