An openFrameworks convex hull / defect index finder using opencv (because there is a bug in the opencv version). bug https://github.com/opencv/opencv/issues/4954

ConvexHull.cpp

//
// Copyright (c) 2017 Christopher Baker <https://christopherbaker.net>
//
// SPDX-License-Identifier:	MIT
//


#include "ConvexHull.h"


ConvexHull::ConvexHull()
{
}


ConvexHull::ConvexHull(const ofPolyline& contour)
{
    setContour(contour);
}


void ConvexHull::setContour(const ofPolyline& contour)
{
    _contour = contour;

    std::vector<cv::Point2i> contour2i;

    cv::Mat(ofxCv::toCv(_contour)).copyTo(contour2i);

    cv::Mat contourMat(contour2i);

    // By passing std::vector<int> we get indices.
    std::vector<int> hullIndices;
    cv::convexHull(contourMat, hullIndices);

    if (hullIndices.size() > 0 && contour.size() > 0)
    {
        std::vector<cv::Vec4i> convexityDefects;

        // TODO: has a bug
        // cv::convexityDefects(contourMat, hullIndices, convexityDefects);

        // Our local, drop-in, inefficient (?) hack.
        ConvexHull::convexityDefects(contourMat, hullIndices, convexityDefects);

        _convexHullDefects.clear();

        for (auto defect: convexityDefects)
        {
            Defect _defect;
            _defect.startIndex = defect[0];
            _defect.endIndex = defect[1];
            _defect.index = defect[2];
            _defect.depth = defect[3] / 256.0;
            _convexHullDefects.push_back(_defect);
        }
    }

    _convexHullIndices.clear();
    
    for (auto index: hullIndices)
    {
        if (index > -1)
        {
            _convexHullIndices.push_back(index);
        }
        else
        {
            ofLogWarning("ConvexHull::setContour") << "Found negative convex hull index, skipping.";
        }
    }
}


ofPolyline ConvexHull::contour() const
{
    return _contour;
}


ofPolyline ConvexHull::convexHull() const
{
    if (_convexHullIndices.size() != 0 && _convexHull.size() != _convexHullIndices.size())
    {
        _convexHull.clear();
        _convexHull.setClosed(true);

        for (auto index: _convexHullIndices)
        {
            _convexHull.addVertex(_contour[index]);
        }
    }

    return _convexHull;
}


std::vector<std::size_t> ConvexHull::convexHullIndices() const
{
    return _convexHullIndices;
}


std::vector<ConvexHull::Defect> ConvexHull::convexHullDefects() const
{
    return _convexHullDefects;
}


void ConvexHull::convexityDefects(const cv::Mat& _contourMat,
                                  const std::vector<int>& _hullIndices,
                                  std::vector<cv::Vec4i>& convexityDefects)
{
    // Make a sorted version of the hull indices.
    auto hullIndices = _hullIndices;
    std::sort(hullIndices.begin(), hullIndices.end());

    CvMat contourMat = _contourMat;
    CvMat hullMat = cvMat(1, hullIndices.size(), CV_32SC1, reinterpret_cast<void*>(&hullIndices[0]));
    CvMemStorage* storage = cvCreateMemStorage(0);
    CvSeq* defects = cvConvexityDefects(&contourMat, &hullMat, storage);

    for (auto i = 0; i < defects->total; ++i)
    {
        CvConvexityDefect* cur = reinterpret_cast<CvConvexityDefect*>(cvGetSeqElem(defects, i));

        // Find the two convex hull indices.
        int startIndex = -1;
        int endIndex = -1;

        for (int hullIndex: hullIndices)
        {
            auto point = _contourMat.at<cv::Point2i>(hullIndex);

            if (startIndex < 0 || endIndex < 0)
            {
                if (startIndex < 0 &&
                    cur->start->x == point.x &&
                    cur->start->y == point.y)
                {
                    startIndex = hullIndex;
                }

                if (endIndex < 0 &&
                    cur->end->x == point.x &&
                    cur->end->y == point.y)
                {
                    endIndex = hullIndex;
                }
            }
            else
            {
                break;
            }
        }

        // Find the defec index by searching between the start and end hull indices.
        int defectIndex = -1;

        for (int index = startIndex; index <= std::max(_contourMat.total(), std::size_t(endIndex)); ++index)
        {
            auto point = _contourMat.at<cv::Point2i>(index);

            if (cur->depth_point->x == point.x &&
                cur->depth_point->y == point.y)
            {
                defectIndex = index;
                break;
            }
        }

        if (startIndex >= 0 && endIndex >= 0 && defectIndex >= 0)
        {
            cv::Vec4i defect;
            defect[0] = startIndex;
            defect[1] = endIndex;
            defect[2] = defectIndex;
            defect[3] = cur->depth * 256.0; // Convert it, to match C++ api.
            convexityDefects.push_back(defect);
        }
        else
        {
            ofLogError("ConvexHull::convexityDefects") << "Invalid index.";
        }
    }

    cvReleaseMemStorage(&storage);
}

ConvexHull.h

//
// Copyright (c) 2017 Christopher Baker <https://christopherbaker.net>
//
// SPDX-License-Identifier:	MIT
//


#pragma once


#include "ofxCv.h"


class ConvexHull
{
public:
    struct Defect
    {
        /// \brief The index of the point in the original contour.
        std::size_t index = 0;

        /// \brief The index of the defect start point in the original contour.
        ///
        /// This point corresponds to a point in the convex hull.
        std::size_t startIndex = 0;

        /// \brief The index of the defect end point in the original contour.
        ///
        /// This point corresponds to a point in the convex hull.
        std::size_t endIndex = 0;

        /// \brief The perpendicular intercept between.
        ///
        /// The vector describing the perpendicular
        glm::vec2 defectHullChordNormal;

        /// \brief The depth of the defect 
        ///
        /// The normal distance from the hull to the defect.
        float depth = 0;

    };

    /// \brief Create a convex hull.
    ConvexHull();

    /// \brief Create a cotext hull.
    /// \param contour The contour to analyze.
    ConvexHull(const ofPolyline& contour);

    void setContour(const ofPolyline& contour);

    /// \brief Get the original contour.
    ofPolyline contour() const;

    /// \brief Create the convex hull from the contour.
    ofPolyline convexHull() const;

    /// \returns the indices of the contour that represent the convex hull.
    std::vector<std::size_t> convexHullIndices() const;

    /// \returns the indices of the contour that represent the convex hull defects.
    std::vector<Defect> convexHullDefects() const;


private:
    /// \brief The original contour.
    ofPolyline _contour;

    /// \brief The indices of the contour that represent the convex hull.
    std::vector<std::size_t> _convexHullIndices;

    /// \brief The convex hull.
    mutable ofPolyline _convexHull;

    /// \brief the indices of the contour that represent the convex hull defects.
    std::vector<Defect> _convexHullDefects;

    /// \brief Custom C++ implementation that finds indices.
    static void convexityDefects(const cv::Mat& contourMat,
                                 const std::vector<int>& hullIndices,
                                 std::vector<cv::Vec4i>& convexityDefects);

};

Yes, I developed and tested on macos and Linux. So it definitely sounds like a VS bug.

sweet! was a VS bug, thanks a lot for your help!