Video of Valve's Portal

Seeing as how you can probably place your portals anywhere, my guess is that it's not a VIS thing at all, but some render-to-texture type stuff. This way, there is no need to alter BSP/walls/geometry in realtime.

Notice in the video how when you place the first portal, you can't see through it until you place the second. The second acts like a camera that shows what the first one sees, and probably the other way around once you get the second placed.

It's probably no different than realtime video screens/cameras that you've seen in many different games, except you can now walk through them.

Shoot, Prey probably does it the same exact way.

Quote from TomWithTheWeather

Seeing as how you can probably place your portals anywhere, my guess is that it's not a VIS thing at all, but some render-to-texture type stuff. This way, there is no need to alter BSP/walls/geometry in realtime. Notice in the video how when you place the first portal, you can't see through it until you place the second. The second acts like a camera that shows what the first one sees, and probably the other way around once you get the second placed.

It's probably no different than realtime video screens/cameras that you've seen in many different games, except you can now walk through them.

Shoot, Prey probably does it the same exact way.

Now that's a better answer.

The camera/viewable portal functionality is already in Source. It just eats resources like Oprah in a cake shop. They probably just found a way to take that existing tech and make it smoother.

Yeah, a lot of engines have had very similar camera/portal tech. Duke3D is probably the earliest I can think of with cameras and Unreal Engine 1 with portals.

Someone has already made a portal mode for gmod. As Tom said, it's probably mainly down to render to texture Tomfoolery with some extensions (making the world seamless at the border etc).