Description

This bug happens when try to generate python-bindings for a function which takes cv::Vec as default arguments.
For example, I designed a function randomCrop which takes padding of dtype cv::Vec4i as default argument:

randomCrop(InputArray src, OutputArray dst, const Size& sz, const Vec4i& padding=Vec4i() , bool pad_if_need=false, int fill=0, int padding_mode=BORDER_CONSTANT);

When I trying to generate python-binding for this function, nothing wrong happens.
But when the user use this function in python environment, and they treat padding as default argument so they don't pass value for this argument. They may call this function like this:

# padding is default argument, it should be Vec4i() if we don't pass any value for it
randomCrop(img, np.array([100, 100]))

Then wired thing happens, the python generator calls pyopencv_to on line 63 of cv2_convert.hpp, trying to convert the python object into Vec4i. But this function pyopencv_to omit the case when this argument has default value and user passes no value for it. In this case, the function trying to copy a None object tmp to the target mx, which will raise error in the copyTo procedure.

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake