Why Backplate too dark in Refractions/Reflections?

[JulianT_CD]

Hi!

I'm trying to render an agricultural machine over a backplate, all straight in 3D for preview purposes.

I placed 3 large planes around the machine orthogonal to each other and made them a matte object so they catch the background and can provide backplate reflections and refractions on the machine.

The machine has a driver's cabin with clear glass windows through which you can see backplate projected onto one of the planes.

I am also using the Physical Camera with a high EV + VRay Sun. Exposure control is set to not affect the background. However, while the backplate is at normal/unaffected intensity for primary rays, the backplate visible through the refractions of the cabin glass is WAY too dark. Same goes for the reflections.

I expected the backplate to appear at normal intensity when seen through the clear cabin glass. But it seems like I am seeing the backplate affected by exposure control through refractions. Why is this happening? Or what have I done wrong?

I can't post any images due to strict NDA.

[svetlozar.draganov]

Can you replicate that behavior on a sample scene and send it for investigation?
Thank you very much in advance.

[Art48]

1) Max Ray intensity set too low?
2) Refraction override map or other map in background environment slot?

Just some pointers maybe.

[JulianT_CD]

Can you replicate that behavior on a sample scene and send it for investigation?
Thank you very much in advance.

Sure, I uploaded a simple test scene + maps to our servers:

Link

Notice how the reflections in the chrome sphere and the refractions in the clear glass are way too dark as compared to the backplate for the primary rays. During the Lightcache Prepass you can see that the backplate seems to be affected by the Physical Camera Exposure Control even though I unchecked "Process Background and Environment Maps".

[svetlozar.draganov]

Thanks for the scene.
The difference comes because Physical Camera exposure Control is not applied to the background image visible directly through the camera but it is applied when seen through refractive object.
You could unify the result if you activate Affect Background checkbox under the Color Mapping rollout:

[JulianT_CD]

Ok thanks, i considered that solution, too, but I had hoped that there was a way to simply tell VRay to not affect the background by exposure control in reflections/refractions. Anyhow, that's good enough!

Btw, what's the difference between "Affect background" in "Color Mapping" and "Process background and environment" in the 3ds max Exposure Control settings?

[svetlozar.draganov]

There is no difference between both, they basically does the same thing but Affect Background option is with higher priority.