Fast Element Access
===================

.. highlight:: cpp


Historically, OpenCV provided many different ways to access image and matrix elements, and none of them was both fast and convenient.
With the new data structures, OpenCV 2.x introduces a few more alternatives, hopefully more convenient than before. For detailed description of the operations, please, check 
:ref:`Mat`
and 
description. Here is part of the retro-photo-styling example rewritten (in simplified form) using the element access operations:




::


    
    ...
    // split the image into separate color planes
    vector<Mat> planes;
    split(img_yuv, planes);
    
    // method 1. process Y plane using an iterator
    MatIterator_<uchar> it = planes[0].begin<uchar>(),
                        it_end = planes[0].end<uchar>();
    for(; it != it_end; ++it)
    {
        double v = *it*1.7 + rand()
        *it = saturate_cast<uchar>(v*v/255.);
    }
    
    // method 2. process the first chroma plane using pre-stored row pointer.
    // method 3. process the second chroma plane using
    //           individual element access operations
    for( int y = 0; y < img_yuv.rows; y++ )
    {
        uchar* Uptr = planes[1].ptr<uchar>(y);
        for( int x = 0; x < img_yuv.cols; x++ )
        {
            Uptr[x] = saturate_cast<uchar>((Uptr[x]-128)/2 + 128);
            uchar& Vxy = planes[2].at<uchar>(y, x);
            Vxy = saturate_cast<uchar>((Vxy-128)/2 + 128);
        }
    }
    
    merge(planes, img_yuv);
    ...
    

..