you should start with one of the existing android samples for face detection, either using cascades or a detection dnn
if you got that running, time to move to the recognition.
you can use the FaceRecognizer classes from opencv_contrib repo (but you would need to reebuild the sdk with contrib modules)
or, alternatively use the pretrained OpenFace dnn model, with code like:
import org.opencv.core.*;
import org.opencv.dnn.*;
import org.opencv.imgcodecs.*;
import org.opencv.imgproc.*;
public class FaceRecognition {
public static Mat process(Net net, Mat img) {
Mat inputBlob = Dnn.blobFromImage(img, 1./255,
new Size(96,96), new Scalar(0,0,0), true, false);
net.setInput(inputBlob);
return net.forward().clone();
}
public static void main(String[] args) {
System.loadLibrary(Core.NATIVE_LIBRARY_NAME);
Net net = Dnn.readNetFromTorch("openface.nn4.small2.v1.t7");
Mat feature1 = process(net, Imgcodecs.imread("Abdullah_Gul_0004.jpg"));
Mat feature2 = process(net, Imgcodecs.imread("Abdullah_Gul_0007.jpg"));
double dist = Core.norm(feature1, feature2);
if (dist < 0.6)
System.out.println("SAME !");
}
}