HDRI lighting and Glass

Question

I want to create something like picture 1 (credits to the Artists of Stellaris Game)

As you can see in picture 2 there is nothing to see because I want to take an image in front for the horizontal background scene.

But if I take a look from inside of this object through the glass front, the HDRI is visible (picture 3) and curved because of the IOR. I use the standard Glass BSDF shader.

In picture 4 you can see the nodes I use to hide the HDRI for render but not for lighting.

Why is it visible through the glass?

Answer 1

The HDRI is visible through the glass because the world background is not reached directly through a Camera Ray, but a Transmission Ray. So usually you would use the Is Transmission Ray as a mix factor.

In your case when you don't want to see the HDRI neither from the inside of the object through the glass nor outside the object, you can combine those two ray informations. Simply take a Math node set to Maximum and plug in Is Camera Ray and Is Transmission Ray. Since both outputs are either 0 or 1 you will get a camera ray = 1 from the outside and transmission ray = 1 from the inside.

By the way, this will leave the HDRI still be visible in reflections and glossy surfaces. So if you don't want that either, you can add more Math nodes set to Maximum and plug in the other Is [...] Ray outputs as well. But if you cancel out everything there won't be much left that is using the light of the HDRI.