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 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);
...