Reflection of a hidden wall.

[bartholdsen]

I want to take a shot through the wall and get the reflection of the wall in the mirror. But I can not see through the wall without removing it. How can I make the wall invisible to see through but still get the reflection in the mirror?

[LayC42]

Perhaps move the wall a bit behind the camera position. The geometry editor will help.
Or render the wall (opposite direction of the camera angle) and use the image as a 'label' on the mirror surface?!

[bdesign]

Here is a node network which with which you can toggle object, shadow, and reflection visibility. The various states of object visibility are controlled by the "Output From" and "Output To" values of a Color To Number node, which has a Surface Backside Mask node as its Input. Esben Oxholm made a post recently with a similar method for achieving basically the opposite of what you are trying to do. As with his technique, you will need to enable Experimental Features for this one as well (Edit>Preferences>Advanced>Experimental Features), in order to access the Backside Mask node under Utilities in the Material Graph.

The basic flow is this: Surface Backside Mask>(Input)Color To Number>(Opacity)Emissive material>(Label)Main material>(Label)Emissive material.

Both Emissive materials have all options unchecked. The downstream Emissive is used as the shadow toggle (Visible In Shadows checked or unchecked).
For your situation (frontface visibility off, backface vis/reflection on), set the C2N "Output From" to 1, "Output To" to 0. Other combinations are:
All Off: 1/1; All On: 0/0; Object on/Reflection Off: 0/1. Below are some quick renders of a demonstration setup. I hope this is helpful.

Eric