安卓人脸检测技术详解：2023年最新实现方法与应用案例

3 年 ago

宇, 华

8 minutes

随着Google Play服务7.8的发布，Google引入了移动视觉API，可以实现人脸检测、条形码检测和文本检测。在本教程中，我们将开发一个安卓人脸检测应用程序，可以在图像中检测出人脸。

Android人脸识别

Android人脸识别API通过使用眼睛、鼻子、耳朵、脸颊和嘴巴等一些特征点来跟踪照片和视频中的人脸。与检测单个特征不同，API会一次性检测整张人脸，然后再检测特征点和分类（如果定义了）。此外，该API还可以检测各种角度的人脸。

Android人脸检测 – 特征点识别

特征点是面部中的一个具有代表性的点。左眼、右眼和鼻子底部都是特征点的例子。以下是目前可以通过API找到的特征点：

左眼和右眼
左耳和右耳
左耳尖和右耳尖
鼻子的底部
左脸颊和右脸颊
嘴角的左边和右边
口腔的底部

当使用”左”和”右”时，它们是相对于对象的。例如，LEFT_EYE特征点是对象的左眼，而不是在查看图像时位于左边的眼睛。

脸部朝向

脸部的定位是使用欧拉角来确定的。这些欧拉角指的是脸部围绕X轴、Y轴和Z轴旋转的角度。欧拉角Y告诉我们脸部是朝左还是朝右看。

欧拉角Z告诉我们脸部是否旋转/倾斜
欧拉角X告诉我们脸部是朝上看还是朝下看（目前不支持）

注意：如果无法计算概率，则将其设置为-1。让我们开始本教程的实践部分。我们的应用程序将包含一些示例图像以及捕获自己图像的功能。注意：该API仅支持人脸检测。当前版本的移动视觉API不提供人脸识别功能。

Android人脸检测示例项目的结构

android face detection api project structure

安卓人脸检测代码

在你的应用的build.gradle文件中添加以下依赖：

compile 'com.google.android.gms:play-services-vision:11.0.4'

请按照下面所示，在AndroidManifest.xml文件的application标签内添加以下元数据：

<meta-data
            android:name="com.google.android.gms.vision.DEPENDENCIES"
            android:value="face"/>

这样，Vision库就能知道你打算在应用程序中检测人脸。在AndroidManifest.xml文件的manifest标签中添加以下权限来获取相机权限：

<uses-feature
        android:name="android.hardware.camera"
        android:required="true"/>
    <uses-permission
        android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

下面提供了activity_main.xml布局文件的代码。

这是文章《安卓人脸检测》的第2部分（共4部分）。

<?xml version="1.0" encoding="utf-8"?>

<ScrollView xmlns:android="https://schemas.android.com/apk/res/android"
    xmlns:app="https://schemas.android.com/apk/res-auto"
    xmlns:tools="https://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <android.support.constraint.ConstraintLayout xmlns:app="https://schemas.android.com/apk/res-auto"
        xmlns:tools="https://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        tools:context="com.Olivia.facedetectionapi.MainActivity">

        <ImageView
            android:id="@+id/imageView"
            android:layout_width="300dp"
            android:layout_height="300dp"
            android:layout_marginTop="8dp"
            android:src="@drawable/sample_1"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toTopOf="parent" />

        <Button
            android:id="@+id/btnProcessNext"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="8dp"
            android:text="处理下一张"
            app:layout_constraintHorizontal_bias="0.501"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/imageView" />

        <ImageView
            android:id="@+id/imgTakePic"
            android:layout_width="250dp"
            android:layout_height="250dp"
            android:layout_marginTop="8dp"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/txtSampleDescription"
            app:srcCompat="@android:drawable/ic_menu_camera" />

        <Button
            android:id="@+id/btnTakePicture"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="8dp"
            android:text="拍照"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/imgTakePic" />

        <TextView
            android:id="@+id/txtSampleDescription"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginBottom="8dp"
            android:layout_marginTop="8dp"
            android:gravity="center"
            app:layout_constraintBottom_toTopOf="@+id/txtTakePicture"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/btnProcessNext"
            app:layout_constraintVertical_bias="0.0" />

        <TextView
            android:id="@+id/txtTakePicture"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="8dp"
            android:gravity="center"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/btnTakePicture" />

    </android.support.constraint.ConstraintLayout>

</ScrollView>

我们定义了两个ImageView、TextView和Button组件。其中一个用于循环遍历示例图片并显示人脸检测结果，另一个用于从相机捕获图片进行实时人脸检测。下面是MainActivity.java文件的代码实现。

package com.Olivia.facedetectionapi;

import android.Manifest;
import android.content.Context;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.net.Uri;
import android.os.Environment;
import android.provider.MediaStore;
import android.support.annotation.NonNull;
import android.support.v4.app.ActivityCompat;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.SparseArray;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;

import com.google.android.gms.vision.Frame;
import com.google.android.gms.vision.face.Face;
import com.google.android.gms.vision.face.FaceDetector;
import com.google.android.gms.vision.face.Landmark;

import java.io.File;
import java.io.FileNotFoundException;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    ImageView imageView, imgTakePicture;
    Button btnProcessNext, btnTakePicture;
    TextView txtSampleDesc, txtTakenPicDesc;
    private FaceDetector detector;
    Bitmap editedBitmap;
    int currentIndex = 0;
    int[] imageArray;
    private Uri imageUri;
    private static final int REQUEST_WRITE_PERMISSION = 200;
    private static final int CAMERA_REQUEST = 101;

    private static final String SAVED_INSTANCE_URI = "uri";
    private static final String SAVED_INSTANCE_BITMAP = "bitmap";

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

        imageArray = new int[]{R.drawable.sample_1, R.drawable.sample_2, R.drawable.sample_3};
        detector = new FaceDetector.Builder(getApplicationContext())
                .setTrackingEnabled(false)
                .setLandmarkType(FaceDetector.ALL_CLASSIFICATIONS)
                .setClassificationType(FaceDetector.ALL_CLASSIFICATIONS)
                .build();

        initViews();

    }

    private void initViews() {
        imageView = (ImageView) findViewById(R.id.imageView);
        imgTakePicture = (ImageView) findViewById(R.id.imgTakePic);
        btnProcessNext = (Button) findViewById(R.id.btnProcessNext);
        btnTakePicture = (Button) findViewById(R.id.btnTakePicture);
        txtSampleDesc = (TextView) findViewById(R.id.txtSampleDescription);
        txtTakenPicDesc = (TextView) findViewById(R.id.txtTakePicture);

        processImage(imageArray[currentIndex]);
        currentIndex++;

        btnProcessNext.setOnClickListener(this);
        btnTakePicture.setOnClickListener(this);
        imgTakePicture.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.btnProcessNext:
                imageView.setImageResource(imageArray[currentIndex]);
                processImage(imageArray[currentIndex]);
                if (currentIndex == imageArray.length - 1)
                    currentIndex = 0;
                else
                    currentIndex++;

                break;

            case R.id.btnTakePicture:
                ActivityCompat.requestPermissions(MainActivity.this, new
                        String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, REQUEST_WRITE_PERMISSION);
                break;

            case R.id.imgTakePic:
                ActivityCompat.requestPermissions(MainActivity.this, new
                        String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, REQUEST_WRITE_PERMISSION);
                break;
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        switch (requestCode) {
            case REQUEST_WRITE_PERMISSION:
                if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                    startCamera();
                } else {
                    Toast.makeText(getApplicationContext(), "Permission Denied!", Toast.LENGTH_SHORT).show();
                }
        }
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        if (requestCode == CAMERA_REQUEST && resultCode == RESULT_OK) {
            launchMediaScanIntent();
            try {
                processCameraPicture();
            } catch (Exception e) {
                Toast.makeText(getApplicationContext(), "Failed to load Image", Toast.LENGTH_SHORT).show();
            }
        }
    }

    private void launchMediaScanIntent() {
        Intent mediaScanIntent = new Intent(Intent.ACTION_MEDIA_SCANNER_SCAN_FILE);
        mediaScanIntent.setData(imageUri);
        this.sendBroadcast(mediaScanIntent);
    }

    private void startCamera() {
        Intent intent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
        File photo = new File(Environment.getExternalStorageDirectory(), "photo.jpg");
        imageUri = Uri.fromFile(photo);
        intent.putExtra(MediaStore.EXTRA_OUTPUT, imageUri);
        startActivityForResult(intent, CAMERA_REQUEST);
    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        if (imageUri != null) {
            outState.putParcelable(SAVED_INSTANCE_BITMAP, editedBitmap);
            outState.putString(SAVED_INSTANCE_URI, imageUri.toString());
        }
        super.onSaveInstanceState(outState);
    }


    private void processImage(int image) {

        Bitmap bitmap = decodeBitmapImage(image);
        if (detector.isOperational() && bitmap != null) {
            editedBitmap = Bitmap.createBitmap(bitmap.getWidth(), bitmap
                    .getHeight(), bitmap.getConfig());
            float scale = getResources().getDisplayMetrics().density;
            Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
            paint.setColor(Color.GREEN);
            paint.setTextSize((int) (16 * scale));
            paint.setShadowLayer(1f, 0f, 1f, Color.WHITE);
            paint.setStyle(Paint.Style.STROKE);
            paint.setStrokeWidth(6f);
            Canvas canvas = new Canvas(editedBitmap);
            canvas.drawBitmap(bitmap, 0, 0, paint);
            Frame frame = new Frame.Builder().setBitmap(editedBitmap).build();
            SparseArray<Face> faces = detector.detect(frame);
            txtSampleDesc.setText(null);

            for (int index = 0; index < faces.size(); ++index) {
                Face face = faces.valueAt(index);
                canvas.drawRect(
                        face.getPosition().x,
                        face.getPosition().y,
                        face.getPosition().x + face.getWidth(),
                        face.getPosition().y + face.getHeight(), paint);


                canvas.drawText("Face " + (index + 1), face.getPosition().x + face.getWidth(), face.getPosition().y + face.getHeight(), paint);

                txtSampleDesc.setText(txtSampleDesc.getText() + "FACE " + (index + 1) + "\n");
                txtSampleDesc.setText(txtSampleDesc.getText() + "Smile probability:" + " " + face.getIsSmilingProbability() + "\n");
                txtSampleDesc.setText(txtSampleDesc.getText() + "Left Eye Is Open Probability: " + " " + face.getIsLeftEyeOpenProbability() + "\n");
                txtSampleDesc.setText(txtSampleDesc.getText() + "Right Eye Is Open Probability: " + " " + face.getIsRightEyeOpenProbability() + "\n\n");

                for (Landmark landmark : face.getLandmarks()) {
                    int cx = (int) (landmark.getPosition().x);
                    int cy = (int) (landmark.getPosition().y);
                    canvas.drawCircle(cx, cy, 8, paint);
                }


            }

            if (faces.size() == 0) {
                txtSampleDesc.setText("Scan Failed: Found nothing to scan");
            } else {
                imageView.setImageBitmap(editedBitmap);
                txtSampleDesc.setText(txtSampleDesc.getText() + "No of Faces Detected: " + " " + String.valueOf(faces.size()));
            }
        } else {
            txtSampleDesc.setText("Could not set up the detector!");
        }
    }

    private Bitmap decodeBitmapImage(int image) {
        int targetW = 300;
        int targetH = 300;
        BitmapFactory.Options bmOptions = new BitmapFactory.Options();
        bmOptions.inJustDecodeBounds = true;

        BitmapFactory.decodeResource(getResources(), image,
                bmOptions);

        int photoW = bmOptions.outWidth;
        int photoH = bmOptions.outHeight;

        int scaleFactor = Math.min(photoW / targetW, photoH / targetH);
        bmOptions.inJustDecodeBounds = false;
        bmOptions.inSampleSize = scaleFactor;

        return BitmapFactory.decodeResource(getResources(), image,
                bmOptions);
    }

    private void processCameraPicture() throws Exception {
        Bitmap bitmap = decodeBitmapUri(this, imageUri);
        if (detector.isOperational() && bitmap != null) {
            editedBitmap = Bitmap.createBitmap(bitmap.getWidth(), bitmap
                    .getHeight(), bitmap.getConfig());
            float scale = getResources().getDisplayMetrics().density;
            Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
            paint.setColor(Color.GREEN);
            paint.setTextSize((int) (16 * scale));
            paint.setShadowLayer(1f, 0f, 1f, Color.WHITE);
            paint.setStyle(Paint.Style.STROKE);
            paint.setStrokeWidth(6f);
            Canvas canvas = new Canvas(editedBitmap);
            canvas.drawBitmap(bitmap, 0, 0, paint);
            Frame frame = new Frame.Builder().setBitmap(editedBitmap).build();
            SparseArray<Face> faces = detector.detect(frame);
            txtTakenPicDesc.setText(null);

            for (int index = 0; index < faces.size(); ++index) {
                Face face = faces.valueAt(index);
                canvas.drawRect(
                        face.getPosition().x,
                        face.getPosition().y,
                        face.getPosition().x + face.getWidth(),
                        face.getPosition().y + face.getHeight(), paint);


                canvas.drawText("Face " + (index + 1), face.getPosition().x + face.getWidth(), face.getPosition().y + face.getHeight(), paint);

                txtTakenPicDesc.setText("FACE " + (index + 1) + "\n");
                txtTakenPicDesc.setText(txtTakenPicDesc.getText() + "Smile probability:" + " " + face.getIsSmilingProbability() + "\n");
                txtTakenPicDesc.setText(txtTakenPicDesc.getText() + "Left Eye Is Open Probability: " + " " + face.getIsLeftEyeOpenProbability() + "\n");
                txtTakenPicDesc.setText(txtTakenPicDesc.getText() + "Right Eye Is Open Probability: " + " " + face.getIsRightEyeOpenProbability() + "\n\n");

                for (Landmark landmark : face.getLandmarks()) {
                    int cx = (int) (landmark.getPosition().x);
                    int cy = (int) (landmark.getPosition().y);
                    canvas.drawCircle(cx, cy, 8, paint);
                }


            }

            if (faces.size() == 0) {
                txtTakenPicDesc.setText("Scan Failed: Found nothing to scan");
            } else {
                imgTakePicture.setImageBitmap(editedBitmap);
                txtTakenPicDesc.setText(txtTakenPicDesc.getText() + "No of Faces Detected: " + " " + String.valueOf(faces.size()));
            }
        } else {
            txtTakenPicDesc.setText("Could not set up the detector!");
        }
    }

    private Bitmap decodeBitmapUri(Context ctx, Uri uri) throws FileNotFoundException {
        int targetW = 300;
        int targetH = 300;
        BitmapFactory.Options bmOptions = new BitmapFactory.Options();
        bmOptions.inJustDecodeBounds = true;
        BitmapFactory.decodeStream(ctx.getContentResolver().openInputStream(uri), null, bmOptions);
        int photoW = bmOptions.outWidth;
        int photoH = bmOptions.outHeight;

        int scaleFactor = Math.min(photoW / targetW, photoH / targetH);
        bmOptions.inJustDecodeBounds = false;
        bmOptions.inSampleSize = scaleFactor;

        return BitmapFactory.decodeStream(ctx.getContentResolver()
                .openInputStream(uri), null, bmOptions);
    }


    @Override
    protected void onDestroy() {
        super.onDestroy();
        detector.release();
    }
}

从上述代码中得出的一些推论是：

imageArray存储了示例图像，当点击”处理下一张”按钮时，这些图像将被扫描以检测人脸。
检测器通过以下代码片段实例化：
```
FaceDetector detector = new FaceDetector.Builder( getContext() )
        .setTrackingEnabled(false)
        .setLandmarkType(FaceDetector.ALL_LANDMARKS)
        .setMode(FaceDetector.FAST_MODE)
        .build();
```
特征点会增加计算时间，因此需要明确设置。人脸检测器可以根据需求设置为FAST_MODE（快速模式）或ACCURATE_MODE（精确模式）。在上述代码中，我们将跟踪设置为false，因为我们处理的是静态图像。对于视频中的人脸检测，可以将其设置为true。
processImage()和processCameraPicture()方法包含了我们实际检测人脸并在其上绘制矩形的代码。
detector.isOperational()用于检查手机中的当前Google Play服务库是否支持视觉API（如果不支持，Google Play会下载所需的本机库以提供支持）。

实际执行人脸检测工作的代码片段是：

Frame frame = new Frame.Builder().setBitmap(editedBitmap).build();
SparseArray faces = detector.detect(frame);

检测到人脸后，我们遍历faces数组以找到每张人脸的位置和属性。
每张人脸的属性会附加在按钮下方的TextView中。
当通过相机捕获图像时，工作方式相同，只是我们需要在运行时请求相机权限，并保存相机应用程序返回的uri和bitmap。

#AI技术 #人脸识别 #安卓开发 #移动应用开发 #计算机视觉