I have a quite simple need but cannot find the right way to do it.
I have a scene, and I only want to see what is inside the borders of an (imaginary) cube.
I tried to make a cube with a transparent material, and put a compositing tag on my scene. I switched off the "seen by camera" and switched on the "seen by transparency". The result is almost good, except that I can see what is behind the transparent cube when I only want to see what is inside the borders of my cube.