OpenCV：arcLength断言用cv :: Mat失败

Question

我尝试使用arcLength计算轮廓周长。轮廓从文件中读取到Mat，这是一张仅有轮廓的黑白图片。 但是，当我将此Mat传递给函数时，它会抛出错误：

Assertion failed (curve.checkVector(2) >= 0 && (curve.depth() == CV_32F || curve.depth() == CV_32S)) in arcLength

我已经发现实际原因是curve.checkVector(2)返回-1。虽然我已经阅读了有关此方法的文档，但我仍然不明白如何修复此错误 这是带角点（1,1），（1,21），（21,21），（21,1）的测试图像

Answer 1

轮廓应该是（来自OpenCV doc）：

  

输入2D点的矢量，存储在std :: vector或Mat。

中

不是黑白图像。

你可以通过不同的方式计算周长。最强大的是使用findContours仅查找外部轮廓（RETR_EXTERNAL），并在该轮廓上调用arcLength。

一些例子：

#include <opencv2\opencv.hpp>
#include <vector>

using namespace std;
using namespace cv;

int main()
{
    Mat1b img = imread("path_to_image", IMREAD_GRAYSCALE);

    // Method 1: length of unsorted points
    // NOTE: doesn't work!

    vector<Point> points;
    findNonZero(img, points);
    double len1 = arcLength(points, true);
    // 848.78

    // Method 2: length of the external contour

    vector<vector<Point>> contours;
    findContours(img.clone(), contours, RETR_EXTERNAL, CHAIN_APPROX_NONE); // Retrieve only external contour
    double len2 = arcLength(contours[0], true);
    // 80



    // Method 3: length of convex hull of contour
    // NOTE: convex hull based methods work reliably only for convex shapes.

    vector<Point> hull1;
    convexHull(contours[0], hull1);
    double len3 = arcLength(hull1, true);
    // 80

    // Method 4: length of convex hull of unsorted points
    // NOTE: convex hull based methods work reliably only for convex shapes.

    vector<Point> hull2;
    convexHull(points, hull2);
    double len4 = arcLength(hull2, true);
    // 80


    // Method 5: number of points in the contour
    // NOTE: this will simply count the number of points in the contour.
    // It works only if: 
    //     1) findContours was used with option CHAIN_APPROX_NONE.
    //     2) the contours is thin (has thickness of 1 pixel).

    double len5 = contours[0].size(); 
    // 80

    return 0;
}