Creating a look through mask

Hi guys,

I’ve been looking for a way to do this for a while but keep drawing blanks so thought I’d try the forums. Hope I can explain this well enough! I want to create a portal effect very cheaply for use in VR. Because it’s stereo VR the “render to texture” option is too expensive so what I’m hoping for is something along the following lines.

I have a sphere with the normals flipped and a space texture applied so you can look in to it and see stars etc. I want to put a planar piece of geometry in front of this so that only the parts of the sphere seen through this are actually visible. so the effect is like looking through a door in to space. The sphere must only be visible through the planar “Door” so that if you walk around the door there’s nothing behind it!

I’ve tried the Sphere Mask approach but this relies on the sphere intersecting with the mesh where as I need a look through mask effect. The player pawn will never actually enter the portal but NPC’s will get pulled in to it (and disappear with it).

Any ideas on this? any help would be very, very welcomed!

Thanks,

Dave