<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    android:orientation="vertical"
    >

    <Button
        android:id="@+id/select_btn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="選擇圖片" />
    <Button
        android:id="@+id/process_btn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="處理" />

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

</LinearLayout>

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;

import android.content.Intent;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.net.Uri;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;

import org.opencv.android.OpenCVLoader;
import org.opencv.android.Utils;
import org.opencv.core.CvType;
import org.opencv.core.Mat;
import org.opencv.core.Point;
import org.opencv.imgproc.Imgproc;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;

import org.opencv.core.MatOfRect;
import org.opencv.core.Rect;
import org.opencv.core.Scalar;
import org.opencv.core.Size;
import org.opencv.objdetect.CascadeClassifier;

import android.content.Context;

public class MainActivity extends AppCompatActivity {

    private double max_size = 1024;
    private int PICK_IMAGE_REQUEST = 1;
    private ImageView myImageView;
    private Bitmap selectbp;

    private static final String    TAG                 = "OCVSample::Activity";
    private static final Scalar    FACE_RECT_COLOR     = new Scalar(0, 255, 0, 255);
    public static final int        JAVA_DETECTOR       = 0;
    public static final int        NATIVE_DETECTOR     = 1;

    private Mat                    mGray;
    private File                   mCascadeFile;
    private CascadeClassifier      mJavaDetector,mNoseDetector;

    private int                    mDetectorType       = JAVA_DETECTOR;

    private float                  mRelativeFaceSize   = 0.2f;
    private int                    mAbsoluteFaceSize   = 0;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        staticLoadCVLibraries();
        myImageView = (ImageView)findViewById(R.id.imageView);
        myImageView.setScaleType(ImageView.ScaleType.FIT_CENTER);
        Button selectImageBtn = (Button)findViewById(R.id.select_btn);
        selectImageBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // makeText(MainActivity.this.getApplicationContext(), "start to browser image", Toast.LENGTH_SHORT).show();
                selectImage();
            }

            private void selectImage() {
                Intent intent = new Intent();
                intent.setType("image/*");
                intent.setAction(Intent.ACTION_GET_CONTENT);
                startActivityForResult(Intent.createChooser(intent,"選擇圖像..."), PICK_IMAGE_REQUEST);
            }
        });

        Button processBtn = (Button)findViewById(R.id.process_btn);
        processBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // makeText(MainActivity.this.getApplicationContext(), "hello, image process", Toast.LENGTH_SHORT).show();
                convertGray();
            }
        });

    }

    private void staticLoadCVLibraries() {
        boolean load = OpenCVLoader.initDebug();
        if(load) {
            Log.i("CV", "Open CV Libraries loaded...");
        }

    }

    private void convertGray() {
        Mat src = new Mat();
        Mat temp = new Mat();
        Mat dst = new Mat();
        Utils.bitmapToMat(selectbp, src);
        Imgproc.cvtColor(src, temp, Imgproc.COLOR_BGRA2BGR);
        Log.i("CV", "image type:" + (temp.type() == CvType.CV_8UC3));
        Imgproc.cvtColor(temp, dst, Imgproc.COLOR_BGR2GRAY);
        Utils.matToBitmap(dst, selectbp);
        myImageView.setImageBitmap(selectbp);

        mGray = dst;
        mJavaDetector = loadDetector(R.raw.lbpcascade_frontalface,"lbpcascade_frontalface.xml");
        mNoseDetector = loadDetector(R.raw.haarcascade_mcs_nose,"haarcascade_mcs_nose.xml");

        if (mAbsoluteFaceSize == 0) {
            int height = mGray.rows();
            if (Math.round(height * mRelativeFaceSize) > 0) {
                mAbsoluteFaceSize = Math.round(height * mRelativeFaceSize);
            }
        }

        MatOfRect faces = new MatOfRect();

        if (mJavaDetector != null) {
            mJavaDetector.detectMultiScale(mGray, faces, 1.1, 2, 2, // TODO: objdetect.CV_HAAR_SCALE_IMAGE
                        new Size(mAbsoluteFaceSize, mAbsoluteFaceSize), new Size());

        }

        Rect[] facesArray = faces.toArray();
        for (int i = 0; i < facesArray.length; i++) {

            Log.e(TAG, "start to detect nose!");

            Mat faceROI = mGray.submat(facesArray[i]);

            MatOfRect noses = new MatOfRect();
            mNoseDetector.detectMultiScale(faceROI, noses, 1.1, 2, 2,
                    new Size(30, 30));

            Rect[] nosesArray = noses.toArray();
            Imgproc.rectangle(src,
                    new Point(facesArray[i].tl().x + nosesArray[0].tl().x, facesArray[i].tl().y + nosesArray[0].tl().y) ,
                    new Point(facesArray[i].tl().x + nosesArray[0].br().x, facesArray[i].tl().y + nosesArray[0].br().y) ,
                    FACE_RECT_COLOR, 3);

            Imgproc.rectangle(src, facesArray[i].tl(), facesArray[i].br(), FACE_RECT_COLOR, 3);
        }

        Utils.matToBitmap(src, selectbp);
        myImageView.setImageBitmap(selectbp);

    }

    private CascadeClassifier loadDetector(int rawID,String fileName) {
        CascadeClassifier classifier = null;
        try {

            // load cascade file from application resources
            InputStream is = getResources().openRawResource(rawID);
            File cascadeDir = getDir("cascade", Context.MODE_PRIVATE);
            mCascadeFile = new File(cascadeDir, fileName);
            FileOutputStream os = new FileOutputStream(mCascadeFile);

            byte[] buffer = new byte[4096];
            int bytesRead;
            while ((bytesRead = is.read(buffer)) != -1) {
                os.write(buffer, 0, bytesRead);
            }
            is.close();
            os.close();

            Log.e(TAG, "start to load file:  " + mCascadeFile.getAbsolutePath());
            classifier = new CascadeClassifier(mCascadeFile.getAbsolutePath());

            if (classifier.empty()) {
                Log.e(TAG, "Failed to load cascade classifier");
                classifier = null;
            } else
                Log.i(TAG, "Loaded cascade classifier from " + mCascadeFile.getAbsolutePath());

            cascadeDir.delete();


        } catch (IOException e) {
            e.printStackTrace();
            Log.e(TAG, "Failed to load cascade. Exception thrown: " + e);
        }
        return classifier;
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (requestCode == PICK_IMAGE_REQUEST && resultCode == RESULT_OK && data != null && data.getData() != null) {
            Uri uri = data.getData();
            try {
                Log.d("image-tag", "start to decode selected image now...");
                InputStream input = getContentResolver().openInputStream(uri);
                BitmapFactory.Options options = new BitmapFactory.Options();
                options.inJustDecodeBounds = true;
                BitmapFactory.decodeStream(input, null, options);
                int raw_width = options.outWidth;
                int raw_height = options.outHeight;
                int max = Math.max(raw_width, raw_height);
                int newWidth = raw_width;
                int newHeight = raw_height;
                int inSampleSize = 1;
                if (max > max_size) {
                    newWidth = raw_width / 2;
                    newHeight = raw_height / 2;
                    while ((newWidth / inSampleSize) > max_size || (newHeight / inSampleSize) > max_size) {
                        inSampleSize *= 2;
                    }
                }

                options.inSampleSize = inSampleSize;
                options.inJustDecodeBounds = false;
                options.inPreferredConfig = Bitmap.Config.ARGB_8888;
                selectbp = BitmapFactory.decodeStream(getContentResolver().openInputStream(uri), null, options);

                myImageView.setImageBitmap(selectbp);

            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}