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image data access in OpenCV 2 or later.

23 Nov

Scrapped from : http://tech.groups.yahoo.com/group/OpenCV/message/84478

Since 2009 with the launch of OpenCV 2, you can count with the new C++ interface, much better than the C style older version. I suggest two ways you can access pixels values. My notes are based on the book OpenCV 2 Computer Vision Application Programming Cookbook, by Robert Laganière.
1. Direct access to pixels values:
     // Test for gray level image
    if (image.channels() == 1) {
        image.at<uchar>(j, i) = …;           // j runs on rows, i runs on cols
    } else if (image.channels() == 3)   // color image
       image.at<cv::Vec3b>(j, i)[0] = …;
       image.at<cv::Vec3b>(j, i)[1] = …;
       image.at<cv::Vec3b>(j, i)[2] = …;
   }
   The image was created as: cv::Mat image = cv::imread(“img.???”);
2. Scanning image with pointer:
    uchar *pixel;
     cv::Mat image = cv::imread(“img.???”);
     . . .
    for (int j = 0; j < nRow; j++) {
          // Get the address of row j
          pixel = image.ptr<uchar>(j);
          for (int i = 0; i < nCol; i++) {
                // process each pixel
               pixel[i] = …;
               …
          }
     }
choose one method and good job!
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1 Comment

Posted by on November 23, 2011 in OpenCV

 

One response to “image data access in OpenCV 2 or later.

  1. kusemanohar

    April 17, 2013 at 11:59 am

    In the same book there is yet another method to access

    uchar * data = image.data;
    //Gives row-major format on the image
    //After this you can access the data in the same way as we did with opencv1

    This is especially useful if you are doing computation on GPUs

     

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