I came across an application that:
[...] shows the distance to objects seen through the camera lens of your [device]. Simply aim your [device] to any object in your surroundings, and [the application] displays the distance towards that object on top of the camera image. [...]
How does it work? [The application] uses the height of the camera lens and its tilt angle to calculate the distance to the object.
I am confused. How does the height of a camera lens and a tilt angle allow to calculate a 3D distance? I thought that a point on a 2D space (what a camera sees) represents a line in 3D so it's impossible to calculate real distance with 1 camera and no objects of reference.