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OpenCV Error: Assertion failed

asked 2014-11-05 09:03:36 -0500

mp352 gravatar image


hopefully my english is good enough for describing my problem. I am working only a short time on opencv. In a little project I tried to detect the number plates from cars. The detection works very well. Only the last function in my script throws an exception.

I tried to create a new mat from the picture and the rectangle but that throws the following exception: OpenCV Error: Assertion failed (0 <= roi.x && 0 <= roi.width && roi.x + roi.width <= m.cols && 0 <= roi.y && 0 <= roi.height && roi.y + roi.height <= m.rows) in cv::Mat::Mat, file ........\opencv\modules\core\src\matrix.cpp, line 323 Exception in thread "main" CvException [org.opencv.core.CvException: cv::Exception: ........\opencv\modules\core\src\matrix.cpp:323: error: (-215) 0 <= roi.x && 0 <= roi.width && roi.x + roi.width <= m.cols && 0 <= roi.y && 0 <= roi.height && roi.y + roi.height <= m.rows in function cv::Mat::Mat

But the detection works fine. the picture shows that the script creates rectangles around them. image description

Could somebody help me?


* To change this license header, choose License Headers in Project Properties. * To change this template file, choose Tools | Templates * and open the template in the editor. */ package demoOpenCV;

import java.util.ArrayList; import java.util.Iterator; import java.util.ListIterator; import org.opencv.core.Core; import org.opencv.core.CvType; import org.opencv.core.Mat; import org.opencv.core.MatOfPoint; import org.opencv.core.MatOfPoint2f; import org.opencv.core.Point; import org.opencv.core.Rect; import org.opencv.core.MatOfRect; import org.opencv.core.RotatedRect; import org.opencv.core.Scalar; import org.opencv.core.Size; import org.opencv.highgui.Highgui; import org.opencv.imgproc.Imgproc; import static org.opencv.imgproc.Imgproc.MORPH_RECT;


  • @author Manuel */ public class NumberPlate {

    public void run() { System.out.println("Kennzeichenerkennung is running...");

    Mat img = Highgui.imread("c:/tmp/plate2.jpg");
    Mat kopie = new Mat();
    Mat img_grey = new Mat();
    Mat img_sobel = new Mat();
    Mat img_threshold = new Mat();
    Mat element = Imgproc.getStructuringElement(MORPH_RECT, new Size(17, 3));
    ArrayList<RotatedRect> rects = new ArrayList<RotatedRect>();
    ArrayList<MatOfPoint> contors = new ArrayList<MatOfPoint>();
    Mat schild;
    Imgproc.cvtColor(img, img_grey, Imgproc.COLOR_BGR2GRAY);
    Imgproc.blur(img_grey, img_grey, new Size(5, 5));
    Imgproc.Sobel(img_grey, img_sobel, CvType.CV_8U, 1, 0, 3, 1, 0);
    Imgproc.threshold(img_sobel, img_threshold, 0, 255,
            ((Imgproc.THRESH_OTSU) + (Imgproc.THRESH_BINARY)));
    Imgproc.morphologyEx(img_threshold, img_threshold, Imgproc.MORPH_CLOSE, element);
    Imgproc.findContours(img_threshold, contors, new Mat(), Imgproc.RETR_EXTERNAL, Imgproc.CHAIN_APPROX_NONE);
    ListIterator<MatOfPoint> it = contors.listIterator();
    while (it.hasNext()) {
        MatOfPoint2f mp2f = new MatOfPoint2f(;
        RotatedRect mr = Imgproc.minAreaRect(mp2f);
        double area = Math.abs(Imgproc.contourArea(mp2f));
    double bbArea = mr.size.area();            
    double ratio = area / bbArea;            
    if ((ratio &lt; 0.45) || (bbArea &lt; 400)) {                
    } else {                
    } for (Iterator<RotatedRect> it1 = rects.iterator(); it1.hasNext();) { RotatedRect roRe =; Rect r = roRe.boundingRect();
    Core.rectangle(kopie, new Point((r.x), (r.y)),
            new Point((r.x + r.width), (r.y + r.height)),
            new Scalar(0, 0, 255));
    ***schild = kopie.submat(r);***
    Imgproc.blur ...
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answered 2014-11-05 09:12:37 -0500

unxnut gravatar image

You should print the dimensions of the rectangles prior to calling the OpenCV function and see if they are as expected. I have a feeling that the thresholding and closing operations may give you some different results than what you expect and that may result in an exception.

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Thanks for answering. I have checked the koordinates, widths and heights of the detected rects. One of them has a calculated koordinate r.width+r.x which is wider than the orignial. The original has a width of 505px and one of the rects 507px. How could I avoid this exception?


mp352 gravatar imagemp352 ( 2014-11-05 10:04:06 -0500 )edit

You will need to investigate why such a condition arises, especially if it should not arise. The problem may be in the code preceding the exception point. Debugger can help.

unxnut gravatar imageunxnut ( 2014-11-05 13:09:58 -0500 )edit
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Asked: 2014-11-05 09:03:36 -0500

Seen: 12,665 times

Last updated: Nov 05 '14