Camera pose estimation is very useful to create amusing toy applications, allowing the insertion of virtual objects in the scene. Let's show a simple example. First we take a sheet of paper with known aspect ratio (A4), which serves as our 3D reference frame:
When the rectangle is properly detected we insert the desired virtual objects as seen by the estimated camera:
Interestingly, the objects are actually "alive" in their small virtual world. If we lift the book the ball moves down and bounces against the walls:
There are many variations, games, etc, based on this idea. This kind of application is a good practical exercise for our computer vision students.
Here is video showing how it works under extremely adverse conditions: a cheap webcam, poor light, etc.