Double Sided vray lights have wrong sampling cutoff on back side

[dlparisi]

I ran into a small problem with a vray light if it's double sided. The back side cutoff seems to have a problem with it's cutoff values:

Real simple scene: 3 planes and 1 double sided disc light with directionality of .8 and the standard cutoff of .001. I added some fog so you could see where the light is but the problem is present whether it's off or on. Weirdly the hard edge seems to become less pronounced as that plane gets closer to the light source. If you push the right plane back far enough the light actually no longer directly illuminates it at all. It also seems to get affected by the size of the light since only small lights seem to be affected (this is probably related to how close the light is to the plane). If I drop the cutoff down to 0, the problem fixes itself regardless.

Scene attached for easy testing. Max 2020 and Vray 6.2.20.02

[hermit.crab]

Thanks for the report. I've reproduced and logged the issue (internal bug-tracker id: VMAX-13935). It seems to be caused by small, but intense double-sided lights, where the second side is cutoff differently. For now, as a workaround, set the Cutoff value to 0.