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Each contour is stored as a vector of points (e.g. std::vector<std::vector<cv::Point> >)

the problem in your code is the line below

    Mat contours;
    findContours(skin,contours,RETR_EXTERNAL,CHAIN_APPROX_SIMPLE);

correct it like:

    vector<vector<Point> > contours;
    findContours(skin,contours,RETR_EXTERNAL,CHAIN_APPROX_SIMPLE);