When dealing with fog, the z distance used to compute the fog density. However, which z?

Turns out, the z depth used is the z depth that is computed after applying the GL_MODELVIEW matrix but before applying the GL_PROJECTION matrix. One result of this is that through the careful manipulation of both matrixes you can create the effect of fog on the valley floor below the viewpoint: fog doesn’t just have to be haze in the distance.