Terragen 4 User Guide Part 2: Fundamental Rendering and Shader Concepts
This part of the User Guide explains some of the fundamental concepts used for rendering and by shaders in Terragen 2.
Global Illumination
This document from the Terragen 2 Reference explains global illumination (GI) and how it works in Terragen 2.
Translucency
This page explains the translucency effect available in several shaders.