I’m looking into ways to implement a portal-like window (as a much simplified portal to help my understanding and minimise the code required) (https://forum.playcanvas.com/t/portal-like-mirror/23894) and I’d like to render a volume off to one side of from the camera’s forward.
This is for a perspective camera.
The API documentation states that the frustum is read only.(https://developer.playcanvas.com/en/api/pc.CameraComponent.html#frustum)
That’s a daunting amount of code. 
Can a custom projection matrix alter the viewing frustum?