GI - GI means Global Illumination. Global illumination enhances the realism of a scene by simulating radiosity, or the interreflection of light in a scene. It generates such effects as "color bleeding," where for example, a white shirt next to a red wall will appear to have a slight red tint.

In this video tutorial (without voice) Aziz khan will try to explain how to acheive a realistic effects through Global Illumination and how to calculate through most widely used renders i.e Mental Ray, Vray.