CODEWITHSUNDEEP
opencvc++-cliglobal-variables
Novfrank
Novfrank

Reputation: 5

OpenCV & C++: global cv::Mat object returns wrong values

I am trying to use OpenCV in Windows Form. I created a global cv::Mat object and is expecting the object for other functions to use. However every time I initialized the global cv::Mat object and quit/return the function, the values of my global cv::Mat object will change automatically with unknown reason.

One sample code snippet is like:

private: cv::Mat *cvImage;

For some reason, I created a function called decode() which returns cv::Mat.

void copy(){cvImage = &decode();}

the cvImage cannot get the correct values by the above codes. Usually is data, dataend, datalimit, datastart and rows being 0 or unexpected values.

Even I tried to use a local variable, say cv::Mat img, to do everything then run cvImage = &img; , whenever the function quits or returns, the value of cvImage will also change unexpectedly.

I am wondering why would this happen??

Upvotes: 0

Views: 2264

Answers (1)

berak
berak

Reputation: 39796

your use of pointers there creates undefined behaviour. don't use them with Mats !

DONT cv::Mat *cvImage;

DO cv::Mat cvImage;

cv::Mats are refcounted, those pointers will wreck havoc on them.

as a reminder:

Mat a,b;  b=a;         // shallow copy, shared pixels (that's probably, what you wanted)
Mat a,b;  b=a.clone(); // deep copy, b will have y copy of a's pixels

Edit:

"For some reason, I created a function called decode() which returns cv::Mat."

that's your first problem here decode creates a temporary( which gets destroyed after leaving copy() )

void copy(){cvImage = &decode();}

and you return the address of it.

since it seems, that cli can handle only POD objects(like pointers)

  1. let decode return a a Mat*, instead of taking the address of a temp

    Mat * decode() { Mat *m = new Mat(100,100,CV_UC8); ... return m; }

  2. let copy dlete the old one

    void copy(){ if (cvImage) delete cvImage; cvImage = decode(); }

Upvotes: 2

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