精心制作图像时的例外情况：OpenCV

Question

我是OpenCV的新手，我不知道为什么会发生这种异常。

if (!frame.empty())
{
    List<MatOfPoint> contours = new ArrayList<>();
    Mat hierarchy = new Mat();
    // convert the image to gray scale
    Mat dilateElement = Imgproc.getStructuringElement(Imgproc.MORPH_RECT, new Size(24,24));
    Mat erodeElement = Imgproc.getStructuringElement(Imgproc.MORPH_RECT, new Size(12,12));
    Imgproc.cvtColor(frame, gray, Imgproc.COLOR_BGR2GRAY);
    Imgproc.threshold(gray, gray,128, 255, Imgproc.THRESH_BINARY);
    Imgproc.blur(gray, gray, new Size(7,7));
    Imgproc.erode(gray, gray, erodeElement);
    Imgproc.dilate(gray, gray, dilateElement);                                  
    Imgproc.findContours(gray, contours, hierarchy, Imgproc.RETR_LIST, Imgproc.CHAIN_APPROX_SIMPLE);
    while(contours != null)
    {
        MatOfPoint thisContour = new MatOfPoint();
        for (int idx = 0; idx >= 0; idx = (int) hierarchy.get(0, idx)[0]
        {
            MatOfPoint2f thisContour2f = new MatOfPoint2f();
            MatOfPoint approxContour = new MatOfPoint();
            MatOfPoint2f approxContour2f = new MatOfPoint2f();
            thisContour.convertTo(thisContour2f, CvType.CV_32FC2);
            Imgproc.approxPolyDP(thisContour2f,approxContour2f,0.02,true);
            approxContour2f.convertTo(approxContour, CvType.CV_32S);
            if(approxContour.size().height==4)
            {
                Rect ret = Imgproc.boundingRect(approxContour);
                text_field.setText("nice");
            }

我已经从c ++转换了代码。这就是错误。

    OpenCV Error: Assertion failed (npoints >= 0 && (depth == CV_32S || depth == CV_32F)) in cv::approxPolyDP, file C:\builds\master_PackSlaveAddon-win32-vc12-static\opencv\modules\imgproc\src\approx.cpp, line 679 Exception during the image elaboration: CvException [org.opencv.core.CvException: cv::Exception: C:\builds\master_PackSlaveAddon-win32-vc12-static\opencv\modules\imgproc\src\approx.cpp:679: error: (-215) npoints >= 0 && (depth == CV_32S || depth == CV_32F) in function cv::approxPolyDP]

我已经尝试了一切来解决它但没有成功。