FaceRecognizer cannot be resolved to a type ?

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the FaceRecognizer classes are not part of the main opencv distribution.

to use that, you'll have to rebuild the opencv libs with the opencv_contrib repo ,

then it is also in a cv::face namespace, like:

Ptr<face::FaceRecognizer> fr = face::LBPHFaceRecognizer::create();

in the meantime, you should also try the new, builtin dnn based face recognition (with a pretrained network).

have a look at the answer here, please.