Downward facing surfaces stay black

[RRIS]

I've noticed this issue since forever on Keyshot and never figured out how to fix it. Surfaces on my models that face downwards stay black, no matter if I elevate them, turn on global illumination, ground illumination and up the bounces. (very easily tested by adding a cube, moving it upwards and observing the bottom face staying black). It's especially noticeable on diffuse materials..

Anybody here figured out if this is a bug, or because of something I'm not understanding properly? Is there a workaround?

[DriesV]

This happens for Product Mode with Ground Shadows active in the Environment tab.
If you turn off Ground Shadows, then bottom-facing diffuse surfaces will receive lighting.

Dries

[RRIS]

Ahh, so then if I do want ground shadows, I need to add a proper plane and let that receive shadows like any other object?
The downside is that I'd have to render the same image twice if I need to drop them into a background where I want visible ground shadows..

[DMerz III]

I may be reading your post wrong, but if you use the 'ground material' shader on your plane it will still give you the transparent ground shadow you want.

I could be misunderstanding what you're looking for, perhaps some screenshots for clarity?