This is a cloud demo Jeff Russell and I (Andres Reinot) wrote over the
It renders translucent clouds as sprites in openGL with ARB vertex and
Each sprite has a normal map and a transmissivity map, and the sprites
are sorted and alpha blended.
The complete lighting equation involves doing a diffuse calculation
with the surface of the cloud defined by a normalmap. To the diffuse
term we add the illumination of the light transmitted through the
cloud from any light sources behind it. This is approximated by the
dot product of the eye and light vectors, and modulated by the
transmissivity map of the sprite.
Illum = Transmissivity * ( -Eye . Light ) + diffuse
The diffuse term is calculated from a normal map in the standard
We used wrap lighting for the transmissive term, which maps the values
generated by the dot product from the range [-1,1] to [0,1]. This
results in the hilight "wrapping" around edges to emulate subsurface
scattering and helps the cloud look good when the light is at a 90
degree angle to the eye.
So the final equation with the scale and bias for the warp lighting ends up as:
Illum = Transmissivity * ( ( -Eye . Light ) * 0.5 + 0.5 ) + diffuse
with contributions from Bill Llib