OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Lighting Coordinate Systems

To make any sense, all the normal, direction, and position coordinates used in a lighting calculation must come from the same coordinate system. If light-position coordinates come after model-view transforms but before perspective projection, so should the surface coordinates that will be compared against them. In this typical case, both are in eye space. That is, the eye is at (0, 0, 0) looking in the negative z direction. This is a regular 3D-coordinate system, not the 4-component homogeneous space needed for perspective. (See the first block diagrams in Chapter 5, “Viewing Transformations, Clipping, and Feedback” to see where in the stack of transformations eye space resides.) This is why, in the examples above, ...