I’ve searched the length and breadth of Google but not actually found any real help on using GL_EXT_texture_env_combine. Is there a book somewhere that goes beyond copying the (rather unhelpful) ascii extension specification?
Cas 
go to opengl.org and there to the extension section ( or go to nvidia.com/developer and search there… )
sorry, no time currently…
BTDT!
Looks like I will have to figure it out meself by chance & experiment!
I suppose I could put my experiments up on the web for all to see when I suss it.
Cas
see my site for an example
ATI has a nice demo on their website that illustrates how the GL_EXT_texture_env_combine works, probably worth checking it out.