Weizhen Huang
6f8011edf7
Implements the paper [A Microfacet-based Hair Scattering Model](https://onlinelibrary.wiley.com/doi/full/10.1111/cgf.14588) by Weizhen Huang, Matthias B. Hullin and Johannes Hanika. ### Features: - This is a far-field model, as opposed to the previous near-field Principled Hair BSDF model. The hair is expected to be less noisy, but lower roughness values takes longer to render due to numerical integration along the hair width. The hair also appears to be flat when viewed up-close. - The longitudinal width of the scattering lobe differs along the azimuth, providing a higher contrast compared to the evenly spread scattering in the near-field Principled Hair BSDF model. For a more detailed comparison, please refer to the original paper. - Supports elliptical cross-sections, adding more realism as human hairs are usually elliptical. The orientation of the cross-section is aligned with the curve normal, which can be adjusted using geometry nodes. Default is minimal twist. During sampling, light rays that hit outside the hair width will continue propogating as if the material is transparent. - There is non-physical modulation factors for the first three lobes (Reflection, Transmission, Secondary Reflection). ### Missing: - A good default for cross-section orientation. There was an attempt (9039f76928) to default the orientation to align with the curve normal in the mathematical sense, but the stability (when animated) is unclear and it would be a hassle to generalise to all curve types. After the model is in main, we could experiment with the geometry nodes team to see what works the best as a default. Co-authored-by: Lukas Stockner <lukas.stockner@freenet.de> Pull Request: https://projects.blender.org/blender/blender/pulls/105600
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