Yongtao Hu's blog

Data continuity of OpenCV Mat

Posted on Edited on Disqus:

Data stored in OpenCV cv::Mats are not always continuous in memory, which can be verified via API Mat::isContinuous(). Instead, it follows the following rules:

  1. Matrices created by imread(), clone(), or a constructor will always be continuous.
  2. The only time a matrix will not be continuous is when it borrows data from an existing matrix (i.e. created out of an ROI of a big mat), with the exception that the data borrowed is continuous in the big matrix, including
    • borrow a single row;
    • borrow multiple rows but with full original width.

The following code demonstrates this in a better way (see the inline comments for further explanation).

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#include <opencv2/opencv.hpp>

int main()
{
  std::vector<cv::Mat> mats(7);

  mats[0] = cv::Mat::ones(1000, 800, CV_32FC3);         // continuous as created using constructor
  mats[1] = mats[0](cv::Rect(100, 100, 300, 200));      // NOT continuous as borrowed data is not continuous (multiple rows and not full original width)
  mats[2] = mats[1].clone();                            // continuous as created using clone()
  mats[3] = mats[2].row(10);                            // continuous for single row always
  mats[4] = mats[2](cv::Rect(5, 5, 100, 2));            // NOT continuous as borrowed data is not continuous (multiple rows and not full original width) 
  mats[5] = mats[2](cv::Rect(0, 5, mats[2].cols, 2));   // continuous as borrowed data is continuous (multiple rows with full original width)
  mats[6] = mats[2].col(10);                            // NOT continuous as borrowed data is not continuous (multiple rows and not full original width) 

  for (size_t i = 0; i < mats.size(); ++i) {
    if (mats[i].isContinuous()) {
      std::cout << "Data of mats[" << int(i) << "] is continuous with size =" << mats[i].size() << std::endl;
    } else {
      std::cout << "Data of mats[" << int(i) << "] is NOT continuous with size =" << mats[i].size() << std::endl;
    }
  }

  system("pause");
  return 0;
}