Light probes and reflection probes

A light probe stores the light passing through one point in space, so that objects moving through that point can be lit to match the baked scene around them. A reflection probe captures what the surroundings look like from one point, usually as a cube map, and feeds it to shiny surfaces nearby. Unity's manual defines light probes as "a way to capture and use information about light that is passing through the empty space in your scene" (Unity Manual: Light Probes).

Probes are the reason a character walking from a sunny courtyard into a dark corridor darkens correctly in a game with baked global illumination.

#Light probes

Baked lighting stores light on static surfaces in lightmaps. Anything that moves has no lightmap. Unity's manual states the solution: "The primary use of light probes is to provide high quality lighting (including indirect bounced light) on moving objects in your scene" (Unity Manual: Light Probes). A second use is lighting static scenery that uses Unity's LOD system (Unity Manual: Light Probes); see level of detail.

Probes are placed in two main ways:

• By hand, per object. Unity's classic Light Probe Groups are placed manually, and each object samples them as a whole (High Definition Render Pipeline manual: Understanding Adaptive Probe Volumes).
• Automatically, per pixel. Unity's Adaptive Probe Volumes place probes "automatically based on the geometry density in your Scene" and sample them "per-pixel rather than per GameObject" (High Definition Render Pipeline manual: Understanding Adaptive Probe Volumes). Probes come in bricks of 64 (4 × 4 × 4), with default spacings of 1, 3, 9 or 27 m, and "each pixel of a GameObject samples lighting data from the eight closest Light Probes around it" (High Definition Render Pipeline manual: Understanding Adaptive Probe Volumes).

#Reflection probes

A reflection probe renders the scene around it into a cube map. Godot's documentation describes them as "a source of reflected and ambient light for objects inside their area of influence" (Godot Engine documentation: Reflection probes). Key settings, which exist in some form in every engine:

• Update mode. Godot's default, Once, "only renders the scene once every time the ReflectionProbe is moved"; Always "forces the probe to re-render everything around it every frame" and costs much more (Godot Engine documentation: Reflection probes).
• Box projection. A cube map assumes the reflected world is infinitely far away, which looks wrong in a room. Box projection corrects the parallax; Godot says it "has a small performance cost, but the quality increase is often worth it in box-shaped rooms" (Godot Engine documentation: Reflection probes).
• Interior. When enabled, "ambient lighting will not be sourced from the environment sky", which stops a bright sky leaking into rooms (Godot Engine documentation: Reflection probes).
• Blending and limits. Godot blends up to 4 probes at a location and fades to environment lighting at the edges. Its Mobile renderer applies at most 8 reflection probes per mesh, the Compatibility renderer 2 (Godot Engine documentation: Reflection probes).

#Common mistakes

• Probes inside geometry. Unity calls these "invalid probes" and offers Virtual Offset to push them out (High Definition Render Pipeline manual: Fix issues with Adaptive Probe Volumes). See light leaking.
• Every reflection probe set to update every frame. Use Once unless something in view really changes (Godot Engine documentation: Reflection probes).
• Unprojected cube maps in rooms. Reflections float; turn on box projection (Godot Engine documentation: Reflection probes).

Related: real-time lighting, lightmap UVs, physically based rendering.

#Further reading and watching