Global illumination (GI)

Global illumination (GI) takes into account light that bounces between surfaces. Direct lighting only reaches surfaces the light source can see; GI carries reflected light into shaded corners, onto the ceiling, under the table. Real light always behaves this way, which is why a render without GI looks harsh and artificial.

What GI does to an image

• Daylight from a window gently filling the whole room
• Colour bleeding: a red rug giving a white wall a faint pink cast
• Natural darkening in corners and where objects meet

How GI is calculated

Path tracing computes GI directly by physically following the path of light; it is the most accurate method and the most expensive. Faster solutions approximate it with caching techniques such as lightmaps, light caches and irradiance maps. Real-time engines use practical screen-space or voxel-based methods.

In interior renders, the quality of GI is one of the biggest factors in how believable a scene feels. If you want a free option, Render Fun computes full GI with path tracing inside SketchUp.