java.lang.NullPointerException: Attempt to read from field 'long org.opencv.core.Mat.nativeObj' on a null object reference

问题

I am new to openCV and android. I am trying to load an image from Resources using bitmap and process the image using open CV. It gave me this error:

java.lang.NullPointerException: 
Attempt to read from field 'long org.opencv.core.Mat.nativeObj' on a null object reference

Code:

    Bitmap bm = BitmapFactory.decodeResource(getResources(),R.drawable.a1);
    ((ImageView)findViewById(R.id.imageView)).setImageBitmap(bm);
    Mat trainImage = new Mat(bm.getHeight(),bm.getWidth(),CvType.CV_32S);
    Utils.bitmapToMat(bm,trainImage);
    if(trainImage.empty()){
        Log.e(TAG,"Empty image");
    }
    else{
        Log.e(TAG,"Image Loaded");
    }
    Imgproc.cvtColor(trainImage, trainGray, Imgproc.COLOR_RGBA2GRAY);

Here is the Exception message:

E/SVM: Image Loaded
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.pragyan.facefeatures, PID: 3308
java.lang.NullPointerException: Attempt to read from field 'long org.opencv.core.Mat.nativeObj' on a null object reference
  at org.opencv.imgproc.Imgproc.cvtColor(Imgproc.java:1724)
  at com.example.pragyan.facefeatures.ExtractFeature.trainSvm(ExtractFeature.java:242)
  at com.example.pragyan.facefeatures.ExtractFeature.onOptionsItemSelected(ExtractFeature.java:313)
  at android.app.Activity.onMenuItemSelected(Activity.java:2885)
  at android.support.v4.app.FragmentActivity.onMenuItemSelected(FragmentActivity.java:403)
  at android.support.v7.app.AppCompatActivity.onMenuItemSelected(AppCompatActivity.java:189)
  at android.support.v7.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:100)
  at android.support.v7.app.AppCompatDelegateImplV7.onMenuItemSelected(AppCompatDelegateImplV7.java:663)
  at android.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:811)
  at android.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:152)
  at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:958)
  at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:948)
  at android.support.v7.view.menu.MenuPopupHelper.onItemClick(MenuPopupHelper.java:191)
  at android.widget.AdapterView.performItemClick(AdapterView.java:305)
  at android.widget.AbsListView.performItemClick(AbsListView.java:1146)
  at android.widget.AbsListView$PerformClick.run(AbsListView.java:3053)
  at android.widget.AbsListView$3.run(AbsListView.java:3860)
  at android.os.Handler.handleCallback(Handler.java:739)
  at android.os.Handler.dispatchMessage(Handler.java:95)
  at android.os.Looper.loop(Looper.java:135)
  at android.app.ActivityThread.main(ActivityThread.java:5254)
  at java.lang.reflect.Method.invoke(Native Method)
  at java.lang.reflect.Method.invoke(Method.java:372)
  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

Can anyone tell me how to fix this. The image is loaded successfully and it still gives a NullPointerException.

回答1:

I think you've not initialized the Mat trainGray. Try the following.

Mat trainGray = new Mat();
Imgproc.cvtColor(trainImage, trainGray, Imgproc.COLOR_RGBA2GRAY);

来源：https://stackoverflow.com/questions/38195782/java-lang-nullpointerexception-attempt-to-read-from-field-long-org-opencv-core

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