/sci/ - Science & Math

If anyone could help me with that it would be lovely, my problem goes the follow, find a relation for the distance D so that the intensity(or highlight?) of the specular light stay constant through f(x). I already know the normal vector r, c and n at all time. Also the position of the camera never change. I've already tried to play with this attenuation of light model: 1/(kc+kl*d+kq*d*d) but didn't found any relation with my current problem. If anyone can even just give me a hint, would be cool. Sorry for bad english.