OpenCV: findContours function error

Question

I am using 2.4.3 version of opencv, and trying to use "findContours" function after canny edge detection like this:

struct Component
{
    cv::Rect boundingBox;
    double area;
    double circularity;
}

cv::vector < Component > components;
cv::vector < cv::Vec4i > hierarchy;
cv::findContours ( cannyEdges, components, hierarchy, CV_RETR_CCOMP, CV_CHAIN_APPROX_NONE);

Then it throws an error for line line "cv::findContours" like this:

OpenCV Error: Assertion failed (mtype == type0 || ( CV_MAT_CN(mtype) == CV_MAT_CN(type0) && ((1((type0) & fixedDepthMask) != 0 )) in unknown function, file ...\opencv\modeuls\core\src\matrix.cpp, line 1421

How can I solve this?

Answer 1

cv::findcontours returns every contour as a vector of points (see http://docs.opencv.org/modules/imgproc/doc/structural_analysis_and_shape_descriptors.html#findcontours).

You have to convert these vectors to your data structure (Component) by yourself like in this minimal example I created:

#include <opencv2/opencv.hpp>
#include <iostream>
struct Component
{
    cv::Rect boundingBox;
    double area;
    double circularity;
};
int main()
{
    // Create a small image with a circle in it.
    cv::Mat image(256, 256, CV_8UC3, cv::Scalar(0, 0, 0));
    cv::circle(image, cv::Point(80, 110), 42, cv::Scalar(255,127, 63), -1);

    // Find canny edges.
    cv::Mat cannyEdges;
    cv::Canny(image, cannyEdges, 80, 60);

    // Show the images.
    cv::imshow("img", image);
    cv::imshow("cannyEdges", cannyEdges);

    // Find the contours in the canny image.
    cv::vector<cv::Vec4i> hierarchy;

    // "Each contour is stored as a vector of points."
    // http://docs.opencv.org/modules/imgproc/doc/structural_analysis_and_shape_descriptors.html#findcontours
    typedef cv::vector<cv::vector<cv::Point> > TContours;
    TContours contours;
    cv::findContours(cannyEdges, contours, hierarchy, CV_RETR_CCOMP, CV_CHAIN_APPROX_NONE);
    // cannyEdges is destroyed after calling cv::findContours

    // Print number of found contours.
    std::cout << "Found " << contours.size() << " contours." << std::endl;

    // Convert contours to Components.
    typedef cv::vector<Component> TComponents;
    TComponents components;
    for (TContours::const_iterator it( contours.begin() ); it != contours.end(); ++it)
    {
        Component c;
        c.area = cv::contourArea(*it);
        c.boundingBox = cv::boundingRect(*it);
        c.circularity = 0.0; // Insert whatever you mean by circularity;
        components.push_back(c);
    }

    for (TComponents::const_iterator it( components.begin() ); it != components.end(); ++it)
        std::cout << it->area << std::endl; // and whatever you want.

    // Wait for user input.
    cv::waitKey();
}