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I want to detect edges from images using Canny in Android, but it keeps throwing this error:

E/cv::error(): OpenCV Error: Assertion failed (_dx.type() == CV_16SC1 || _dx.type() == CV_16SC3) in void cv::Canny(cv::InputArray, cv::InputArray, cv::OutputArray, double, double, bool), file /build/master_pack-android/opencv/modules/imgproc/src/canny.cpp, line 959
    E/org.opencv.imgproc: imgproc::Canny_11() caught cv::Exception: /build/master_pack-android/opencv/modules/imgproc/src/canny.cpp:959: error: (-215) _dx.type() == CV_16SC1 || _dx.type() == CV_16SC3 in function void cv::Canny(cv::InputArray, cv::InputArray, cv::OutputArray, double, double, bool)
    W/System.err: CvException [org.opencv.core.CvException: cv::Exception: /build/master_pack-android/opencv/modules/imgproc/src/canny.cpp:959: error: (-215) _dx.type() == CV_16SC1 || _dx.type() == CV_16SC3 in function void cv::Canny(cv::InputArray, cv::InputArray, cv::OutputArray, double, double, bool)
    W/System.err: ]
    W/System.err:     at org.opencv.imgproc.Imgproc.Canny_1(Native Method)
    W/System.err:     at org.opencv.imgproc.Imgproc.Canny(Imgproc.java:984)
    W/System.err:     at app.android.opencvproject.OpenCVText.detectContours(OpenCVText.java:185)
    W/System.err:     at app.android.opencvproject.CameraPreview$1$1.run(CameraPreview.java:428)
    W/System.err:     at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234)
    W/System.err:     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
    W/System.err:     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
    W/System.err:     at java.lang.Thread.run(Thread.java:818)

I don't know what causes the error and I did not find any answer on the internet.

Here is the code I used:

public void detectContours(Bitmap bitmap){
        try{
            Mat img = new Mat();
            Utils.bitmapToMat(bitmap, img);

            Mat gray = new Mat(img.size(), CvType.CV_8UC1);
            Imgproc.cvtColor(img, gray, Imgproc.COLOR_RGB2GRAY, 4);
            Imgproc.Canny(gray, gray, 80, 100);

            Bitmap outputBitmap = Bitmap.createBitmap(gray.cols(), gray.rows(), Bitmap.Config.ARGB_8888);
            Utils.matToBitmap(gray, outputBitmap);
            MediaStore.Images.Media.insertImage(context.getContentResolver(), outputBitmap, "Opencv" , "Hello");
            Log.d("OPENCV", "Image stored in your gallery");
        }catch (Exception e){
            e.printStackTrace();
        }
    }

Can someone help me to solve this problem?

Thanks in advance!