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.

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