Detect Mouse Clicks and Moves on Image Window

void setMouseCallback(const string& winname, MouseCallback onMouse, void* userdata = 0)

This function sets a callback function to be called every time any mouse events occurs in the specified window. Here is the detailed explanation of the each parameters of the above OpenCV function.

• winname - Name of the OpenCV window. All mouse events related to this window will be registered
• onMouse - Name of the callback function. Whenever mouse events related to the above window occur, this callback function will be called. This function should have the signature like the following
• void FunctionName(int event, int x, int y, int flags, void* userdata)
• event - Type of the mouse event. These are the entire list of mouse events
• EVENT_MOUSEMOVE
• EVENT_LBUTTONDOWN
• EVENT_RBUTTONDOWN
• EVENT_MBUTTONDOWN
• EVENT_LBUTTONUP
• EVENT_RBUTTONUP
• EVENT_MBUTTONUP
• EVENT_LBUTTONDBLCLK
• EVENT_RBUTTONDBLCLK
• EVENT_MBUTTONDBLCLK
• x - x coordinate of the mouse event
• y - y coordinate of the mouse event
• flags - Specific condition whenever a mouse event occurs. See the next OpenCV example code for the usage of this parameter. Here is the entire list of enum values which will be possesed by "flags"
• EVENT_FLAG_LBUTTON
• EVENT_FLAG_RBUTTON
• EVENT_FLAG_MBUTTON
• EVENT_FLAG_CTRLKEY
• EVENT_FLAG_SHIFTKEY
• EVENT_FLAG_ALTKEY
• userdata - Any pointer passes to the "setMouseCallback" function as the 3rd parameter (see below)
• userdata - This pointer will be passed to the callback function

This program takes care of mouse button press and mouse movements.

This may be helpful while you want to select a region on an image and perform certain task in that region only.

Example:

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#include "opencv2/highgui/highgui.hpp"
#include <iostream>

using namespace std;
using namespace cv;

void CallBackFunc(int event, int x, int y, int flags, void* userdata)
{
    if  ( event == EVENT_LBUTTONDOWN )
    {
        cout << "Left button of the mouse is clicked - position (" << x << ", " << y << ")" << endl;
    }
    else if  ( event == EVENT_RBUTTONDOWN )
    {
        cout << "Right button of the mouse is clicked - position (" << x << ", " << y << ")" << endl;
    }
    else if  ( event == EVENT_MBUTTONDOWN )
    {
        cout << "Middle button of the mouse is clicked - position (" << x << ", " << y << ")" << endl;
    }
    else if ( event == EVENT_MOUSEMOVE )
    {
        cout << "Mouse move over the window - position (" << x << ", " << y << ")" << endl;
    }    
}

int main()
{
    // Read image from file 
    Mat img = imread("lena.JPG");

    //if fail to read the image
    if ( img.empty() ) 
    { 
        cout << "Error loading the image" << endl;
        return -1; 
    }

    //Create a window
    namedWindow("ImageDisplay", 1);

    //set the callback function for any mouse event
    setMouseCallback("ImageDisplay", CallBackFunc, NULL);

    //show the image
    imshow("ImageDisplay", img);

    // Wait until user press some key
    waitKey(0);

    return 0;

}

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Example 2:

This code will detect left mouse clicks while pressing the "CTRL" key , right mouse clicks while pressing the "SHIFT" key and movements of the mouse over the OpenCV window while pressing the "ALT" key.

------------

#include "opencv2/highgui/highgui.hpp"
#include <iostream>

using namespace std;
using namespace cv;

void CallBackFunc(int event, int x, int y, int flags, void* userdata)
{
    if ( flags == (EVENT_FLAG_CTRLKEY + EVENT_FLAG_LBUTTON) )
    {
        cout << "Left mouse button is clicked while pressing CTRL key - position (" << x << ", " << y << ")" << endl;
    }
    else if ( flags == (EVENT_FLAG_RBUTTON + EVENT_FLAG_SHIFTKEY) )
    {
        cout << "Right mouse button is clicked while pressing SHIFT key - position (" << x << ", " << y << ")" << endl;
    }
    else if ( event == EVENT_MOUSEMOVE && flags == EVENT_FLAG_ALTKEY)
    {
        cout << "Mouse is moved over the window while pressing ALT key - position (" << x << ", " << y << ")" << endl;
    }
}

int main(int argc, char** argv)
{
    // Read image from file 
    Mat img = imread("lena.JPG");

    //if fail to read the image
    if ( img.empty() ) 
    { 
        cout << "Error loading the image" << endl;
        return -1; 
    }

    //Create a window
    namedWindow("My Window", 1);

    //set the callback function for any mouse event
    setMouseCallback("My Window", CallBackFunc, NULL);

    //show the image
    imshow("My Window", img);

    // Wait until user press some key
    waitKey(0);

    return 0;
}

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Example 3: 

This code will extract color information at a pixel location in an image.
------------

#include "opencv2/highgui/highgui.hpp"
#include <iostream>

using namespace std;
using namespace cv;

void mouseEvent(int evt, int x, int y, int flags, void* param) 
{                    
    Mat* rgb = (Mat*) param;
    if (evt == CV_EVENT_LBUTTONDOWN) 
    { 
        printf("%d %d: %d, %d, %d\n", 
        x, y, 
        (int)(*rgb).at<Vec3b>(y, x)[0], 
        (int)(*rgb).at<Vec3b>(y, x)[1], 
        (int)(*rgb).at<Vec3b>(y, x)[2]); 
    }         
}

int main(int argc, char** argv)
{
    // Read image from file
    Mat img = imread("lena.JPG");

    //if fail to read the image
    if ( img.empty() )
    {
        cout << "Error loading the image" << endl;
        return -1;
    }

    //Create a window
    namedWindow("My Window", 1);

    //set the callback function for any mouse event
    setMouseCallback("My Window", mouseEvent, &img);

    //show the image
    imshow("My Window", img);

    // Wait until user press some key
    waitKey(0);

    return 0;
} 

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Example 4:

------------

#include "opencv2/highgui/highgui.hpp"
#include <iostream>

using namespace std;
using namespace cv;

void on_mouse( int e, int x, int y, int d, void *ptr )
{
    Point*p = (Point*)ptr;
    p->x = x;
    p->y = y;
    cout<<*p;
}

int main(int argc, char** argv)
{
    // Read image from file
    Mat img = imread("lena.JPG");

    //if fail to read the image
    if ( img.empty() )
    {
        cout << "Error loading the image" << endl;
        return -1;
    }

    //Create a window
    namedWindow("My Window", 1);

    //set the callback function for any mouse event
    Point p;
    setMouseCallback("My Window", on_mouse, &p );

    //show the image
    imshow("My Window", img);
    
    // Wait until user press some key
    waitKey(0);
    return 0;
}

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Sources:
http://opencv-srf.blogspot.in/2011/11/mouse-events.html
http://stackoverflow.com/questions/14874449/opencv-set-mouse-callback
http://stackoverflow.com/questions/15570431/opencv-return-value-from-mouse-callback-function