Improving the overlay

In the previous section you created a perspective camera, which renders your model with a vertical field of view of 30 degrees. However, to increase the realism of your scene, you actually want to match the field of view of your virtual and physical cameras as well as possible. This field of view in a general imaging system such as your phone's camera is dependent both on the size of the camera sensor and the focal length of the optics used. The focal length is a measure of how strongly the camera lens bends incoming parallel light rays until they come into focus (on the sensor plane), it is basically the distance between the sensor plane and the optical elements of your lens.

The FOV can be computed from the formula α = 2 arctan d/2f, where d is the (vertical, horizontal, or diagonal) extent of the camera sensor and 2 is the focal length. Sounds easy, right? There is only a small challenge. You most often do not know the (physical) sensor size or the focal length of...