Roi of a shared_ptr<mat> is not correct

C++
1

hi all,
I have a big image defined as :

img_whole_ = std::make_sharedcv::Mat(800, 5120, CV_8UC1)

and I want to divide it into four 800x1280 images.
If I do the following, my four images come out screwed up:

img1->data = ((*img_whole_)(cv::Range(0, 800),cv::Range(0 , 1280))).data;
img2->data = ((*img_whole_)(cv::Range(0, 800),cv::Range(1280, 2560))).data;
img3->data = ((*img_whole_)(cv::Range(0, 800),cv::Range(2560, 3840))).data;
img4->data = ((*img_whole_)(cv::Range(0, 800),cv::Range(3840, 5120))).data;

While if I create a new Mat this way:

cv::Mat pd1 = ((*img_whole_)(cv::Range(0, 800),cv::Range(0 , 1280)));

it works correctly.
The Step value of every Mat is correct.
What am I doing wrong?
Thanks

2

You don’t need std::make_sharedcv::Mat(800, 5120, CV_8UC1)
Mat already use shared data

instead of roi you should try

◆ Mat() [13/29]

cv::Mat::Mat ( int ndims,
const int * sizes,
int type,
void * data,
const size_t * steps = 0
)
OpenCV: cv::Mat Class Reference
3

I would strongly recommend formatting code as code, not as a comment. you will notice how your comment swallows some syntax.

5

thanks i will give it a try

6

using pointers.

this correctly saves the offsets of the roi within the ‘parent’ Mat

this only copies a (private !) data pointer, it neither updates refcounts or offsets/strides

7 
cv::Mat img_whole { 800, 5120, CV_8UC1 };
cv::Mat img1 { img_whole, cv::Range(0, 800), cv::Range(0, 1280) };
// ...

img1 is a view into img_whole. data can be altered through either of them.