Fallbacks

intermediate concepts

Understanding Fallbacks

Radiant GI is a screen-space effect — it can only use lighting information from objects visible on screen. To complement this with off-screen data, Radiant provides several fallback systems that supply indirect light when screen-space rays miss their targets.

You can enable any combination of fallbacks. The system automatically selects the best available data source for each pixel.

Important: When adding light from fallbacks, reduce Unity's Ambient Lighting value in the Environment settings to avoid double-counting ambient contribution. Ambient lighting should be replaced by screen-space GI + fallbacks, not added on top.

APV / Sky

Adaptive Probe Volumes store baked GI diffuse data throughout your scene. If your scene uses APV, enable this fallback to blend that baked data with the real-time screen-space GI. If APV is not present, this fallback adds sky/ambient contribution instead.

Aspect	Details
Performance cost	Medium
Best for	Scenes with baked APV data or when sky ambient is desired
Note	If APV is available, prefer this over Reflection Probes

Reuse Rays

When a ray misses in the current frame, this fallback checks whether a ray from the same screen position succeeded in a previous frame and reuses its result. This produces smoother GI composition at almost zero cost.

Aspect	Details
Performance cost	Very fast
Best for	All scenes — virtually free improvement
Note	Keep values moderate to avoid over-brightening the image

Reflection Probes

Uses cubemaps from nearby reflection probes when rays miss in screen space. Radiant automatically selects the 2 nearest probes to the camera. In Forward+/Deferred+ with probe atlas enabled in the URP asset, Radiant uses the probe atlas automatically.

Aspect	Details
Performance cost	Fast
Best for	Scenes that already use reflection probes
Tip	Place reflection probes in the center of rooms with bake mode set to Awake for a good balance of quality and cost. A real-time probe parented to the camera provides the most accurate result but is expensive.

Radiant uses each probe's Importance, Intensity, Blending Distance, and Box Size properties to weight the cubemap sampling.

Reflective Shadow Maps

Renders the scene from the directional light's point of view, capturing flux (emitted light), normals, and world positions in addition to the usual depth map. This data is used to compute light bounces that can reach surfaces outside the camera frustum.

Aspect	Details
Performance cost	Expensive when updating, medium when idle
Setup	Add the Radiant Shadow Map script to the main directional light
Update frequency	Only re-renders when the directional light rotates or the camera moves more than 5 meters

Selection Priority & Performance

When multiple fallbacks are enabled, the system picks the best available source per pixel:

Fallback	Priority	Cost
Reflection Probes (if available)	1 (highest)	Fast
Reflective Shadow Maps (if updated)	2	Medium–Expensive
APV / Sky	3	Medium
Reuse Rays	4 (lowest)	Very fast

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