OpenCV speed up Mat iteration - c++

Question

I need to iterate throug a 3 channel Matrix in the HSV color space and it's awfully slow. It takes about 2.5 sec.
cv::Mat img = cv::imread( "image.jpg" );
cv::Mat img32FC3;
img.convertTo( img32FC3, CV_32FC3 );
cv::cvtColor( img32FC3, img32FC3, CV_BGR2HSV );
int height = img32FC3.rows;
int width = img32FC3.cols;
cv::Size size = img32FC3.size();
if( img32FC3.isContinuous() ) {
size.width *= size.height;
size.height = 1;
}
size.width *= 3;
for( int i = 0; i < size.height; i ++ ) {
float* ptr = img32FC3.ptr<float>(i);
for( int j = 0; j < size.width; j += 3 ) {
h = (ptr[ j ]);
s = (ptr[j +1 ]);
v = (ptr[j +2 ]);
}
}
cv::cvtColor( img32FC3, img32FC3, CV_HSV2BGR );
img32FC3.convertTo( img, CV_8UC3 );
imwrite("test.jpg", img );
The code above is adapted from the documentation of openCV where it states that it's performant. So I wonder how I can speed this up because 2.5 sec is really very very slow :(.
BTW: the image is 3744x5616 pixel

Answer 1

You should not directly access the data of a mat since sometimes it is not stored as you would expect. Check my answer here.
Another way is using the Mat iterator. According to the [opencv_tutorials.pdf, ver 2.4.2 pp. 89-92]
it is slightly slower than the nested loops from my link. (For a large image you are 5% slower but note that they used just MatIterator, not the const one which can be further optimised by a nice compiler.)
MatConstIterator_<Vec3b> it = M.begin<Vec3b>(), it_end = M.end<Vec3b>();
for(; it != it_end; ++it)
{
//do sth read-only otherwise use MatIterator_<Vec3b>
b = (*it)[0];
g = (*it)[1];
r = (*it)[2];
}
<double> if grayscale, from ocv2.4.2 refman p19.

Answer 2

Best chance to speed up is to parallelize the loop. OpenCV uses TBB as multithreading environment, you might want to check it. By the way, you dont need to do all your calculations about the size, etc. You already checked that your matrix isContinuous(), so you can get the pointer as float ptr = reinterpret_cast<float>(img32FC3.data) then your loop:
for (size_t i = ; i < img32FC3.rows*img32FC3.cols; ++i, ptr +=3) {
// do something
}