FindContours轮廓发现

//函数原型1
void FindContours(InputArray image,
    out Mat[] contours,
    OutputArray hierarchy,
    RetrievalModes mode,
    ContourApproximationModes method,
    Point? offset = null)

//函数原型2
void FindContours(InputArray image,
    out Point[][] contours,
    out HierarchyIndex[] hierarchy,
    RetrievalModes mode,
    ContourApproximationModes method,
    Point? offset = null)

原灰度图

各参数图例对比

下面图例对比RetrievalModes和ContourApproximationModes不同时的不同结果

窗口标题是对应的RetrievalModes、ContourApproximationModes取值及总轮廓数

轮廓左上角的文字格式说明

第一个数字：表示当前轮廓序号

Next：下一个轮廓序号

Pre:Previous前一个轮廓序号

Child:子轮廓序号

Pa:Parent:父轮廓序号

Count:该轮廓包含的坐标个数

外轮廓为坐标为逆针，内轮廓坐标为顺时针

RetrievalModes为External时

只有外轮廓，没有层次关系(Child，Parent为-1）

RetrievalModes为List时

内、外轮廓都有，无层次关系

RetrievalModes为CComp时

内、外轮廓都有，也有层次关系，注意孔内轮廓3的Parent为轮廓2，轮廓2的Parent为-1（即也认为是顶层）

RetrievalModes为Tree时

内、外轮廓都有，完整的树形层次关系。注意轮廓9（对应上图的轮廓3）的Parent为轮廓8（对应上图的轮廓2），轮廓8的Parent为轮廓7。

RetrievalModes为FloodFill时

加了offset=(5,15),

DrawContours绘制轮廓

//函数原型1
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)

//函数原型2
void DrawContours(InputOutputArray image,
    IEnumerable<Mat> contours,
    int contourIdx,
    Scalar color,
    int thickness = 1,
    LineTypes lineType = LineTypes.Link8,
    Mat? hierarchy = null,
    int maxLevel = int.MaxValue,
    Point? offset = null)

contourIdx和maxLevel参数图例

测试图像

contourIdx和maxLevel分别为0,1,3,6时的示例(轮廓检索方式为Tree)

源码示例

public void Run()
{
    using var srcGray = Cv2.ImRead(ImagePath.Contours, ImreadModes.Grayscale);
    if (srcGray.Empty()) throw new Exception("图像读取有误");
    Cv2.ImShow("srcGray", srcGray);

    //需要去除噪声的话，可应用相关的模糊函数
    //Cv2.GaussianBlur(srcGray, srcGray, new Size(3, 3), 0);
    //用大律法二值化
    Cv2.Threshold(srcGray, srcGray, 0, 255, ThresholdTypes.Otsu | ThresholdTypes.BinaryInv);

    //轮廓偏移量
    var offset = new Point(5, 15);

    //测试各种RetrievalModes
    foreach (RetrievalModes mode in Enum.GetValues(typeof(RetrievalModes)))
    {
        //记录窗口时，为了自动排列窗口用
        List<string> winNames = new List<string>();
        //测试各种ContourApproximationModes
        foreach (ContourApproximationModes caMode in Enum.GetValues(typeof(ContourApproximationModes)))
        {
            try
            {
                Point[][] contoursP;
                HierarchyIndex[] hierarchyI;
                if (mode== RetrievalModes.FloodFill)
                {
                    using var floodFileMat = new Mat();
                    //用FloodFill方式时，需转为CV_32SC1
                    srcGray.ConvertTo(floodFileMat, MatType.CV_32SC1);
                    Cv2.FindContours(floodFileMat, out contoursP, out hierarchyI, mode, caMode, offset);
                }
                else
                {
                    //检索轮廓
                    Cv2.FindContours(srcGray, out contoursP, out hierarchyI, mode, caMode);              
                }                        
                //另一种调用方式
                //FindContours(InputArray image, out Mat[] contours, OutputArray hierarchy, RetrievalModes mode,
                //      ContourApproximationModes method, Point? offset = null)

                using var result = srcGray.CvtColor(ColorConversionCodes.GRAY2BGR);
                //绘制所有轮廓
                Cv2.DrawContours(result, contoursP, -1, Scalar.Red, 2);
                /*
                int index = 0;
                foreach (Point[] points in contoursP)
                {
                    var h = hierarchyI[index];
                    //在轮廓左上角加白底黑字提示文字
                    var textPoint = new Point(points[0].X - 30, points[0].Y - 12);
                    Helper.PutTextWithBack(result, $"{index},Next:{h.Next},Pre:{h.Previous},Child:{h.Child},Pa:{h.Parent},Count:{points.Count()}",
                        textPoint, fontScale: 0.3);
                    //文字与轮廓连一条线
                    Cv2.Line(result, points[0], textPoint, Scalar.Green, 1);
                    index++;
                }*/
                var winName = $"RetrievalModes:{mode},ContourApproximationModes:{caMode},contoursCount:{contoursP.Count()}";
                if (mode == RetrievalModes.FloodFill)
                {//测试参数offset
                    result.PutText($"offset=({offset.X},{offset.Y})", offset, HersheyFonts.HersheySimplex, 0.3, Scalar.Red);
                }
                Cv2.ImShow(winName, result);
                winNames.Add(winName);
            }
            catch(Exception ex)
            {
                Console.WriteLine(ex.Message);
            }                    
        }
        //窗口对齐
        //Helper.ArrangeWins(winNames, 2);
        Cv2.WaitKey();
        Cv2.DestroyAllWindows();
    }

    //用于测试多层次轮廓图像
    using var nContours = Cv2.ImRead(ImagePath.mContours, ImreadModes.Grayscale);
    Cv2.Threshold(nContours, nContours, 0, 255, ThresholdTypes.Otsu | ThresholdTypes.BinaryInv);
    //检索轮廓
    Cv2.FindContours(nContours, out Point[][] contoursPs, out HierarchyIndex[] hierarchyIs, RetrievalModes.Tree, ContourApproximationModes.ApproxSimple);

    for (int contourIdx = 0; contourIdx < contoursPs.Length; contourIdx++)
    {
        using var nContoursMat = nContours.CvtColor(ColorConversionCodes.GRAY2BGR);
        //绘制轮廓，测试contourIdx和maxLevel
        Cv2.DrawContours(nContoursMat, contoursPs, contourIdx, Scalar.Red, 2, hierarchy: hierarchyIs, maxLevel: contourIdx);
        //var textPoint = contoursPs[contourIdx][0];
        //var h = hierarchyIs[contourIdx];
        //Helper.PutTextWithBack(nContoursMat, $"{contourIdx},Next:{h.Next},Pre:{h.Previous},Child:{h.Child},Pa:{h.Parent}",
        //                textPoint, fontScale: 0.3);
        Cv2.ImShow($"contourIdx={contourIdx},maxLevel={contourIdx}", nContoursMat);
    }
    Cv2.WaitKey();
    Cv2.DestroyAllWindows();
}

OpenCvSharp函数示例(目录)

参考

https://edu.csdn.net/learn/38286/608275

https://docs.opencv.org/4.7.0/