obtaining Q matrix from matlab stereoparams

Question

I am working on 3d image reconstrcution using stereo camera. I started with opencv 3.2 and visual studio. I was unable to correctly register two point clouds from two scenes with an overlap correctly. So, I have my doubt on the Q matrix obtained from camera calibration process. So I did the camera calibration using the matlab calibrator app. I want to manually create the Q matrix from the calibration parameters obtained from matlab and then use it in opencv. I found from this post how to create a Q matrix. Now the problem is i don't know the focal length i should use in this matrix. Matlab provides the calibration parameters in a stereoparam object which contains camera parameters for both camera sensors separately. So i have fx and fy from camera1 and fx and fy from camera2. So how do i obtain a single focal length for the stereo camera?

Answer 1

As reported here, fx and fy are expressed in pixels.

F, the focal length in world unit (typically in millimeters) can be computed as

F = fx * px or F = fy * py,

where px and py are the size of the pixel along x and y, respectively.

In particular,

px = image width [pixel] / image sensor width [mm]

py = image height [pixel] / image sensor height [mm].

Answer 2

This is the Q matrix aka reprojection matrix

Since you have the camera intrinsic matrix and extrinsic matrix, then just fill it in accordingly.

Take note the Tx should be in mm.