What's the difference between Mat::clone and Mat::copyTo?

Question

I know 'copyTo' can handle mask. But when mask is not needed, can I use both equally?

http://docs.opencv.org/modules/core/doc/basic_structures.html#mat-clone

without mask both are same.. – rotating_image Mar 28 '13 at 03:14 — rotating_image, Mar 28 '13 at 03:14
Thanks. I should read source code not just documents. – ITO Yosei Mar 30 '13 at 05:30 — ITO Yosei, Mar 30 '13 at 05:30

score 78 · Accepted Answer · answered Jul 06 '15 at 01:23

Actually, they are NOT the same even without mask.

The major difference is that when the destination matrix and the source matrix have the same type and size, copyTo will not change the address of the destination matrix, while clone will always allocate a new address for the destination matrix.

This is important when the destination matrix is copied using copy assignment operator before copyTo or clone. For example,

Using copyTo:

Mat mat1 = Mat::ones(1, 5, CV_32F);
Mat mat2 = mat1;
Mat mat3 = Mat::zeros(1, 5, CV_32F);
mat3.copyTo(mat1);
cout << mat1 << endl;
cout << mat2 << endl;

Output:

[0, 0, 0, 0, 0]
[0, 0, 0, 0, 0]

Using clone:

Mat mat1 = Mat::ones(1, 5, CV_32F);
Mat mat2 = mat1;
Mat mat3 = Mat::zeros(1, 5, CV_32F);
mat1 = mat3.clone();
cout << mat1 << endl;
cout << mat2 << endl;

Output:

[0, 0, 0, 0, 0]
[1, 1, 1, 1, 1]

score 33 · Answer 2 · answered Mar 28 '13 at 17:19

33

This is the implementation of Mat::clone() function:

inline Mat Mat::clone() const
{
  Mat m;
  copyTo(m);
  return m;
}

So, as @rotating_image had mentioned, if you don't provide mask for copyTo() function, it's same as clone().

answered Mar 28 '13 at 17:19

Safir

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Whats the difference from clone to assignment? Like: Mat m1; Mat m2; m1 = m2; – Derzu May 06 '18 at 00:59
What is the `const` after `Mat::clone()` ? – Hasani Jun 17 '18 at 08:55
What you wrote is not right. See the answer below of @yangjie and OpenCV documentation: "When the operation mask is specified, if the Mat::create call shown above reallocates the matrix, the newly allocated matrix is initialized with all zeros before copying the data." but when you DO NOT provide a mask data are copied without reallocation, this means that other Mat pointing to that matrix will have values modified! – giuseppe Sep 24 '18 at 13:39

score 24 · Answer 3 · answered Mar 28 '13 at 17:22

24

Mat::copyTo is for when you already have a destination cv::Mat that (may be or) is already allocated with the right data size. Mat::clone is a convenience for when you know you have to allocate a new cv::Mat.

answered Mar 28 '13 at 17:22

Timothy Shields

75,459
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score 3 · Answer 4 · answered Jan 21 '19 at 15:59

3

copyTo doesn't allocate new memory in the heap which is faster.

answered Jan 21 '19 at 15:59

Nathan B

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What's the difference between Mat::clone and Mat::copyTo?

4 Answers4

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