cv::Mat + cv::Range does not work as expected for column assignment (Bug #1686)

Added by Nghia Ho almost 13 years ago. Updated almost 13 years ago.

Status:Cancelled Start date:2012-03-14
Priority:Normal Due date:
Assignee:Marina Kolpakova % Done:

0%
Category:core
Target version:2.4.0
Affected version: Operating System:
Difficulty: HW Platform:
Pull request:

Description

I have the following code that performs a column assignment to a matrix, but does not work as expected

 1#include <iostream>
 2#include <opencv2/core/core.hpp>
 3
 4using namespace std;
 5
 6int main()
 7{
 8    cv::Mat X = cv::Mat::zeros(3,4,CV_64F);
 9    cv::Mat A = cv::Mat::ones(3,1,CV_64F);
10
11    // This does not work as expected
12    X(cv::Range::all(), cv::Range(3,4)) = A;
13    cout << X << endl;
14
15    cout << endl;
16
17    // This works, but why?
18    X(cv::Range::all(), cv::Range(3,4)) = A*1;
19    cout << X << endl;
20
21    return 0;
22}

The output is:

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

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

History

#1
Updated by Marina Kolpakova almost 13 years ago

  • Description changed from I have the following code that performs a column assignment to a matrix, but ... to I have the following code that performs a column assignment to a matrix, but ... More

#2
Updated by Marina Kolpakova almost 13 years ago

This behavior is expected and documented.

Firstly, X(cv::Range::all(), cv::Range(3,4)) is a sub-matrix with its own header (size, type information and so on) and data referred to original X.

In following line

1X(cv::Range::all(), cv::Range(3,4)) = A;

the first signature of assignment operator is used Mat& Mat::operator=(const Mat& m). Header and data reference of temporal sub-matrix are updated.

In the second case 

1X(cv::Range::all(), cv::Range(3,4)) = A * 1;

the second signature Mat& Mat::operator=(const MatExpr_Base& expr) cause A * 1 returned MatExpr. So you actually copy values of A * 1 in current data storrade of matrix X

For more information see at opencv documentation Mat::operator =

  • Status changed from Open to Cancelled
  • Target version set to 2.4.0
  • Assignee set to Marina Kolpakova

#3
Updated by Nghia Ho almost 13 years ago

Thanks. Maybe worth mentioning in the document. Feels like an easy 'gotcha'.

#4
Updated by Kirill Kornyakov almost 13 years ago

It is actually documented: http://opencv.itseez.com/trunk/modules/core/doc/intro.html#automatic-memory-management. But if you feel that some other place should be updated, feel free to point us. If you think that operator= documentation should be updated, please provide your version of the text...

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