Question

Using OpenCV 2.3.1. I am running cv::minMaxLoc() and it is returning the point (-1, -1) for both the minimum and maximum location, with 0 as the value for both. What does this mean?

Answer 1

Below are the relevant sections of code for the minMaxLoc function:

static void ofs2idx(const Mat& a, size_t ofs, int* idx)
{
    int i, d = a.dims;
    if( ofs > 0 )
    {
        ofs--;
        for( i = d-1; i >= 0; i-- )
        {
            int sz = a.size[i];
            idx[i] = (int)(ofs % sz);
            ofs /= sz;
        }
    }
    else
    {
        for( i = d-1; i >= 0; i-- )
            idx[i] = -1;
    }
}

}

void cv::minMaxIdx(InputArray _src, double* minVal,
                   double* maxVal, int* minIdx, int* maxIdx,
                   InputArray _mask)
{
    Mat src = _src.getMat(), mask = _mask.getMat();
    int depth = src.depth(), cn = src.channels();

    CV_Assert( (cn == 1 && (mask.empty() || mask.type() == CV_8U)) ||
               (cn >= 1 && mask.empty() && !minIdx && !maxIdx) );
    MinMaxIdxFunc func = minmaxTab[depth];
    CV_Assert( func != 0 );

    const Mat* arrays[] = {&src, &mask, 0};
    uchar* ptrs[2];
    NAryMatIterator it(arrays, ptrs);

    size_t minidx = 0, maxidx = 0;
    int iminval = INT_MAX, imaxval = INT_MIN;
    float fminval = FLT_MAX, fmaxval = -FLT_MAX;
    double dminval = DBL_MAX, dmaxval = -DBL_MAX;
    size_t startidx = 1;
    int *minval = &iminval, *maxval = &imaxval;
    int planeSize = (int)it.size*cn;

    if( depth == CV_32F )
        minval = (int*)&fminval, maxval = (int*)&fmaxval;
    else if( depth == CV_64F )
        minval = (int*)&dminval, maxval = (int*)&dmaxval;

    for( size_t i = 0; i < it.nplanes; i++, ++it, startidx += planeSize )
        func( ptrs[0], ptrs[1], minval, maxval, &minidx, &maxidx, planeSize, startidx );

    if( minidx == 0 )
        dminval = dmaxval = 0;
    else if( depth == CV_32F )
        dminval = fminval, dmaxval = fmaxval;
    else if( depth <= CV_32S )
        dminval = iminval, dmaxval = imaxval;

    if( minVal )
        *minVal = dminval;
    if( maxVal )
        *maxVal = dmaxval;

    if( minIdx )
        ofs2idx(src, minidx, minIdx);
    if( maxIdx )
        ofs2idx(src, maxidx, maxIdx);
}    

void cv::minMaxLoc( InputArray _img, double* minVal, double* maxVal,
                Point* minLoc, Point* maxLoc, InputArray mask )
{
    Mat img = _img.getMat();
    CV_Assert(img.dims <= 2);

    minMaxIdx(_img, minVal, maxVal, (int*)minLoc, (int*)maxLoc, mask);
    if( minLoc )
        std::swap(minLoc->x, minLoc->y);
    if( maxLoc )
        std::swap(maxLoc->x, maxLoc->y);
}

Judging from the code flow it appears you have the case of minidx equal to zero, which then sets dminval = dmaxval = 0. Also, within the ofs2idx function, when minidx (i.e., ofs parameter in ofs2idx) is equal to zero the logic sets the min and max points to (-1, -1) (i.e., idx[i] = -1;). This might happen if your matrix has no elements. What is the size of the matrix you are trying to use?