Global illumination improves realism by simulating the way sunlight interacts with surfaces and objects in the scene. This is especially useful for areas in shadow that are not directly illuminated by the sun.
Global illumination uses a rendering technique called "ray tracing." It does this by casting millions of virtual light rays into the scene to determine how surfaces are illuminated, either by direct sunlight or by light reflecting off other surfaces.
However, these calculations are very time-consuming, even with a fast video card. A complex scene can take anywhere from several seconds to over a minute or more to render each frame when creating a movie or taking a screenshot. Therefore, we recommend leaving global illumination disabled until you are ready to produce your final renders.
Tip: You can preview global illumination while designing by enabling it in the Graphics Settings. When enabled, the global illumination will be calculated as soon as your viewpoint remains stationary for over one second. The result will fade in as the illumination is calculated. As soon as you move the viewpoint, the results will be removed until the next time you stop moving.
The following are examples with and without global illumination.
Without global illumination:
With global illumination:
