omp reduction on vector of cv::Mat or cv::Mat in general

Question

//In other words, this equilavent to cv::Mat1f mat(5,n)
//i.e. a matrix 5xn
std::vector mat(5,cv::Mat1f::zeros(1,n));
std::vector indexes(m         


        

Answer 1

Types such as cv::Mat1f, that use references instead of copying, are indeed dangerous in this context. You make a clear explicit solution by splitting the parallel region and the for loop.

#pragma omp declare reduction(vec_mat1f_plus : std::vector<cv::Mat1f> : \
            std::transform(omp_out.begin(), omp_out.end(), omp_in.begin(), omp_out.begin(), std::plus<cv::Mat1f>()));
// initializer not necessary if you initialize explicitly

std::vector<cv::Mat1f> mat;
#pragma omp parallel reduction(vec_mat1f_plus : mat)
{
  mat = std::vector<cv::Mat1f>(5);
  for (auto& elem : mat) {
    elem = cv:Mat1f::zeros(1, n);
  }
  #pragma omp for
  for(size_t i=0; i<m; i++){
    mat[indexes[m]] += 1;
  }
}

^{I haven't tested whether std::plus<cv::Mat1f> works, but it looks good.}

Your approach with vectMat will also work if you provide an operator= that deep-copies the underlying Mat with clone(), and keep the initializer.