How can I hide the interior frontfaces of a portal when viewed from the outside?

Question

I'm trying to follow This tutorial for a portal mask shader. It's an awesome effect but it comes with a couple of caveats. The shader as he designs it can only exist in a bubble, as:

1. The portal is visible through other objects (this I solved by finagling the queue order):

2. The illusion the portal creates of another space within is broken as soon as you enter:

As loading can't be perfectly instantaneous, this means when a player enters the portal, there will be a brief moment in which they are right back outside it, which I feel ruins immersion.

In order to compensate for this, I figured I would try to extrude the edges of the mask geometry, which works for the inside of the portal..

..but introduces the new problem of the front faces of the inside of the object being visible:

So what I'm hoping to achieve is a shader which:

• Occludes the PortalMask shader/object from the outside only
• Does not occlude other environment objects
• Is not itself visible

Am I asking the impossible?

Answer 1

A simple trick for this is to add a second shader pass, or second material that renders before the main one.

Switch the culling mode to cull front faces and draw back faces instead. Write to the depth buffer but leave the colour buffer unchanged.

This puts an invisible occluder in front of the front faces when viewed from the side, so they don't draw through it.

The downside with this is that it will also include any transparent effects drawn after the portal, like particles. It also only works from outside the portal volume. Inside, you'll see the front faces unoccluded, even if you walked in from the side rather than through the aperture.

I think a better solution is to keep the portal flat. Make a corridor out in empty space with your portal interior effect. As soon as the camera position crosses the portal, teleport the player into the corresponding position in the corridor, so that the player doesn't see any visible cut. (You can have an imposter of the room they just left in case they turn around, obscured by magical portal effects to disguise artifacts)