Problem

The KNN and MOG2 background subtractors extend the Algorithm class but don't override the getDefaultName() method (which returns my_object).

• https://github.com/opencv/opencv/blob/4.5.5/modules/core/src/algorithm.cpp#L80-L84

Proposed solution

Add override for the method, avoiding the . character in the name because persistence disallows it in key names.

• https://github.com/opencv/opencv/blob/4.5.5/modules/core/src/persistence_json.cpp#L223-L224
• https://github.com/opencv/opencv/blob/4.5.5/modules/core/src/persistence_yml.cpp#L260-L261

Usage code snippet

#include <opencv2/opencv.hpp>

int main(int argc, char* argv[])
{
    std::cout << cv::createBackgroundSubtractorKNN()->getDefaultName() << std::endl;
    cv::createBackgroundSubtractorKNN()->save("knn.algo");

    std::cout << cv::createBackgroundSubtractorMOG2()->getDefaultName() << std::endl;
    cv::createBackgroundSubtractorMOG2()->save("mog2.algo");

    return 0;
}

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