1.权限设置

任何版本的安卓应用，所需的权限都要首先在AndroidManifest.xml声明

	<uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.CAMERA"/>
    <uses-feature
        android:name="android.hardware.camera"
        android:required="false" />
    <uses-feature
        android:name="android.hardware.camera.autofocus"
        android:required="false" />
    <uses-feature
        android:name="android.hardware.camera.front"
        android:required="false" />
    <uses-feature
        android:name="android.hardware.camera.front.autofocus"
        android:required="false" />

由于安卓版本关系，Android6之后需要运行时获取权限，这里使用第三方库permissionsdispatcher，
Github：https://github.com/permissions-dispatcher/PermissionsDispatcher
使用教程：
1.依赖PermissionsDispatcher库，在build.gradle(app)里添加：

dependencies {
    implementation fileTree(dir: "libs", include: ["*.jar"])
    implementation 'androidx.appcompat:appcompat:1.1.0'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    implementation project(path: ':OpenCV')
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test.ext:junit:1.1.1'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'

    //引入PermissionsDispatcher库
    implementation 'com.github.hotchemi:permissionsdispatcher:3.2.0'
    annotationProcessor 'com.github.hotchemi:permissionsdispatcher-processor:3.2.0'
}

2.在需要获取动态权限的Activity或者Fragment,添加必要注解：
@RuntimePermissions
@NeedsPermission将所需的权限获取后，再进行下一步操作。

2.实现人脸检测

编写布局文件，加入摄像头预览控件JavaCameraView，

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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"
    xmlns:opencv="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
    
    <org.opencv.android.JavaCameraView
        android:id="@+id/cameraView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        opencv:camera_id="any" />

</androidx.constraintlayout.widget.ConstraintLayout>

在AndroidManifest.xml声明activity,设置横屏

	<activity android:name=".MainActivity"
        android:theme="@style/Theme.AppCompat.NoActionBar"
        android:screenOrientation="landscape"
        >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
			<category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>

添加libc++_shared.so，这个我在网上的其他博客并没有看到，但我这边集成时，确实需要，应该是其内部封装通过一些jni接口调用c++算法时需要集成的一个so，我上传了armeabi-v7a的libc++_shared.so，下面是下载地址：
https://download.csdn.net/download/u013370255/12609010
加入模型文件lbpcascade_frontalface_improved.xml
文件来自于…\opencv-4.3.0-android-sdk\OpenCV-android-sdk\sdk\etc\lbpcascades
主显示界面mainActivity

package com.zhb.opencvdemo;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import android.Manifest;
import android.content.Context;
import android.os.Bundle;
import android.util.Log;
import android.view.SurfaceView;

import org.opencv.android.BaseLoaderCallback;
import org.opencv.android.CameraBridgeViewBase;
import org.opencv.android.LoaderCallbackInterface;
import org.opencv.android.OpenCVLoader;
import org.opencv.core.Core;
import org.opencv.core.Mat;
import org.opencv.core.Point;
import org.opencv.objdetect.CascadeClassifier;

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

import permissions.dispatcher.NeedsPermission;
import permissions.dispatcher.RuntimePermissions;

@RuntimePermissions
public class MainActivity extends AppCompatActivity {

    //预览图
    CameraBridgeViewBase mOpenCvCameraView;
    //模型文件
    File mCascadeFile;
    //级联分类器
    private CascadeClassifier cascadeClassifier = null;
    //4通道的图片矩阵，彩图
    Mat mRgba;
    //单通道的图片矩阵，灰图
    Mat mGray;
    //摄像头索引：-1/0后置摄像头，1前置摄像头
    private int mCameraIndex = 1;

    // 使用opencv自带的分类器文件初始化
    private BaseLoaderCallback mLoaderCallback = new BaseLoaderCallback(this) {
        @Override
        public void onManagerConnected(int status) {
            switch (status) {
                case LoaderCallbackInterface.SUCCESS: {
                    try {
                        InputStream is = getResources().openRawResource(R.raw.lbpcascade_frontalface_improved);
                        File cascadeDir = getDir("cascade", Context.MODE_PRIVATE);
                        mCascadeFile = new File(cascadeDir, "lbpcascade_frontalface_improved.xml");
                        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();

                        // 加载cascadeClassifier
                        cascadeClassifier = new CascadeClassifier(mCascadeFile.getAbsolutePath());

                        cascadeDir.delete();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                    
                    //启用相机
                    mOpenCvCameraView.setCameraPermissionGranted();
                    mOpenCvCameraView.enableView();
                }

                break;
                default: {
                    super.onManagerConnected(status);
                }
            }
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mOpenCvCameraView = findViewById(R.id.cameraView);
        mOpenCvCameraView.setVisibility(SurfaceView.VISIBLE);
        //摄像头索引-1/0：后置双摄1：前置
        mOpenCvCameraView.setCameraIndex(mCameraIndex);
        //openCV预览界面监听
        mOpenCvCameraView.setCvCameraViewListener(new CameraBridgeViewBase.CvCameraViewListener2() {
            @Override
            public void onCameraViewStarted(int width, int height) {
            }

            @Override
            public void onCameraViewStopped() {
                mRgba.release();
                mGray.release();
            }

            @Override
            public Mat onCameraFrame(CameraBridgeViewBase.CvCameraViewFrame inputFrame) {
                mRgba = inputFrame.rgba();
                mGray = inputFrame.gray();

                return mRgba;
            }
        });
    }

    @Override
    public void onPause() {
        super.onPause();
        disableCamera();
    }

    @Override
    public void onResume() {
        super.onResume();
        MainActivityPermissionsDispatcher.showCameraWithPermissionCheck(this);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        disableCamera();
    }

    public void disableCamera() {
        if (mOpenCvCameraView != null)
            mOpenCvCameraView.disableView();
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        // NOTE: delegate the permission handling to generated method
        MainActivityPermissionsDispatcher.onRequestPermissionsResult(this, requestCode, grantResults);
    }


    @NeedsPermission({
            Manifest.permission.CAMERA,
            Manifest.permission.WRITE_EXTERNAL_STORAGE,
            Manifest.permission.READ_EXTERNAL_STORAGE})
    void showCamera() {
        //OpenCV初始化
        if (!OpenCVLoader.initDebug()) {
            OpenCVLoader.initAsync(OpenCVLoader.OPENCV_VERSION, this, mLoaderCallback);
        } else {
            mLoaderCallback.onManagerConnected(LoaderCallbackInterface.SUCCESS);
        }
    }

}

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