OpenCV trackbar callback in C++ class - c++

Question

I have a question about how to define the callback for trackbars in OpenCV when working with classes in C++.
When I define my trackbar let's say in the constructor method of my .cpp class how can I define the callback?
I have been trying to work with function pointers but it doesn't work out. I guess I must be doing something very wrong :-)
This is my header file:
class SliderwithImage {
public:
SliderwithImage(void);
~SliderwithImage(void);
void sliderCallBack(int pos);
};
This is the implementation file:
#include "SliderwithImage.h"
void SliderwithImage::sliderCallBack(int pos) {
}
SliderwithImage::SliderwithImage(void) {
const char* windowName = "window";
int lowvalue =1;
namedWindow(windowName, CV_GUI_EXPANDED);
createTrackbar("mytrackbar", windowName, &lowvalue, 255, sliderCallBack);
}
SliderwithImage::~SliderwithImage(void) {
}
Obviously the createTrackbar method does not recognize sliderCallBack... I guess it's a problem of scope. But I am not sure how to solve this?
Any help would be appreciated.
Thank you very much.

Answer 1

The callback function must be static or global, but you can pass it a reference to an object you want to operate on (see this post on the OpenCV Users mailing list).
The createTrackbar method has a userdata parameter which is passed to the calling function. In C there is an undocumented cvCreateTrackbar2 method, defined in highgui_c.h, which has the same functionality:
CVAPI(int) cvCreateTrackbar2( const char* trackbar_name, const char* window_name,
int* value, int count, CvTrackbarCallback2 on_change,
void* userdata CV_DEFAULT(0));
These methods let you create a class with a static callback function that takes a pointer to an object of that class. You can create the trackbar like so:
cv:createTrackbar("Label", "Window" &variable, MAX_VAL, &MyClass::func, this);
The callback would look something like this:
void MyClass:func(int newValue, void * object) {
MyClass* myClass = (MyClass*) object;
// ...do stuff.
}
Note that you don't need to explicitly update the variable yourself as long as you provided a pointer to it when creating the trackbar (as above), but if you need to process it first I suggest you set it explicitly in the callback function.

Answer 2

You have to implement the callback function either as a global function or a static member function. To make it more OOP look, you might prefer to implement it as a static member function:)

Answer 3

I am using a different solution to obtain the slider value in a class variable (in my case to obtain chosen rotation angle of a live video stream). The int* value in the createTrackbar function is a public class variable which is then used within a loop (while the video is acquired, but this might messily work repeatedly redrawing a single image).
Not the best solution but it works for me.
cv::createTrackbar("Rotation Angle(deg)", "Preview", &rotationAngle,
alpha_slider_max, NULL);
for(;;)
{
int rotAngle = this -> rotationAngle;
cv::Mat frame;
cv::Mat rot_frame;
this -> capture >> frame;
rot_frame = rotateVideo (frame, rotAngle);
imshow("Preview", rot_frame);
if(cv::waitKey(30) >= 0) break;
}