I was looking at this video and saw this image, tried my best to recreate it but it doesn't seem to be the most accurate. Here's the picture i'm referencing. I wanted to rotate my camera like it shows in the video.

cybereality the line that is between a and b is just a wall the portal would be sitting on. my main concern is trying to get angle a from player position. here, i'll post the clip of what the guy says in the video here: Video Clip

    It's just a reflection. Like in 2D (on a straight Y axis, like in the image) you would just flip the sign of the x and y position relative to the intersection point (the portal center).

    But it can be more complex when the wall is in 3D, or not straight on an axis. You could use the classes Transform and Basis to do the reflection.

    https://docs.godotengine.org/en/stable/classes/class_transform.html

    https://docs.godotengine.org/en/stable/classes/class_basis.html

    But I'm about to go to sleep, so this is a bit much right now. Maybe I can write some code tomorrow.