It’s a little difficult to explain what I’m trying to do, so I made a quick picture to explain.
I’m using an orthographic camera in a project, and the camera can rotate and move. The player always moves left and right relative to this camera as if it were a 2D game, but the game logic works in 3D including the collisions. I want to spawn things in the world the same distance from this camera as the player (black dot) so that they will be able to interact with those objects as if the world was 2D. If you consider the case that the camera is staring down the Z axis, what I’d want to do is spawn objects with at the same Z coordinate as the player, but different X coordinates. The line of where I want things to spawn is the yellow line in the picture. Because the camera is not necessarily looking down an axis, what I really want is the length of the blue lines in the picture below (they’re all the same length). Once I have that I plan to use it in combination with ScreenToWorldPoint to place objects.