How to save mouse position in variable using OpenCV and Python?

Question

I'm using Python and OpenCV for some vision application. I need to save mouse position in variables and I don't know how. I can get the current mouse position to print in window, but can't save it to variable.

My problem is similar to this one only I work in python: OpenCV Return value from mouse callback function

I define my function like this (for printing mouse position):

def mousePosition(event,x,y,flags,param):
    if event == cv2.EVENT_MOUSEMOVE:
        print x,y

I use it in my program like this:

cv2.setMouseCallback('Drawing spline',mousePosition)

what is the problem? what is the output? what is your expectation ? — Hadi
– Hadi, Commented May 11, 2014 at 18:56

Abid Rahman K · Accepted Answer · 2014-05-11 19:03:02Z

17

Below is a small modified version of code from : http://docs.opencv.org/trunk/doc/py_tutorials/py_gui/py_mouse_handling/py_mouse_handling.html#mouse-handling

import cv2
import numpy as np

ix,iy = -1,-1
# mouse callback function
def draw_circle(event,x,y,flags,param):
    global ix,iy
    if event == cv2.EVENT_LBUTTONDBLCLK:
        cv2.circle(img,(x,y),100,(255,0,0),-1)
        ix,iy = x,y

# Create a black image, a window and bind the function to window
img = np.zeros((512,512,3), np.uint8)
cv2.namedWindow('image')
cv2.setMouseCallback('image',draw_circle)

while(1):
    cv2.imshow('image',img)
    k = cv2.waitKey(20) & 0xFF
    if k == 27:
        break
    elif k == ord('a'):
        print ix,iy
cv2.destroyAllWindows()

It stores the mouse position in global variables ix,iy. Every time you double-click, it changes the value to new location. Press a to print the new value.

answered May 11, 2014 at 19:03

Abid Rahman K

52.8k31 gold badges149 silver badges158 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

page47250 Over a year ago

Thank you. Haven't thought of using global variables. I thought they shouldn't be used. Does it stand for Python too?

Abid Rahman K Over a year ago

I use them whenever I absolutely need them. Never thought much about 'protocols' :)

mjul Over a year ago

If you want to avoid globals and you write functional style Python, you can use local ix and iy variables and capture them in draw_circle in a closure by making it a local function (Python uses pass-by-reference). If you use an object-oriented style, you you can encapsulate ix and iy in a class as members and make the draw_circle a method on an instance of that class.

Abid Rahman K Over a year ago

@mjul : +1 - I think it would be really great if you could make this comment to an answer (with examples) and share it here (Personally me too like to see different approaches). Then people can select whatever approach they like. You can use the same example in my answer and show how it can be done without globals by the two methods you mentioned.

user3522371 Over a year ago

why the parameters in def draw_circle(event,x,y,flags,param): are never used but the program works ?

|

Mr Vinagi · Accepted Answer · 2021-01-08 11:38:54Z

15

Avoid global variables by storing as class members:

import cv2
import numpy as np  



class CoordinateStore:
    def __init__(self):
        self.points = []

    def select_point(self,event,x,y,flags,param):
            if event == cv2.EVENT_LBUTTONDBLCLK:
                cv2.circle(img,(x,y),3,(255,0,0),-1)
                self.points.append((x,y))


#instantiate class
coordinateStore1 = CoordinateStore()


# Create a black image, a window and bind the function to window
img = np.zeros((512,512,3), np.uint8)
cv2.namedWindow('image')
cv2.setMouseCallback('image',coordinateStore1.select_point)

while(1):
    cv2.imshow('image',img)
    k = cv2.waitKey(20) & 0xFF
    if k == 27:
        break
cv2.destroyAllWindows()


print "Selected Coordinates: "
for i in coordinateStore1.points:
    print i

edited Jan 8, 2021 at 11:38

answered Aug 19, 2016 at 22:08

Mr Vinagi

4305 silver badges14 bronze badges

1 Comment

eric Over a year ago

This is a really clever way to avoid globals, which everyone uses for these kinds of mouse callbacks in open cv. I wonder what the downside of using objects versus globals are, specifically for this kind of use case, as you scale up to more complicated projects.

Musakkhir Sayyed · Accepted Answer · 2016-02-18 07:40:16Z

3

You can try this Code:

def mousePosition(event,x,y,flags,param):

    if event == cv2.EVENT_MOUSEMOVE:
        print x,y
        param = (x,y)

cv2.setMouseCallback('Drawing spline',mousePosition,param)

edited Feb 18, 2016 at 7:40

Musakkhir Sayyed

7,21813 gold badges44 silver badges67 bronze badges

answered Feb 18, 2016 at 6:17

user5944131

311 bronze badge

Collectives™ on Stack Overflow

How to save mouse position in variable using OpenCV and Python?

3 Answers 3

6 Comments

1 Comment

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

6 Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related