Radiant GI is a screen space effect. It provides instant global illumination, but it’s limited to the screen content. Non-visible objects (i.e. objects behind the camera or out of screen) can’t illuminate other objects in the scene. On the other hand, Radiant is 100% GPU based, requires no pre-computation, works with any kind of 3D scene including dynamic/procedural content and any surface or light, and it’s very easy to use.
To complement light data from off-screen objects, we’ve provided “fallbacks” methods. These fallbacks can be enabled in the Radiant GI inspector and are described here.
You can enable any number of fallbacks. Some are more expensive than others and the system will automatically pick the best one automatically.
APV / Sky
Adaptative Probe Volumes store baked Gi diffuse in your scene and Radiant can leverage that information. If your scene uses APV, enable this fallback to improve the results. Otherwise this fallback will add sky/ambient contribution to the scene.
Important: when adding lighting from this fallback or others, reduce Unity Ambient Lighting value in the General Settings to avoid double-counting ambient lighting (reason is that ambient lighting should be replaced by screen sapce GI + fallbacks, not just added).
Reuse Rays
This technique is very cheap and uses data from previous frames. When a ray misses a target during the main Radiant raytrace pass, it can look for rays from previous frames that were emitted from the same positions and reuse them. The result is a smoother GI composition. Don’t push this value too high to avoid burning the image.
Using Radiant GI with reflection probes
When using the reflection probes option in the Fallbacks section, Radiant GI will automatically use the cubemap rendered by nearby reflection probes when rays miss their target in screen space. Although this can’t be 100% accurate, it will improve the effect.
Radiant will automatically use the 2 nearest reflection probes to the camera. Of course, this technique requires that you use reflection probes in the scene. If you’re using forward+/deferred+ and probe atlas is enabled in the URP asset, Radiant will use the probe atlas automatically.
You can add a reflection probe as a child to the camera and make it real-time probe to provide the most accurate result possible although this will be quite expensive. Or just add reflection probes to the center of rooms with bake mode set to Awake.
Radiant GI will use the importance and intensity values of each reflection probes as well as the blending distance and boundaries defined by box size to determine the weight of the sampling of the probe cubemap texture.
Note: if APV are present, it’s preferable to use APV instead of reflection probes.
Using Reflective Shadow Maps
This feature can be enabled in the Fallback section of Radiant GI inspector. When you enable this option, make sure the script “Radiant Shadow Map” is added to the main directional light.
Reflective Shadow Maps is a technique used to compute global illumination for certain lights. It works like shadow maps, but instead of capturing only the depth map from the light point of view, it also captures the emitted light (also called flux), the normals and world positions of those pixels. Using this data, the technique computes the bounces of the light onto nearby pixels or surfaces generating a GI map which is later used during the main step of Radiant GI.
The benefit of this technique is that it can supply GI data to Radiant GI for objects that are not within the camera frustum.
For optimization purposes, Radiant GI only computes the buffers (flux, normal, world positions) of the reflective shadow maps when the directional light rotates or when the camera position changes at least 5 meters.
Selection Criteria and Performance
If reflection probes are available, the system will always use their data. If they’re not available, it will try to use data from the reflective shadow maps. And finally, if no reflection probes or reflective shadow map data is available, it will use the “Reuse Rays” fallback (only if these fallbacks are enabled of course, you can decide which fallbacks can use the system).
- APV/Sky: medium.
- Reflection Probes: fast.
- Reuse Rays: very fast.
- Reflective Shadow Maps: expensive when it’s updated, but medium normally (it’s updated only when directional light rotates or camera moves a certain distance so while not updating,
