How to use cv::createButton prototype in OpenCV

Question

I'd like to understand how to use cv::createButton which is defined in OpenCV documentation:

http://opencv.jp/opencv-2svn_org/cpp/highgui_qt_new_functions.html#cv-createbutton

It says that the prototype is:

createButton(const string& button_name CV_DEFAULT(NULL), ButtonCallback on_change CV_DEFAULT(NULL), void* userdata CV_DEFAULT(NULL), int button_type CV_DEFAULT(CV_PUSH_BUTTON), int initial_button_state CV_DEFAULT(0)

but i don't know how to define the function ButtonCallback in order to trap the button event.

I do:

cvCreateButton("button6", callbackButton2, pointer, CV_PUSH_BUTTON, 0);

to declare the button and

void callbackButton2(int state, void *pointer){

    printf("ok");

}

but it doesn't work.

I don't know the meaning of the third parameter "void* userdata".

Can someone help me, please?

Thanks.

Answer 1

We still don't know what doesn't work means to you, but I'll provide some information on how to use the callback and what userdata is.

As the signature suggests, void* userdata is a parameter that you can use to send/receive data to the callback. This is purely optional, so if you don't have any use for it just pass NULL .

On the example below I'll be using userdata to retrieve data from the callback. You might have noticed that the callback receives a state information from OpenCV. I'm interested in storing this value and making it available to main() .

For this purpose, I define a custom data type and declare a variable of this type on main() . The custom type has an int member to store the state received by our callback and a mutex that we are going to use to protect the custom type from being read/written simultaneously by the 2 threads (callback and main() ).

#include <iostream>
#include <cv.h>
#include <highgui.h>
#include <pthread.h>
#include <string.h>

using namespace cv;

typedef struct custom_data
{
    int state;
    pthread_mutex_t mtx;
} custom_data_t;


void my_button_cb(int state, void* userdata)
{
    std::cout << "@my_button_cb" << std::endl;   

    // convert userdata to the right type
    custom_data_t* ptr = (custom_data_t*)userdata;
    if (!ptr)
    {
        std::cout << "@my_button_cb userdata is empty" << std::endl;
        return;
    }

    // lock mutex to protect data from being modified by the
    // main() thread
    pthread_mutex_lock(&ptr->mtx);

    ptr->state = state;

    // unlock mutex
    pthread_mutex_unlock(&ptr->mtx);
}    

int main()
{
    // declare and initialize our userdata
    custom_data_t my_data = { 0 };

    createButton("dummy_button", my_button_cb, &my_data, CV_PUSH_BUTTON, 0);

    // For testing purposes, go ahead and click the button to activate
    // our callback.

    // waiting for key press <enter> on the console to continue the execution
    getchar(); 

    // At this point the button exists, and our callback was called 
    // (if you clicked the button). In a real application, the button is 
    // probably going to be pressed again, and again, so we need to protect 
    // our data from being modified while we are accessing it. 
    pthread_mutex_lock(&my_data.mtx);

    std::cout << "The state retrieved by the callback is: " << my_data.state << std::endl;

    // unlock mutex    
    pthread_mutex_unlock(&my_data.mtx);

    return 0;
}

Answer 2

The reason it didn't work, is because the cvCreateButton implementation had a bug which caused button clicks to be ignored :

https://code.ros.org/trac/opencv/ticket/786

This is now fixed in trunk, so a button click should now be working fine again if you use trunk. It should eventually make its way into an official release.