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OpenCV: arcLength assert fails with cv::Mat

I tried to calculate contour perimeter using arcLength. The contour is read from the file into Mat which is a black and white picture of contour only. However, when I pass this Mat into function it throws an error:

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

I have figured out that the actual cause is that curve.checkVector(2) returns -1. Although I have read the documentation about this method I still do not understand how to fix this error.
Here is the test image with corner points (1,1), (1,21), (21,21), (21,1)

Solution

  • The contour should be (from OpenCV doc):

    Input vector of 2D points, stored in std::vector or Mat.

    not a b/w image.

    You can compute the perimeter is different ways. The most robust is to use findContours to find external contours only (RETR_EXTERNAL), and call arcLength on that contour.

    A few examples:

    #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;
}