Search code examples
opencvvoid-pointerstrackbar

In OpenCV createTrackbar function what is the 6th paramter(User data) how it will be useful.(Please find the parameters in the explanation)

C++: int createTrackbar(const string& trackbarname, const string& winname, int* value, int count, TrackbarCallback onChange=0, void* userdata=0) In above mention function what is the userdata how it will used in the callback function

Solution

  • that param is useful together with a callback function, which looks like this:

    void onChange (int trackpos , void *userdata);

    you can e.g pass (the address of) an image to it:

    Mat img; // e.g from a webcam
int b=3; // blur value
namedWindow("win");
createTrackBar("blur","win", &b, 100, onChange, (void*)(&img) );

    and use it inside the callback:

    void onChange (int trackpos , void *userdata)
{
   Mat img = *((Mat*)userdata); // 1st cast, then deref
   Mat b2;
   blur( img,b2, Size(trackpos,trackpos));
   imshow("win",b2);
   waitKey(10);
}