To do physically-correct shading, one must perform a global calculation to see how light bounces (possibly multiple times) off of surfaces.  This would include:

• All the light sources
• All the objects and their materials

Real lighting requires that objects be lit not only by light sources but by objects themselves – both the same object and other objects.  These operations tend to be recursive and computationally expensive.

In real-time computer games, speed is of the utmost importance.

Instead, OpenGL processes each primitive independently, or locally.  As a consequence, objects are lit only by light sources – not by light from other objects. 

This is a highly simplified model compared the complexity of the real world.  However, it is suitable for hardware implementation and produces compelling shading for games.

High