calibrateCamera with CALIB_FIX_ASPECT_RATIO + CALIB_RATIONAL_MODEL yields zero distortion coefficients

[bacalfa]

System information (version)

• Python => 3.6.8
• OpenCV => 4.0.1 (Anaconda package build version: py36hce2de41_201)
• Operating System / Platform => Windows and Linux (RHEL) 64 Bit
• Compiler => Visual Studio 2015 and g++ 8.2.0

Detailed description

When I call function calibrateCamera with the flags argument equal to cv2.CALIB_FIX_ASPECT_RATIO + cv2.CALIB_RATIONAL_MODEL, I get all distortion coefficients equal to zero. When calling the function, I pass an array of shape (8, 1) full of zeros.

I also noticed that the following cases do not yield distortion coefficients equal to zero after the optimization finishes (properly sized arrays are passed in each):

• flags = cv2.CALIB_FIX_ASPECT_RATIO + cv2.CALIB_RATIONAL_MODEL + cv2.CALIB_THIN_PRISM_MODEL
• flags = cv2.CALIB_FIX_ASPECT_RATIO + cv2.CALIB_RATIONAL_MODEL + cv2.CALIB_THIN_PRISM_MODEL + cv2.CALIB_TILTED_MODEL

Steps to reproduce

Unfortunately, I cannot share the data or the code. But I'm wondering if others also experience this issue. I also initialize the camera matrix elements [0, 0] and [1, 1] to the value 1. All other arguments take on their default values.