Rays from the Camera

In the section Understanding the View Frustum, it was explained that any point in the camera’s view corresponds to a line in world space. It is sometimes useful to have a mathematical representation of that line and Unity can provide this in the form of a Ray object. The Ray always corresponds to a point in the view, so the Camera class provides the ScreenPointToRay and ViewportPointToRay functions. The difference between the two is that ScreenPointToRay expects the point to be provided as a pixel coordinate, while ViewportPointToRay takes normalized coordinates in the range 0..1 (where 0 represents the bottom or left and 1 represents the top or right of the view). Each of these functions returns a Ray which consists of a point of origin and a vector which shows the direction of the line from that origin. The Ray originates from the near clipping plane rather than the Camera’s transform.position point.

Raycasting

The most common use of a Ray from the camera is to perform a raycast out into the scene. A raycast sends an imaginary “laser beam” along the ray from its origin until it hits a collider in the scene. Information is then returned about the object and the point that was hit in a RaycastHit object. This is a very useful way to locate an object based on its onscreen image. For example, the object at the mouse position can be determined with the following code:-

var hit: RaycastHit;
var ray: Ray = camera.ScreenPointToRay(Input.mousePosition);

if (Physics.Raycast(ray, hit)) {
    var objectHit: Transform = hit.transform;
    
    // Do something with the object that was hit by the raycast.
}

Moving the Camera Along a Ray

It is sometimes useful to get a ray corresponding to a screen position and then move the camera along that ray. For example, you may want to allow the user to select an object with the mouse and then zoom in on it while keeping it “pinned” to the same screen position under the mouse (this might be useful when the camera is looking at a tactical map, say). The code to do this is fairly straightforward:-

var zooming: boolean;
var zoomSpeed: float;

if (zooming) {
    var ray: Ray = camera.ScreenPointToRay(Input.mousePosition);
    zoomDistance = zoomSpeed * Input.GetAxis("Vertical") * Time.deltaTime;
    camera.transform.Translate(ray.direction * zoomDistance, Space.World);
}