OpenCV：arcLength 断言失败并出现 cv::Mat

Question

我尝试使用arcLength 计算轮廓周长。轮廓从文件中读入Mat，它只是轮廓的黑白图片。 但是，当我将此 Mat 传递给函数时，它会引发错误：

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

我发现真正的原因是curve.checkVector(2) 返回-1。尽管我已阅读有关此方法的文档，但我仍然不明白如何修复此错误。
这是带有角点的测试图像 (1,1), (1,21), (21,21), (21,1)

Answer 1

轮廓应该是（来自OpenCV doc）：

二维点的输入向量，存储在 std::vector 或 Mat 中。

不是黑白图像。

您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。

几个例子：

#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;
}