storing images in database for template matching?

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If you can get good enough performance storing them as png or jpg in directories, that is the thing to do. If that isn't fast enough, you may need a real database or a custom file format. No xml or yml approach will be as good as either of those options.

You may need to use OpenCV XML/YAML file storages to store Mat in to file and later read back.

Writing Mat to file

Mat src = imread( "img.jpg",1);
FileStorage Data_W("Data.yml",FileStorage::WRITE); // Store the  data in a file
Data_W << "data" << src;
Data_W.release();

Reading back from file

Mat dst;
FileStorage Data_R("Data.yml",FileStorage::READ); // Read traing data to a Mat
Data_R["data"] >> dst;
Data_R.release();
imshow("dst",dst);
waitKey();