title: Error on countNonZero when used to compare two Mats
body: <p>I took screenshots of each slide of the powerpoint and stored them into an <code>ArrayList</code> of Mats, I am trying to see when I reach the end of the slide show so I use a compare function. However, the implementation gives an exception on the <code>countNonZero</code> function: </p>
  
  <blockquote>
    <p>E/cv::error(): OpenCV(3.4.5) Error: Assertion failed (cn == 1) in int
    cv::countNonZero(cv::InputArray), file
    /build/3_4_pack-android/opencv/modules/core/src/count_non_zero.cpp,
    line 298 E/org.opencv.core: core::countNonZero_10() caught
    cv::Exception: OpenCV(3.4.5)
    /build/3_4_pack-android/opencv/modules/core/src/count_non_zero.cpp:298:
    error: (-215:Assertion failed) cn == 1 in function 'int
    cv::countNonZero(cv::InputArray)' D/AndroidRuntime: Shutting down VM
    E/AndroidRuntime: FATAL EXCEPTION: main
        Process: com.example.autoslide, PID: 27129
        CvException [org.opencv.core.CvException: cv::Exception: OpenCV(3.4.5)
    /build/3_4_pack-android/opencv/modules/core/src/count_non_zero.cpp:298:
    error: (-215:Assertion failed) cn == 1 in function 'int
    cv::countNonZero(cv::InputArray)'
        ]</p>
  </blockquote>
  
  <p><strong>Here is the code:</strong></p>
  
  <pre><code>private void getScreens(WebView view) {
          int i = 0;
          do {
              Bitmap b = Screenshot.takeScreenshot(view);
              //imageView.setImageBitmap(b);
              Mat mat = new Mat();
              Utils.bitmapToMat(b.copy(Bitmap.Config.ARGB_8888, true), mat);
              screens.add(mat);
              view.dispatchKeyEvent(new KeyEvent(KeyEvent.ACTION_DOWN, KeyEvent.KEYCODE_DPAD_RIGHT));
              i++;
              //System.out.println(screens.size());
          } while(i &lt;= 4 || !(matEquals(screens.get(i-1), screens.get(i-2))));
  
  
  
      }
  
      public static boolean matEquals(Mat img1, Mat img2){
          Mat out = new Mat();
          Core.compare(img1, img2, out, Core.CMP_NE);
          return Core.countNonZero(out) == 0;
      } 
  </code></pre>