Global illumination, also called GI for short, simulates the way light reflects off objects in the scene, providing more realistic-looking rendering at the cost of potentially reduced performance as the light reflections are calculated.
Global illumination can be set with the Global Illum Quality dropdown menu under Light Settings in the Scene Settings properties panel. This setting determines the quality of global illumination shown in the scene, with increasing detail at each level. Preview provides the lowest detail and the fastest build time for the scene, and Best shows the most detail at the cost of a very extended scene build time.
Global illumination quality settings.
Note: Activating global illumination can substantially extend the processing time required to build a scene.
Tips and best practices
Here are a few tips, tricks, and best practices to help you get the most out of global illumination in its current evolving state:
If light leaks through some of the walls in your scene, consider making your walls thicker.
If your scene seems overly bright, try adjusting Sky Brightness in the scene settings.
If you notice that the shadows in your scene that are lit by global illumination seem "noisy" or look like mosaic artifacts, you may need to consider setting Global Illum Quality higher.
Note: Increasing global illumination quality also substantially increases processing time when building the scene.