Hello!

I'm drawing my FPS as a sprite, and I want to have it drawn to the screen independetly of the camera position. For example, always in the lower left corner. I'm using an orthographic projection and I always have the z value of the sprite greater than the camera, and it always seems to draw it the same distance away from the camera. So far, so good. But if I move the camera in the x or y directions, the sprite moves in those directions too.

Do I need a separate viewport? Any other ideas?

You want to have it shown as a normal 2D overlay? In that case you don't need to touch the camera projection at all. You can just draw it directly in NDC space. If you want to have it in 3D, but follow the camera you can just draw it in view space. That way it will always be "attached" to the camera, regardless of where it is. Other nearby objects might be drawn on top of it tough. In that case you need to draw it last, without depth, ie. using painter's algorithm.