Deep Copy of OpenCV cv::Mat

Question

The behaviour of copying cv::Mat is confusing me.

I understand from the documentation that Mat::copyTo() is deep copy while the assignment ope

Answer 1

I've been using OpenCV for a while now and the cv::Mat confused me too, so I did some reading.

cv::Mat is a header that points to a *data pointer which holds the actual image data. It also implements reference counting. it holds the number of cv::Mat headers currently pointing to that *data pointer. So when you do a regular copy such as:

cv::Mat b; 
cv::Mat a = b;

a will point to b's data and the reference count for it will be incremented. At the same time, the reference count for the data previously pointed to by b will be decremented (and the memory will be freed if it is 0 after decrementing).

Question 1: It depends on your program. Please refer to this question for more details: is-cvmat-class-flawed-by-design

Question 2: the function returns by value. That means return image will copy the Mat and increase the ref count(now ref_count = 2) and return the new Mat. When the function ends, the image will be destroyed and ref_count will be reduced by one. But the memory will not be freed since the ref_count is not 0. So the returned cv::Mat is not pointing to random memory location.

Question 3: A similar thing happens. When you say orgImage2.copyTo(aCopy); The ref_count for the data pointed to by aCopy will be decreased. Then new memory is allocated to store the new data that will be copied. So That is why copyCopy1 was not modified when you did this.