# confused on isotropic BRDF

I am reading a paper and there is a part I don't understand:

I am wondering why the isotropic BRDF only depends on the view direction and roughness? Why is the light direction not related?

• I think that it does depend on it, it just does not depend on $\phi$. Where's the reference from? – lightxbulb Jan 31 at 14:00
• – TIANLUN ZHU Jan 31 at 20:47
• The GGX distribution term they are referring to doesn't take $l$ as argument. However you can see that they do have other terms that take $l$ into account. – lightxbulb Jan 31 at 21:39