Split OpenCV Mat without copying the data - c++

Question

I have a RGB image and I trying to do some modification on R channel. So I do similar to the following:
Mat img;
vector<Mat> chs;
//....
split(img, chs);
//some modification on chs[2]
imshow("Result", img);
But it seems that OpenCV copy data to chs by value (not by reference). As a result the img matrix not changed. But due to memory limitations I don't prefer to use merge function.
Is there any alternative to split the matrix in-place?

Answer 1

split will always copy the data, since it's creating new matrices.
The simplest way to work on, say, red channel will be using split and merge:
Mat3b img(10,10,Vec3b(1,2,3));
vector<Mat1b> planes;
split(img, planes);
// Work on red plane
planes[2](2,3) = 5;
merge(planes, img);
Note that merge doesn't allocate any new memory, so if you're ok with split, there isn't any good reason not to call also merge.
You can always work on the R channel directly:
Mat3b img(10,10,Vec3b(1,2,3));
// Work on red channel, [2]
img(2,3)[2] = 5;
If you want to save the memory used by split, you can work directly on the red channel, but it's more cumbersome:
#include <opencv2\opencv.hpp>
using namespace cv;
int main()
{
Mat3b img(10,10,Vec3b(1,2,3));
// Create a column matrix header with red plane unwound
// No copies here
Mat1b R = img.reshape(1, img.rows*img.cols).colRange(2, 3);
// Work on red plane
int r = 2;
int c = 3;
// You need to access by index, not by (row, col).
// This will also modify img
R(img.rows * r + c) = 5;
return 0;
}
You can probably find a good compromise by copying the red channel only in a new matrix (avoiding to allocate space also for other channels), and then by copying the result back into original image:
#include <opencv2\opencv.hpp>
using namespace cv;
int main()
{
Mat3b img(10,10,Vec3b(1,2,3));
// Allocate space only for red channel
Mat1b R(img.rows, img.cols);
for (int r=0; r<img.rows; ++r)
for(int c=0; c<img.cols; ++c)
R(r, c) = img(r, c)[2];
// Work on red plane
R(2,3) = 5;
// Copy back into img
for (int r = 0; r<img.rows; ++r)
for (int c = 0; c<img.cols; ++c)
img(r, c)[2] = R(r,c);
return 0;
}
Thanks to #sturkmen for reviewing the answer