Why does the flag set in cv2.calibrateCamera() not work?

[Easonyesheng]

Expected behaviour

I want use the cv2.calibrateCamera() to calculate the camera intrinsic parameters using the flag like cv2.CALIB_FIX_FOCAL_LENGTH which works well in C++.

Actual behaviour

The cv2.calibrateCamera() can take in the flag but just not work.
It must get something wrong in the API.

[asmorkalov]

@Easonyesheng Could you provide the issue reproducer and OpenCV version you use. Also please ensure that you have CALIB_USE_INTRINSIC_GUESS mentioned in flags, otherwise the function ignores provided focal length value.

[Easonyesheng]

@asmorkalov

My environment is as follow:

python-opencv 3.4.2
python 3.6

This issue appears in code like this:

def __calibrate_with_flag(self, f, pp):
        """name
            Calibration with fix focal length
        Args:
            f: focal length
            pp: principle points
        Returns:
        """
        mtx = np.array([[f,0,pp],
                        [0,f,pp],
                        [0,0,1]])
        ret, mtx, dist, rvecs, tvecs = cv2.calibrateCamera(self.object_points, self.img_points, (self.img.shape[2], self.img.shape[1]),cv2.CALIB_USE_INTRINSIC_GUESS,cv2.CALIB_FIX_FOCAL_LENGTH)
        return ret, mtx, dist, rvecs, tvecs

This code doesn't work like calibration with fix focal length and it just ignores the two flags I set.
I'm not sure it is the right way to provide focal length cause I cannot find related reference documents.

[asmorkalov]

I see 2 issues in the provided code:

• According to documentation (link) your parameters does not match expected parameters order. Flags are passed to wrong place.
• All flags are passed as single value. You need to concatenate them to single integer like this: cv2.CALIB_USE_INTRINSIC_GUESS + cv2.CALIB_FIX_FOCAL_LENGTH

[Easonyesheng]

Thank you!
I will check these 2 issues.