OpenCV4 调用摄像头黑屏问题

OpenCV 调用 Android 摄像头这一块，我之前研究了好几天，都是一片黑，毫无头绪。后来发现 OpenCV4 要想调用摄像头，必须继承自 OpenCV 的 CameraActivity ！！！

CameraActivity.java 的源码如下，可以看出大部分代码都是为了 Android M（6.0）以上请求权限而生的，只有两个地方非常关键

1. protected List<? extends CameraBridgeViewBase> getCameraViewList() { …… }
   子 Activity 在继承 CameraActivity 后，需要复写该函数，把 JavaCamera2View 或 JavaCameraView 送入 List 作为返回值。

2. cameraBridgeViewBase.setCameraPermissionGranted()
   相机视图初始情况下是黑屏的，即不工作状态。只有当权限授予完毕，调用了 setCameraPermissionGranted 之后，OpenCV 才开始调用相机并把数据输出到 SurfaceView 上。

public class CameraActivity extends Activity {      private static final int CAMERA_PERMISSION_REQUEST_CODE = 200;      protected List<? extends CameraBridgeViewBase> getCameraViewList() {         return new ArrayList<CameraBridgeViewBase>();     }      protected void onCameraPermissionGranted() {         List<? extends CameraBridgeViewBase> cameraViews = getCameraViewList();         if (cameraViews == null) {             return;         }         for (CameraBridgeViewBase cameraBridgeViewBase: cameraViews) {             if (cameraBridgeViewBase != null) {                 cameraBridgeViewBase.setCameraPermissionGranted();             }         }     }      @Override     protected void onStart() {         super.onStart();         boolean havePermission = true;         if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {             if (checkSelfPermission(CAMERA) != PackageManager.PERMISSION_GRANTED) {                 requestPermissions(new String[]{CAMERA}, CAMERA_PERMISSION_REQUEST_CODE);                 havePermission = false;             }         }         if (havePermission) {             onCameraPermissionGranted();         }     }      @Override     @TargetApi(Build.VERSION_CODES.M)     public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {         if (requestCode == CAMERA_PERMISSION_REQUEST_CODE && grantResults.length > 0                 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {             onCameraPermissionGranted();         }         super.onRequestPermissionsResult(requestCode, permissions, grantResults);     } } 

实现要领

1. 首先，要继承 CameraActivity，之前已经说过了。这个基类会去申请权限，然后通知 javaCameraView 已获取到权限，可以正常使用。
2. 复写父类的 getCameraViewList 方法，将 javaCameraView 回送回去，这样当权限已被赋予时，就可以通知到预览界面开始正常工作了。
3. OpenCV 已经为我们实现了 Camera 和 Camera2 的函数，如果应用最低版本 minSdkVersion > 5.0，建议使用 JavaCamera2View 的相关函数，否则使用 JavaCameraView。
4. 在 onResume 时判断 opencv 库是否加载完毕，然后启用预览视图。在 onPause 时由于界面被遮挡，此时应该暂停摄像头的预览以节省手机性能和电量损耗。
5. Camera2 和 Camera 的绝大部分差异 OpenCV 均已经为我们屏蔽在类的内部了，唯一的差别就是两者实现的 CvCameraViewListener 监听器里的预览函数 onCameraFrame 的参数略有不同。从下面的源码可以看出 CvCameraViewListener2 的 inputFrame 由 Mat 类型改为了 CvCameraViewFrame 类型，它额外提供了一个转化为灰度图的接口。

CvCameraViewListener

public interface CvCameraViewListener {    /**      * This method is invoked when camera preview has started. After this method is invoked      * the frames will start to be delivered to client via the onCameraFrame() callback.      * @param width -  the width of the frames that will be delivered      * @param height - the height of the frames that will be delivered      */     public void onCameraViewStarted(int width, int height);      /**      * This method is invoked when camera preview has been stopped for some reason.      * No frames will be delivered via onCameraFrame() callback after this method is called.      */     public void onCameraViewStopped();      /**      * This method is invoked when delivery of the frame needs to be done.      * The returned values - is a modified frame which needs to be displayed on the screen.      * TODO: pass the parameters specifying the format of the frame (BPP, YUV or RGB and etc)      */     public Mat onCameraFrame(Mat inputFrame); } 

CvCameraViewListener2

public interface CvCameraViewListener2 {    /**      * This method is invoked when camera preview has started. After this method is invoked      * the frames will start to be delivered to client via the onCameraFrame() callback.      * @param width -  the width of the frames that will be delivered      * @param height - the height of the frames that will be delivered      */     public void onCameraViewStarted(int width, int height);      /**      * This method is invoked when camera preview has been stopped for some reason.      * No frames will be delivered via onCameraFrame() callback after this method is called.      */     public void onCameraViewStopped();      /**      * This method is invoked when delivery of the frame needs to be done.      * The returned values - is a modified frame which needs to be displayed on the screen.      * TODO: pass the parameters specifying the format of the frame (BPP, YUV or RGB and etc)      */     public Mat onCameraFrame(CvCameraViewFrame inputFrame); };  /**  * This class interface is abstract representation of single frame from camera for onCameraFrame callback  * Attention: Do not use objects, that represents this interface out of onCameraFrame callback!  */ public interface CvCameraViewFrame {      /**      * This method returns RGBA Mat with frame      */     public Mat rgba();      /**      * This method returns single channel gray scale Mat with frame      */     public Mat gray(); }; 

示例程序

下面使用 Camera2 来实现拍照功能（ 注意：Camera2 只能用于 Android 5.0 以上的手机 ）

Java代码

public class OpencvCameraActivity extends CameraActivity {      private static final String TAG = "OpencvCam";      private OpencvCameraActivity activity = this;     private JavaCamera2View javaCameraView;     private CameraBridgeViewBase.CvCameraViewListener2 cvCameraViewListener2 = new CameraBridgeViewBase.CvCameraViewListener2() {         @Override         public void onCameraViewStarted(int width, int height) {             Log.i(TAG, "onCameraViewStarted width=" + width + ", height=" + height);         }          @Override         public void onCameraViewStopped() {             Log.i(TAG, "onCameraViewStopped");         }          @Override         public Mat onCameraFrame(CameraBridgeViewBase.CvCameraViewFrame inputFrame) {             return inputFrame.rgba();      }  }      private BaseLoaderCallback baseLoaderCallback = new BaseLoaderCallback(this) {         @Override         public void onManagerConnected(int status) {             Log.i(TAG, "onManagerConnected status=" + status + ", javaCameraView=" + javaCameraView);             switch (status) {                 case LoaderCallbackInterface.SUCCESS: {                     if (javaCameraView != null) {                         javaCameraView.setCvCameraViewListener(cvCameraViewListener2);                         // 禁用帧率显示                         javaCameraView.disableFpsMeter();                         javaCameraView.enableView();                     }                 }                 break;                 default:                     super.onManagerConnected(status);                     break;             }         }     };   // 复写父类的 getCameraViewList 方法，把 javaCameraView 送到父 Activity，一旦权限被授予之后，javaCameraView 的 setCameraPermissionGranted 就会自动被调用。     @Override     protected List<? extends CameraBridgeViewBase> getCameraViewList() {         Log.i(TAG, "getCameraViewList");         List<CameraBridgeViewBase> list = new ArrayList<>();         list.add(javaCameraView);         return list;     }      @Override     protected void onCreate(Bundle savedInstanceState) {         super.onCreate(savedInstanceState);         setContentView(R.layout.activity_camera3);         javaCameraView = findViewById(R.id.javaCameraView);     }      @Override     public void onPause() {         Log.i(TAG, "onPause");         super.onPause();         if (javaCameraView != null) {             javaCameraView.disableView();         }     }      @Override     public void onResume() {         Log.i(TAG, "onResume");         super.onResume();         if (OpenCVLoader.initDebug()) {             Log.i(TAG, "initDebug true");             baseLoaderCallback.onManagerConnected(LoaderCallbackInterface.SUCCESS);         } else {             Log.i(TAG, "initDebug false");             OpenCVLoader.initAsync(OpenCVLoader.OPENCV_VERSION, this, baseLoaderCallback);         }     } } 

布局文件

布局文件很简单，核心就是这个 JavaCamera2View 视图

<?xml version="1.0" encoding="utf-8"?> <FrameLayout xmlns:android="https://schemas.android.com/apk/res/android"     xmlns:app="https://schemas.android.com/apk/res-auto"     android:layout_width="match_parent"     android:layout_height="match_parent">      <org.opencv.android.JavaCamera2View         android:id="@+id/javaCameraView"         android:layout_width="match_parent"         android:layout_height="match_parent"         app:camera_id="back"         app:show_fps="true" /> </FrameLayout>