Surface Backside Mask: HDRI vs. Physical Light

[cjwidd]

Surface backside mask requires color inversion to control opacity for physical lights, but not HDRI? (Keyshot 8.2.80)

In the attached image there is a ground plane with ground material applied to it and positioned between a light source and a model. The opacity channel of the ground material is being modified with a surface backside mask.

Screenshot (637) - physical light casts light onto underside of model using surface backside node to control opacity of ground plane
Screenshot (638) - identical set-up does not work for HDRI pin light aimed at underside of model
Screenshot (639) - expected behavior using [non-inverted] surface backside mask with HDRI pin light

[cjwidd]

Is this expected behavior?

[INNEO_MWo]

Do you tried to use surface backside mask on a solid body?


I've done a short test with a backdrop ramp (ground material and surface backside mask in the opacity channel) and one physical light source and an "one pin environment".
The ramp blocks the the HDRI light, while the physical light "streams" through the ramp. This can be very useful in several cases.


btw: the surface backside mask shows on one side, what's on the opposite side. So it can be used on solid bodies if the camera is inside of the body (like a room).

[cjwidd]

In my case, the surface backside mask *is* achieving what it is intended to do. My question is simply whether it is been implemented to affect HDRI vs. physical lights in the way that I have described.

Your response seems to indicate that, yes, it is designed in this way on purposes, because it allows for physical lights to be handled differently - it offers more control. Is that correct?

[INNEO_MWo]

Yes