Camera Calibration and 3D reconstruction: Scaling factor

Question

I am following this documentation: http://docs.opencv.org/modules/calib3d/doc/camera_calibration_and_3d_reconstruction.html

In the expression above, I have managed to obtain all the matrices except the scaling factor 's'. I am assuming that my object is planar and hence Z = 0. I tried to solve for 's' by substituting a known 2D to 3D map, and I got a value. However, this value seems to give me a correct mapping only for some points, and not for all. Am I doing anything wrong?

Any help will be appreciated! Thanks!

Answer 1

Do you know the physical size of the object, and is it the same in all images? Then the scale is exactly one, because you are in a calibrated setup.

If either condition above does not apply, then you cannot recover a physically meaningful scaling factor.