Detect an ellipse and get its dimensions

1

Hi,

I don’t have much experience in OpenCV and I would like a bit of help in the program I work on.
I’m working with an image like this one below :

Sample_2
Sample_2800×600 145 KB

The idea would be to detect those shapes and get their dimensions.
For know I’m using a gaussian blur to get rid of the noise and a SimpleBlobDetector to find the shapes on the image.

What can I do now to get theses shapes dimensions knowing that I will only have to process ellipses ?

2

SimpleBlobDetector should be able to filter for circularity.

if you need more control over what’s going on, I’d recommend connectedComponentsWithStats or findContours, and then go from there

there’s fitEllipse, which you’d apply to a contour. this example shows minAreaRect and fitEllipse, which are independent of each other OpenCV: Creating Bounding rotated boxes and ellipses for contours

3

Seems to work,

image
image798×605 37.3 KB

Here is my code :

            Mat imgCanny = new Mat();
            Mat imgContours = new Mat();
            Mat<OpenCvSharp.Point>[] points;

            List<RotatedRect> rectangles = new List<RotatedRect>();

            RotatedRect rect = new RotatedRect();
            OpenCvSharp.Size imgSize = new OpenCvSharp.Size(9, 9);

            HierarchyIndex[] index;

            //Filtrage (inversion → flou gaussien → Canny)
            img = Cv2.ImRead(imagePath, ImreadModes.Grayscale);
            Cv2.BitwiseNot(img, img);
            Cv2.GaussianBlur(img, img, imgSize, 4);
            Cv2.Canny(img, imgCanny, _MinThreshold, _MaxThreshold);

            //Recherche des contours
            points = Cv2.FindContoursAsMat(imgCanny, RetrievalModes.External,ContourApproximationModes.ApproxSimple);

            //Pour chaque forme trouvée, on détermine l'ellipse correspondante
            for(int i = 0; i<points.Length; i++)
            rectangles.Add(Cv2.FitEllipse(points[i]));

            Cv2.CvtColor(img, img, ColorConversionCodes.GRAY2BGR);

            //On affiche toute les éllipses trouvées
            foreach(RotatedRect rectangle in rectangles)
            Cv2.Ellipse(img, rectangle, Scalar.Red);
            
            var w3 = new Window("Ellipses", img);

Thank you very much !