How do I initialize a value in OpenCV CallBack function-CodePudding

I have the following code that displays an image if you have picture.png or similar.

import cv2

def my_event(event, x, y, flags, params):
   
    if event==cv2.EVENT_MOUSEWHEEL:

           # displaying the coordinates
           # on the Shell
           print(x, ' ', y, event, flags, params, img.shape[0])
           if flags >= 0:
               width = height = 400
               width = width   10
               height = height   10
               img_resized = cv2.resize(img, (width,height), interpolation = cv2.INTER_AREA)
              
               cv2.imshow('image', img_resized)
           
if __name__=="__main__":

    img = cv2.imread('picture.png', 1)

    print("doug")

    cv2.imshow('image', img)

    cv2.setMouseCallback('image', my_event) # A call back loop
    
    print("doug2")
    cv2.waitKey(0)

    cv2.destroyAllWindows()

My understanding is that setMouseCallback is an interrupt function that is active per mouse or keyboard action. In my case it calls my_event. I have initialized the dimensions of my image to 400. I would like to change image size per mousewheel motion and not have the 400 override as shown. My logic would dictate that I would put the initialization values at "doug" but it does not work there. I have tried most other places with no success. How do I initialize a variable for the interrupt?

CodePudding user response：

Callback function is not a "interrupt function" by itself.

By specifying cv2.waitKey(0) you have a blocking call in the main thread, and imshow(...) in the callback function can modify the context.

Following code works for me (zoom /- by scrolling):

import cv2

D = dict.fromkeys(['width', 'height'])
D['width'] = 500
D['height'] = 500


def my_event(event, x, y, flags, params):
    if event == cv2.EVENT_MOUSEWHEEL:
        if flags >= 0:
            D['width'] = D['width']   10
            D['height'] = D['height']   10
            print(D['width'], D['height'])
            img_resized = cv2.resize(img, (D['width'], D['height']), interpolation=cv2.INTER_AREA)
            cv2.imshow('image', img_resized)
        else:
            D['width'] = D['width'] - 10
            D['height'] = D['height'] - 10
            print(D['width'], D['height'])
            img_resized = cv2.resize(img, (D['width'], D['height']), interpolation=cv2.INTER_AREA)
            cv2.imshow('image', img_resized)


if __name__ == "__main__":
    img = cv2.imread('image.jpg', 1)
    cv2.imshow('image', img)
    cv2.setMouseCallback('image', my_event, [D['width'], D['height']])  # A call back loop
    cv2.waitKey(0)
    cv2.destroyAllWindows()