Description :A Skylight (3D) procedural texture,
Based on Greeneggs' Renderman shader.
Original code 'A Practical Analytic Model for Daylight'
from A. J. Preetham, Peter Shirley and Brian Smits.
Usage :-Select or Add a Distant light, add a "Sun Spot" motion modifier for sun realistic settings (keep default 1 km or more as distance).
-Apply as procedural texture to color channel of a mesh skydome,
set diffusion to 0%, and luminosity to 100%.
-Or Apply as procedural in the Texture panel of "Texture Environnement" plugin.
-Select above sun light. -Last five parameters let you alterate the original algorithm.
-Turn on Radiosity and at least "Backdrop Only" in Global Illumination Panel.
-Ground "Edge" option is for extending sky texture below horizon line. -Skylight plugin takes data from light rotation only.
In addition a color/intensity channel modifier could be applied to the Sun light
with Suncolor plugin.