I try to determine the distortion of an imaging system. Due to technical reasons, I can only use a grid target. Sample Image of a grid target
I follow the method using cv2 as described here: https://opencv24-python-tutorials.readthedocs.io/en/stable/py_tutorials/py_calib3d/py_calibration/py_calibration.html
Using cv2.findChessboardCorners() does not work. Fair enough, I do not have a checkerboard. BUT I determined the grid points "manually", which works pretty well. Grid points are shown in orange
However, following the code example from cv2, I get this:
ret, mtx, dist, rvecs, tvecs = cv2.calibrateCamera([objp], [corners], img.shape[::-1], None, None)
with
ret = 403.18434311612924
mtx = [[2.15648501e+04 0.00000000e+00 6.60463129e+02]
[0.00000000e+00 4.83041078e+05 5.08963085e+02]
[0.00000000e+00 0.00000000e+00 1.00000000e+00]]
dist = [[ 2.25904915e+03 6.07202692e-03 2.83113951e+00 -9.49974797e+00 1.69976610e-08]]
rvecs = (array([[-0.59541192],
[-1.49725611],
[-0.56402402]]),)
tvecs = (array([[ 1.2836315e+01],
[-1.8945857e+01],
[ 1.7703286e+04]]),)
Then I try to cv2.undistort() and get a very strange image Undistortion gone wrong
What's wrong here? I do not get any errors.
I tried to artificially distort the image using gimp. Maybe the distortion in the original image is too low to work with. However, the results are also not satisfactory.
The problem is, that the image is too large. I wondered why the images I took from the internet were working and the image from the camera did not. After some playing around, I found that only difference were the sizes of the files.
I cropped my original image and then the code worked fine. I added some artificial distortion using gimp and this also worked fine with the cropped image.
TL;DR: CV2 does not like high pixel count images!
Is it a bug in cv2? Maybe... I will try to report this one.
