Ellaboration of BSDF "up-vector" for asymmetrical materials like angled veneitan blinds



Hi all

I have a question regarding the BSDF up-vector that I hope somebody might be able to help with -

My previous understanding was that the BSDF “up-vector” should be any vector that is not perfectly perpindicular to the window surface regardsless of the BSDF.

It also seems to be the conclussion from this post:

However reviewing the updated description for HB+

" An optional vector that sets the hemisphere that the BSDF material faces. For materials that are symmetrical about the HBSrf plane (like non-angled venitian blinds), this can be any vector that is not perfectly normal to the HBSrf. For asymmetrical materials like angled veneitan blinds, this variable should be coordinated with the direction the HBSrfs are facing. The default is set to (0.01, 0.01, 1.00), which should hopefully not be perpendicular to any typical HBSrf."

a question come to mind:

  1. For asymmetrical materials how excatly should the up-vector be “coordinated” with the direction of the HBsrf?

Thank you in advance

Best Tobias


That is indeed how it is.

The description in HB[+] docstrings is somewhat misleading. The direction of the up-vector relates to the window-opening (aperture) to which the BSDF is mapped. With regards to your example for angled venetian blinds, as long as the up-vector is not parallel to the direction of the aperture, it should be fine. A similar question had come up on the Radiance forum a few months ago: https://discourse.radiance-online.org/t/modified-three-phase-odd-radiation-levels/4751/7


Thank you for the clarification Sarith!