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copied!<p>The callback function must be static or global, but you can pass it a reference to an object you want to operate on (see <a href="http://opencv-users.1802565.n2.nabble.com/Member-function-callback-in-cvCreateTrackbar-td3785481.html" rel="noreferrer">this post</a> on the OpenCV Users mailing list).</p> <p>The <a href="http://opencv.willowgarage.com/documentation/cpp/user_interface.html#cv-createtrackbar" rel="noreferrer">createTrackbar</a> method has a <code>userdata</code> parameter which is passed to the calling function. In C there is an undocumented <code>cvCreateTrackbar2</code> method, defined in <code>highgui_c.h</code>, which has the same functionality:</p> <pre><code>CVAPI(int) cvCreateTrackbar2( const char* trackbar_name, const char* window_name, int* value, int count, CvTrackbarCallback2 on_change, void* userdata CV_DEFAULT(0)); </code></pre> <p>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:</p> <pre><code>cv:createTrackbar("Label", "Window" &variable, MAX_VAL, &MyClass::func, this); </code></pre> <p>The callback would look something like this:</p> <pre><code>void MyClass:func(int newValue, void * object) { MyClass* myClass = (MyClass*) object; // ...do stuff. } </code></pre> <p>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.</p>

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