twosided lighting

whats the calculations for doing twosided lighting correctly?
i know its pretty easy but i spent an hour on it and didn’t really get anywhere

cheers zed

Well, the gl simply flips the normal and uses the back face material properties; otherwise, the base lighting calculations are the same.

cheers
as they say here in oz, ‘to easy!’
another case of me trying to over complicate things

got it working my problem was i had the idea in my head i had to do something with the light direction vector.
when i just needed to find out if the polygon is backfacing from the cameras viewpoint