Question about sampleM in HairBCSDF

[Immocat]

Hi, Benedikt(@tunabrain)

Thanks for your rendering resources and papers at first, which I used and learned a lot from. I am implementing your Disney Hair/Fur BSDF(BCSDF), and I found some difference between your implementation in tungsten and pbrt's implementation when sampling the longitudinal M.

As mentioned in an open issue of pbrt, your implementation and the paper Importance Sampling for Physically-based Hair Fiber Models both firstly tilt the angle, and then sampling the M. However, pbrt's version is sampling M and then tilting. I think that your implementation and the paper's way are correct.

Please let me know if my opinion is wrong. Thank you very much for your attention.

Zejian Wang