the hair fake occ code from
Hello,
It’s really easier, it’s really a big fake. I just used the “v direction” that drive the mix of root and tips color to drive a black and white mix along the hair, with a coefficient to drive the color along the hair.
So if you use a very furry creature, works very well because you not see the skin of the character and it’s really fast. But if you use low fur, the better result is to use PTC occlusion with a low shading rate, and a large maxisoangle.
Here the code I putted in my hair shader (the core of fake occ) :
float SHAVEopacity = mix( shaveOpaRoot, shaveOpaTips, (v * shaveOpaOffset));
color OverColor = mix(OverRoot, OverTips, v);
color fakeOcc = mix(baseOcc, tipsOcc, (v*spreadOcc)); // here the fake occ
mixed = mix( rootcolor, tipcolor, v);
It's a trick I explained on 3delight forum, and it's really easier, the more difficulty was just to have the idea 
Best regards,
Matthieu
PS : You can also use a map to control it but it's more difficult to drive it between uv space and vertex color space.