OpenCvSharp函数：ApproxPolyDP轮廓逼近

ApproxPolyDP轮廓逼近

函数说明：使用道格拉斯-普克算法（Douglas–Peucker algorithm）获取一个小于或等于指定精度(epsilon)的更少坐标集的轮廓(或曲线)。图片来自(道格拉斯普克算法)

//函数原型1
void ApproxPolyDP(InputArray curve,
    OutputArray approxCurve,
    double epsilon,
    bool closed)

//函数原型2
Point[] ApproxPolyDP(IEnumerable curve,
    double epsilon,
    bool closed)

//函数原型3
Point2f[] ApproxPolyDP(IEnumerable curve,
    double epsilon,
    bool closed)

参数	说明
InputArray curve IEnumerable curve IEnumerable curve	待计算的轮廓(曲线)
double epsilon	近似精度，为逼近结果与原轮廓(曲线)的最大差值
bool closed	轮廓(曲线)是否闭合
返回值OutputArray approxCurve	返回逼近后的轮廓(曲线)坐标集

图像示例

原图

逼近并矫正后结果

通过轮廓发现的轮廓个数是2864（蓝色边框），轮廓逼近后个数只剩4个（红点）

源代码示例

public void Run()
{
    using (var src = Cv2.ImRead(ImagePath.Book, ImreadModes.Color))
    {
        if (src.Empty()) throw new Exception("图像读取有误");
        var srcWinName = "src";

        ResizeWin(srcWinName, src);
        Cv2.ImShow("src", src);                

        using (var srcGray = new Mat())
        {
            Cv2.CvtColor(src, srcGray, ColorConversionCodes.BGR2GRAY);
            //中值模糊
            Cv2.MedianBlur(srcGray, srcGray, 15);
            //Otsu二值化
            Cv2.Threshold(srcGray, srcGray, 0, 255, ThresholdTypes.Otsu | ThresholdTypes.BinaryInv);

            var srcGrayWinName = "srcGray";
            ResizeWin(srcGrayWinName, srcGray);
            Cv2.ImShow(srcGrayWinName, srcGray);

            //轮廓发现
            Cv2.FindContours(srcGray, out Point[][] contours, out HierarchyIndex[] hierarchy, RetrievalModes.External, ContourApproximationModes.ApproxSimple);

            //找到最大轮廓
            var maxIndex = -1;
            double maxContourArea = 0;
            for (int index = 0; index < contours.Length; index++)
            {
                var area = Cv2.ContourArea(contours[index]);
                if (maxContourArea > area) continue;
                maxIndex = index;
                maxContourArea = area;
            }

            if (maxIndex > -1)
            {
                //绘制未逼近前的轮廓
                Cv2.DrawContours(src, new Point[][] { contours[maxIndex] }, -1, Scalar.Blue, 5);

                //轮廓逼近参数
                var epsilon = Cv2.ArcLength(contours[maxIndex], true) * 0.1;
                //轮廓逼近
                var approxContour = Cv2.ApproxPolyDP(contours[maxIndex], epsilon, true);

                //标记逼近后轮廓点
                foreach (var point in approxContour)
                {
                    Cv2.Circle(src, point, 10, Scalar.Red, -1);
                }

                //获取轮廓最小外接矩形（图像中书不是平行四边形，应该用霍夫变换的，但还没研究，先用这个简单处理吧）
                var RotatedRect = Cv2.MinAreaRect(approxContour);

                //获取旋转矩形，Angle为正数时是逆时针
                var rotMat = Cv2.GetRotationMatrix2D(RotatedRect.Center, RotatedRect.Angle, 1);

                //每个通道分开仿射变换
                var splits = Cv2.Split(src);
                for (int i = 0; i < splits.Length; i++)
                {
                    var dst = new Mat();
                    //仿射变换
                    Cv2.WarpAffine(splits[i], dst, rotMat, srcGray.Size());
                    splits[i] = dst;
                }
                Cv2.Merge(splits, src);

                //转化率逼近前后的个数
                Cv2.PutText(src, $"Before ApproxPolyDP Contour count={contours[maxIndex].Length},after ={approxContour.Length}", new Point(50, 50), HersheyFonts.HersheySimplex, 1.5, Scalar.Red,5);

                var srcResult = "ApproxPolyDP";
                ResizeWin(srcResult, src);
                Cv2.ImShow(srcResult, src);
            }                    
        }
    }
    Cv2.WaitKey();
    Cv2.DestroyAllWindows();
}

/// 

/// 缩放窗口
/// 
/// 
/// 
/// 
private void ResizeWin(string winName, Mat mat, int newWidth = 600)
{
    Cv2.NamedWindow(winName, WindowFlags.KeepRatio);
    Cv2.ResizeWindow(winName, newWidth, (int)(mat.Height * newWidth / mat.Width));
}

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OpenCvSharp函数示例(目录)

参考

https://edu.csdn.net/learn/38286/608280

https://docs.opencv.org/