OpenCV 3.1 convexityDefects 错误修正。

 3.1版本的convexityDefects 结果总是有错，没办法，只好看看源代码了。发现还真是有问题。

问题原因：在每次for(;;)循环开始前，没对变量 j 进行初始化。

修改为：在for(;;) 语句前加：j = hcurr;

其它版本是否有类似问题，大家可以找找看。

3.1版本的原码如下：

void convexityDefects( InputArray _points, InputArray _hull, OutputArray _defects )
{
    Mat points = _points.getMat();
    int i, j = 0, npoints = points.checkVector(2, CV_32S);
    CV_Assert( npoints >= 0 );


    if( npoints <= 3 )
    {
        _defects.release();
        return;
    }


    Mat hull = _hull.getMat();
    int hpoints = hull.checkVector(1, CV_32S);
    CV_Assert( hpoints > 2 );


    const Point* ptr = points.ptr();
    const int* hptr = hull.ptr();
    std::vector defects;


    // 1. recognize co-orientation of the contour and its hull
    bool rev_orientation = ((hptr[1] > hptr[0]) + (hptr[2] > hptr[1]) + (hptr[0] > hptr[2])) != 2;


    // 2. cycle through points and hull, compute defects
    int hcurr = hptr[rev_orientation ? 0 : hpoints-1];
    CV_Assert( 0 <= hcurr && hcurr < npoints );


    for( i = 0; i < hpoints; i++ )
    {
        int hnext = hptr[rev_orientation ? hpoints - i - 1 : i];
        CV_Assert( 0 <= hnext && hnext < npoints );


        Point pt0 = ptr[hcurr], pt1 = ptr[hnext];
        double dx0 = pt1.x - pt0.x;
        double dy0 = pt1.y - pt0.y;
        double scale = dx0 == 0 && dy0 == 0 ? 0. : 1./std::sqrt(dx0*dx0 + dy0*dy0);

        int defect_deepest_point = -1;
        double defect_depth = 0;
        bool is_defect = false;

        for(;;)
        {
            // go through points to achieve next hull point
            j++;
            j &= j >= npoints ? 0 : -1;
            if( j == hnext )
                break;

            // compute distance from current point to hull edge
            double dx = ptr[j].x - pt0.x;
            double dy = ptr[j].y - pt0.y;
            double dist = fabs(-dy0*dx + dx0*dy) * scale;

            if( dist > defect_depth )
            {
                defect_depth = dist;
                defect_deepest_point = j;
                is_defect = true;
            }
        }

        if( is_defect )
        {
            int idepth = cvRound(defect_depth*256);
            defects.push_back(Vec4i(hcurr, hnext, defect_deepest_point, idepth));
        }

        hcurr = hnext;
    }

    Mat(defects).copyTo(_defects);
}

修改完成以后，源代码为：

void convexityDefects( InputArray _points, InputArray _hull, OutputArray _defects )
{
    Mat points = _points.getMat();
    int i, j = 0, npoints = points.checkVector(2, CV_32S);
    CV_Assert( npoints >= 0 );


    if( npoints <= 3 )
    {
        _defects.release();
        return;
    }


    Mat hull = _hull.getMat();
    int hpoints = hull.checkVector(1, CV_32S);
    CV_Assert( hpoints > 2 );


    const Point* ptr = points.ptr();
    const int* hptr = hull.ptr();
    std::vector defects;


    // 1. recognize co-orientation of the contour and its hull
    bool rev_orientation = ((hptr[1] > hptr[0]) + (hptr[2] > hptr[1]) + (hptr[0] > hptr[2])) != 2;


    // 2. cycle through points and hull, compute defects
    int hcurr = hptr[rev_orientation ? 0 : hpoints-1];
    CV_Assert( 0 <= hcurr && hcurr < npoints );


    for( i = 0; i < hpoints; i++ )
    {
        int hnext = hptr[rev_orientation ? hpoints - i - 1 : i];
        CV_Assert( 0 <= hnext && hnext < npoints );


        Point pt0 = ptr[hcurr], pt1 = ptr[hnext];
        double dx0 = pt1.x - pt0.x;
        double dy0 = pt1.y - pt0.y;
        double scale = dx0 == 0 && dy0 == 0 ? 0. : 1./std::sqrt(dx0*dx0 + dy0*dy0);

        int defect_deepest_point = -1;
        double defect_depth = 0;
        bool is_defect = false;

j = hcurr;

        for(;;)
        {
            // go through points to achieve next hull point
            j++;
            j &= j >= npoints ? 0 : -1;
            if( j == hnext )
                break;

            // compute distance from current point to hull edge
            double dx = ptr[j].x - pt0.x;
            double dy = ptr[j].y - pt0.y;
            double dist = fabs(-dy0*dx + dx0*dy) * scale;

            if( dist > defect_depth )
            {
                defect_depth = dist;
                defect_deepest_point = j;
                is_defect = true;
            }
        }

        if( is_defect )
        {
            int idepth = cvRound(defect_depth*256);
            defects.push_back(Vec4i(hcurr, hnext, defect_deepest_point, idepth));
        }

        hcurr = hnext;
    }

    Mat(defects).copyTo(_defects);
}