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OCR是一个很常用的功能，Google ML Kit提供了OCR能力，用起来也很简单，本文介绍一下使用方法。

1. 相关概念

2. 代码实现

在build.gradle中添加相关依赖：

// To recognize Chinese script
implementation 'com.google.mlkit:text-recognition-chinese:16.0.1'

添加布局文件activity_ocr.xml：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"android:layout_width="match_parent"android:layout_height="match_parent"android:orientation="vertical"><FrameLayoutandroid:layout_width="wrap_content"android:layout_height="wrap_content"><SurfaceViewandroid:id="@+id/camera_preview"android:layout_width="wrap_content"android:layout_height="wrap_content" /><com.example.study.views.DrawViewandroid:id="@+id/ocr_area"android:layout_width="wrap_content"android:layout_height="wrap_content" /></FrameLayout><Buttonandroid:id="@+id/ocr_switch"android:layout_width="match_parent"android:layout_height="match_parent"android:layout_gravity="center_horizontal|bottom"android:layout_marginBottom="80dp"android:background="@color/夏云灰"android:text="stop" />
</LinearLayout>

绘制文字的OCRDrawView.java：

package com.example.study.views;import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.Point;
import android.util.AttributeSet;
import android.view.View;import androidx.annotation.Nullable;import java.util.ArrayList;
import java.util.List;public class OCRDrawView extends View {private final Object lock = new Object();protected Paint paint = new Paint();protected Path path = new Path();private final List<ShapeInfo> cornerPointsList = new ArrayList<>();public OCRDrawView(Context context) {super(context);}public OCRDrawView(Context context, @Nullable AttributeSet attrs) {super(context, attrs);}public void clear() {synchronized (lock) {cornerPointsList.clear();}postInvalidate();}public void add(Point[] cornerPoints, String text) {synchronized (lock) {cornerPointsList.add(new ShapeInfo(cornerPoints, text));}}@Overrideprotected void onDraw(Canvas canvas) {super.onDraw(canvas);synchronized (lock) {for (ShapeInfo shapeInfo : cornerPointsList) {drawBackground(shapeInfo, canvas);drawText(shapeInfo, canvas);}}}private void drawText(ShapeInfo shapeInfo, Canvas canvas) {Point[] points = shapeInfo.points;// 根据矩形区域的高度设置文字大小double height = calDistance(points[0], points[3]);double width = calDistance(points[2], points[3]);float textSize = (float) Math.min(height, width / shapeInfo.text.length());paint.setColor(Color.BLUE);paint.setTextSize(textSize);path.reset();path.moveTo(points[3].x, points[3].y);path.lineTo(points[2].x, points[2].y);canvas.drawTextOnPath(shapeInfo.text, path, 0, 0, paint);}private double calDistance(Point start, Point end) {return Math.sqrt(Math.pow(start.x - end.x, 2) + Math.pow(start.y - end.y, 2));}private void drawBackground(ShapeInfo shapeInfo, Canvas canvas) {Point[] shape = shapeInfo.points;path.reset();path.moveTo(shape[3].x, shape[3].y);for (int i = 0; i < shape.length; i++) {path.lineTo(shape[i].x, shape[i].y);}path.close();paint.setColor(Color.WHITE);canvas.drawPath(path, paint);}static class ShapeInfo {Point[] points;String text;public ShapeInfo(Point[] shape, String text) {this.points = shape;this.text = text;}}
}

activity类：

package com.example.study.activities;import android.Manifest;
import android.content.pm.PackageManager;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.ImageFormat;
import android.graphics.Matrix;
import android.graphics.Point;
import android.graphics.Rect;
import android.graphics.YuvImage;
import android.hardware.Camera;
import android.os.Bundle;
import android.util.Log;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.FrameLayout;
import android.widget.Toast;import androidx.activity.ComponentActivity;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;import com.example.study.R;
import com.example.study.views.OCRDrawView;
import com.google.mlkit.vision.text.Text;
import com.google.mlkit.vision.text.TextRecognition;
import com.google.mlkit.vision.text.TextRecognizer;
import com.google.mlkit.vision.text.chinese.ChineseTextRecognizerOptions;import java.io.ByteArrayOutputStream;public class OCRActivity extends ComponentActivity implements Camera.PreviewCallback, SurfaceHolder.Callback {private static final String TAG = "CameraDemoActivity";private static final int REQUEST_CAMERA = 1000;private static final int HEIGHT = 1920;private static final int WIDTH = 1080;private static final int ORIENTATION = 90;private SurfaceView preview;private OCRDrawView ocrArea;private Button ocrSwitch;private Camera camera;private Camera.Parameters parameters;private TextRecognizer recognizer;private Matrix matrix;private boolean isRecognizering = false;private boolean stopRecognizer = false;@Overrideprotected void onCreate(@Nullable Bundle savedInstanceState) {super.onCreate(savedInstanceState);this.setContentView(R.layout.activity_ocr);initView();initVar();// 检查权限if (checkSelfPermission(Manifest.permission.CAMERA) != PackageManager.PERMISSION_GRANTED) {requestPermissions(new String[]{Manifest.permission.CAMERA}, REQUEST_CAMERA);} else {preview.getHolder().addCallback(this);}}private void initVar() {recognizer = TextRecognition.getClient(new ChineseTextRecognizerOptions.Builder().build());matrix = new Matrix();matrix.setRotate(ORIENTATION);// 4个角的坐标是没有旋转过的，所以HEIGHT、WIDTH是反的matrix.preTranslate(-HEIGHT >> 1, -WIDTH >> 1);}private void initView() {preview = findViewById(R.id.camera_preview);ocrArea = findViewById(R.id.ocr_area);ocrSwitch = findViewById(R.id.ocr_switch);ocrSwitch.setOnClickListener(view -> {stopRecognizer = !stopRecognizer;ocrSwitch.setText(stopRecognizer ? "start" : "stop");if (camera == null) {return;}if (stopRecognizer) {camera.stopPreview();} else {camera.startPreview();}});adjustSurface(preview, ocrArea);}private void adjustSurface(SurfaceView cameraPreview, OCRDrawView ocrArea) {FrameLayout.LayoutParams cameraPreviewParams = (FrameLayout.LayoutParams) cameraPreview.getLayoutParams();cameraPreviewParams.width = WIDTH;cameraPreviewParams.height = HEIGHT;ViewGroup.LayoutParams ocrAreaParams = ocrArea.getLayoutParams();ocrAreaParams.width = WIDTH;ocrAreaParams.height = HEIGHT;}@Overridepublic void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {super.onRequestPermissionsResult(requestCode, permissions, grantResults);if (requestCode == REQUEST_CAMERA && grantResults.length > 0) {if (grantResults[0] == PackageManager.PERMISSION_GRANTED) {preview.getHolder().addCallback(this);surfaceCreated(preview.getHolder());camera.setPreviewCallback(this);camera.startPreview();} else {finish();}}}@Overridepublic void onPreviewFrame(byte[] data, Camera camera) {if (isRecognizering || stopRecognizer) {return;}Bitmap bitmap = convertToBitmap(camera, data);isRecognizering = true;recognizer.process(bitmap, ORIENTATION).addOnSuccessListener(text -> {parseOCRResult(text);}).addOnFailureListener(exception -> {Toast.makeText(this, "Failure", Toast.LENGTH_SHORT).show();isRecognizering = false;}).addOnCompleteListener(task -> {isRecognizering = false;}).addOnCanceledListener(() -> {Toast.makeText(this, "Canceled", Toast.LENGTH_SHORT).show();isRecognizering = false;});}private void parseOCRResult(Text text) {// 所有识别到的内容，下同String textContent = text.getText();if (textContent == null || textContent.trim().length() == 0) {return;}ocrArea.clear();// 块，段落for (Text.TextBlock textBlock : text.getTextBlocks()) {// 一行文本for (Text.Line line : textBlock.getLines()) {drawResult(line);// 元素：单词，对汉字来说，需要"开头 (分隔符)中间（分隔符） 结尾"之类比较强烈的分隔符去分隔for (Text.Element element : line.getElements()) {// symbol：字符，字母，字for (Text.Symbol symbol : element.getSymbols()) {symbol.getText();}}}}}private void drawResult(Text.Line line) {// line的旋转角度（以度为单位，顺时针为正，范围为[-180, 180]）float angle = line.getAngle() + ORIENTATION;// 检测到的文本的轴对齐边界矩形Rect boundingBox = line.getBoundingBox();// 从左上角开始顺时针方向的四个角点。不带旋转角度，如果设置过旋转角度camera.setDisplayOrientation，需要进行旋转Point[] cornerPoints = line.getCornerPoints();// 置信度float confidence = line.getConfidence();// 获取文本中的主要语言（如果有的话）String recognizedLanguage = line.getRecognizedLanguage();// 置信度太低的过滤掉if (confidence < 0.3f) {return;}for (Point cornerPoint : cornerPoints) {float[] floats = {cornerPoint.x, cornerPoint.y};matrix.mapPoints(floats);cornerPoint.x = (int) floats[0] + (WIDTH >> 1);cornerPoint.y = (int) floats[1] + (HEIGHT >> 1);}ocrArea.add(cornerPoints, line.getText());ocrArea.postInvalidate();}/*** Convert camera data into bitmap data.*/private Bitmap convertToBitmap(Camera camera, byte[] data) {int width = camera.getParameters().getPreviewSize().width;int height = camera.getParameters().getPreviewSize().height;YuvImage yuv = new YuvImage(data, ImageFormat.NV21, width, height, null);ByteArrayOutputStream stream = new ByteArrayOutputStream();yuv.compressToJpeg(new Rect(0, 0, width, height), 100, stream);return BitmapFactory.decodeByteArray(stream.toByteArray(), 0, stream.toByteArray().length);}@Overrideprotected void onResume() {super.onResume();}@Overrideprotected void onRestart() {super.onRestart();}@Overrideprotected void onDestroy() {super.onDestroy();if (recognizer != null) {recognizer.close();}}@Overridepublic void surfaceCreated(@NonNull SurfaceHolder holder) {try {camera = Camera.open(Camera.CameraInfo.CAMERA_FACING_BACK);parameters = camera.getParameters();// 旋转了90度，所以height、width互换parameters.setPictureSize(HEIGHT, WIDTH);parameters.setFocusMode(Camera.Parameters.FOCUS_MODE_CONTINUOUS_PICTURE);parameters.setPictureFormat(ImageFormat.NV21);camera.setPreviewDisplay(holder);camera.setDisplayOrientation(ORIENTATION);camera.setParameters(parameters);} catch (Exception exception) {Log.i(TAG, exception.getMessage());}}@Overridepublic void surfaceChanged(@NonNull SurfaceHolder holder, int format, int width, int height) {if (camera != null) {camera.stopPreview();camera.setPreviewCallback(null);camera.startPreview();camera.setPreviewCallback(this);ocrArea.clear();stopRecognizer = true;ocrSwitch.performClick();}}@Overridepublic void surfaceDestroyed(@NonNull SurfaceHolder holder) {if (camera != null) {camera.stopPreview();camera.setPreviewCallback(null);camera.release();}}
}

参考文章

1. 文字识别 v2