Projection Mapping - transparent faces I'm trying to do as Ian Hubert does in his -now famous- blender speech at BCON19.

https://www.youtube.com/watch?v=whPWKecazgM

At 3.47, he 'import images as a planes' then extrude it.
I try to do the same, but Blender doesn't react the same at all.

When I extrude my wall photo, it just gets semi transparent in a strange way, as if it was hollow.
When I do an edge loop a part of the picture just disappears.

On the right the original pic, on the left the same plane that I extruded and loopcutted vertically in the middle.



What am I doing wrong?
Thanks for your help.
There are two things to prevent those transparent faces.


For Cycles - add some thicknes to the whole wall, before extruding individual parts
For Eevee  - Set material Blend Mode to Opaque


Cycles - add thickness before extruding. Just extrude the whole plane first.



Eevee - set blend mode to Opaque