MTCNN is one of the best face detection algorithms. Here is inference only for MTCNN face detector on Tensorflow, which is based on davidsandberg's facenet project, include the python version and C++ version.
There are two version for C++.
One is to be build inside tensorflow code repository, so that it needs to be copied to the directory tensorflow/example. please check cpp/tf_embedded/README.md for details.
The other is the standalone one, just needs libtensorflow.so and c_api.h to build and run. Please check cpp/standalone/README.md for more details
Please check out the guide how to build tensorflow on firefly
https://github.com/kpzhang93/MTCNN_face_detection_alignment
https://github.com/wowo200/MTCNN
FaceNet uses MTCNN to align face
https://github.com/davidsandberg/facenet From this directory: facenet/src/align