OpenCV contour discovery and contour drawing (C#)

Implementation process:

1. edge search
2. Contour discovery
3. Draw outline

API:

1. Find contours

public static void FindContours(InputArray image, out Point[][] contours, out HierarchyIndex[] hierarchy, RetrievalModes mode, ContourApproximationModes method, Point? offset = null);

parameter

image	Input image, grayscale image
contours	Contour output
hierarchy	Optional output vector containing information about the image topology
mode	Contour search mode
method	contour approximation method
offset	Optional parameter, offset, such as (-10, 10) means that the contour point is offset by 10 pixels in the negative X direction and 10 pixels in the positive Y direction.

Contour search mode:

• RetrievalModes.EXTERNAL - extract only the outermost contours
• RetrievalModes.LIST - extract all contours and place them in the list
• RetrievalModes.CCOMP - extracts all contours and organizes them into a two-level hierarchy: the top layer is the outer boundary of the connected domain, and the second layer is the inner boundary of the hole.
• RetrievalModes.TREE - extract all contours and reconstruct the entire hierarchy of nested contours

Contour approximation method:

• ContourApproximationModes.ApproxNone - Translate (convert) all points from chain code form to point sequence form
• ContourApproximationModes.ApproxSimple - compresses horizontal, vertical and diagonal divisions, that is, the function only retains the end pixels;
• ContourApproximationModes.ApproxTC89L1/ContourApproximationModes.ApproxTC89KCOS - applies the Teh-Chin chain approximation algorithm. CV_LINK_RUNS - uses a completely different contour extraction algorithm through horizontal fragments connected to 1. Only the CV_RETR_LIST extraction mode can be used in this method.

2. Draw outline

public static void DrawContours(InputOutputArray image, IEnumerable<IEnumerable<Point>> contours, int contourIdx, Scalar color, int thickness = 1, LineTypes lineType = LineTypes.Link8, IEnumerable<HierarchyIndex>? hierarchy = null, int maxLevel = int.MaxValue, Point? offset = null);

parameter

image	input image
contours	Contour output
contourIdx	Contour number
color	outline color
thickness	Line width
lineType	line type
hierarchy	Information about contour image topology
maxLevel	Maximum level for drawing outlines

code demo

private string picFile = "";
private Mat inputMat;

private Mat outMat;
private Mat displayMat;

private void button1_Click(object sender, EventArgs e)
{
    
    
    OpenFileDialog fileDialog = new OpenFileDialog();
    if (fileDialog.ShowDialog() == DialogResult.OK)
    {
    
    


        picFile = fileDialog.FileName;
        inputMat = Cv2.ImRead(picFile, ImreadModes.Grayscale);
        displayMat = Cv2.ImRead(picFile, ImreadModes.AnyColor);
        outMat = new Mat(new Size(inputMat.Cols, inputMat.Rows), inputMat.Type());

        Cv2.Canny(inputMat, outMat, 30, 90);

        //仅查找外轮廓
        Cv2.FindContours(outMat, out OpenCvSharp.Point[][] contours,out HierarchyIndex[] outputArray, RetrievalModes.External,ContourApproximationModes.ApproxNone);

        //绘制轮廓
        Scalar color = new  Scalar(255, 0, 0); 
        for (int i = 0; i < contours.Length; i++)
        {
    
    

            if (contours[i].Length > 100)
            {
    
    
                Cv2.DrawContours(displayMat,contours, i,color,5,LineTypes.Link8, outputArray, 0);
            }
        }





        picBox_Display.Image = OpenCvSharp.Extensions.BitmapConverter.ToBitmap(inputMat);
        picBox_After.Image = OpenCvSharp.Extensions.BitmapConverter.ToBitmap(displayMat);


    }


}

draw all contours

Draw only the outer contour

Additional notes:
The OpenCV library used in .NET in this case is OpenCvSharp4