One bounce of diffuse light

since 0.7.0 Light and environment

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A sky environment answers one question about indirect light: what does the sky look like from here. It says nothing about the room. A red wall lit by a white lamp should tint the white wall facing it, and an environment map has no way to know the red wall is even there. An irradiance field is a grid of probes that do know: each one is filled by casting rays out from it and recording what colour comes back.

Step 1: Lay out the grid #

IrradianceField divides a box into cells, with a probe at each corner. Two probes on every axis is the smallest field that has a cell at all.

_field = IrradianceField(
  origin: Vector3(-1.0, 0.5, -1.0),
  spacing: Vector3(1.0, 1.0, 1.0),
  countX: 2,
  countY: 2,
  countZ: 2,
);

Step 2: Fill it from the scene #

gather casts a few dozen rays from every probe, finds what each one hits, and asks that surface's own colour and the scene's lights what light comes back. One bounce: what a ray finds is a surface lit directly by a lamp, not a surface lit by another bounce. A room lit only by light that has already bounced twice stays dark.

gather(_field, scene, rays: 96);

This is a bake, not a per-frame cost. It runs once, when the scene is built, and the field is read many times after that.

Step 3: Turn it on and dial the strength #

A scene reads at most one field at a time, through Scene.irradianceField. Turning it off falls back to whatever ambient light the scene already had.

@override
void update(DemoContext context, double dt) {
  _scene.irradianceField = baked ? _field : null;
  _scene.ambientIntensity = ambient;
}

Note. The field only sees the room it was baked in. Move the wall after gathering and the probes still answer for where the wall used to be, because nothing here reruns automatically.

Toggle Field on and the room loses its red tint on the side facing the wall; drag Ambient strength and the tint scales with it, the same knob a flat ambient uses. That tint only has somewhere to show up because the floor beside the wall actually falls into shadow. The wall is one surface with nothing behind it, and a shadow pass that records only back faces sees straight through it. The sun records both sides by default since 0.8.2; the page names the setting anyway, because it is what the shadow depends on.

// A one-sided wall, not a closed brush. A pass that records only back
// faces sees straight through a plane like this one, so the floor beside
// the wall would never fall into shadow and the field's bounce light would
// have nothing but full sun to compete with. The sun records both sides by
// default since 0.8.2; named here because the shadow depends on it.
@override
RenderSettings settings(DemoContext context) => const RenderSettings(
  shadows: ShadowSettings(directionalCasterFaces: ShadowCasterFaces.both),
);