python - How to get ROI bounding box coordinates with mouse clicks instead of guess & check

Question

So I am attempting to take a screenshot of my monitor and only grab part of the screen when doing so. I know I can use mss or opencv, pillow or any other screenshotting library that supports bounding boxes... However, instead of randomly guessing on what the coordinate are... and what I mean by this is taking a screenshot with bounding box coordinates set, and seeing if it is anywhere close to what I am actually trying to get the picture of.

---

For example: my trial coordinates would be 10,10,500,500 when in reality my actual coordinates that I need are 15,40,200,300 (these coordinates are made up)

---

My idea to solve this problem is to either have a tool that allows me to click and drag a bounding box around the image (part of the screen) that I need and have the program return the results, such as 15,40,200,300. Also, if I box could be drawn as it is shown that would be really helpful! If there is another way of doing achieving this goal I would be open to this as well.

---

Thank you.

Answer 1

The idea is to click-and-drag a bounding box around a region of interest to obtain the coordinates. To do this, we must capture the event actions of a mouse click and record the starting and ending coordinates of the ROI. OpenCV allows us to do this by processing mouse click events. Anytime a mouse click event is triggered, OpenCV will relay the information to our extract_coordinates callback function. In order to handle the event, OpenCV requires various arguments:

• event: Event that took place (left/right pressed or released mouse click)
• x: The x-coordinate of event
• y: The y-coordinate of event
• flags: Relevant flags passed by OpenCV
• Parameters: Extra parameters passed by OpenCV

A pressed left click records the top left coordinates while a released left click records the bottom right coordinates. We then draw a bounding box around the ROI and print the coordinates of the top left and bottom right rectangular region to the console. A right click will reset the image.

Extract bounding box coordinates widget:

import cv2

class BoundingBoxWidget(object):
    def __init__(self):
        self.original_image = cv2.imread('1.jpg')
        self.clone = self.original_image.copy()

        cv2.namedWindow('image')
        cv2.setMouseCallback('image', self.extract_coordinates)

        # Bounding box reference points
        self.image_coordinates = []

    def extract_coordinates(self, event, x, y, flags, parameters):
        # Record starting (x,y) coordinates on left mouse button click
        if event == cv2.EVENT_LBUTTONDOWN:
            self.image_coordinates = [(x,y)]

        # Record ending (x,y) coordintes on left mouse button release
        elif event == cv2.EVENT_LBUTTONUP:
            self.image_coordinates.append((x,y))
            print('top left: {}, bottom right: {}'.format(self.image_coordinates[0], self.image_coordinates[1]))
            print('x,y,w,h : ({}, {}, {}, {})'.format(self.image_coordinates[0][0], self.image_coordinates[0][1], self.image_coordinates[1][0] - self.image_coordinates[0][0], self.image_coordinates[1][1] - self.image_coordinates[0][1]))

            # Draw rectangle 
            cv2.rectangle(self.clone, self.image_coordinates[0], self.image_coordinates[1], (36,255,12), 2)
            cv2.imshow("image", self.clone) 

        # Clear drawing boxes on right mouse button click
        elif event == cv2.EVENT_RBUTTONDOWN:
            self.clone = self.original_image.copy()

    def show_image(self):
        return self.clone

if __name__ == '__main__':
    boundingbox_widget = BoundingBoxWidget()
    while True:
        cv2.imshow('image', boundingbox_widget.show_image())
        key = cv2.waitKey(1)

        # Close program with keyboard 'q'
        if key == ord('q'):
            cv2.destroyAllWindows()
            exit(1)