Getting x,y coordinates from an object in a video

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Software: python, opencv,ubunu 16.04

Hello, I am working on a project where I am trying to get the real world x,y coordinates for some cubes using the region of interest. I was able to get the region of interest and find the center coordinates for my objects. The part I am stuck on is getting the center coordinates relative to the region of interest in my case it is a white sheet of paper that is about 8 1/4 x 11 3/4 inches. How can I find the real world center coordinates of the objects(shown in picture) relative to the size of the region of interest? This is only a prototype and I later be using a region of interest that is 25cmx25cm. I have included a picture of my work and my code for reference.

Thank you

https://imgur.com/a/r5MtAxO

import cv2
import numpy as np

#find the center of an object
def center(contours):
    # calculate moments for each contour
    for c in contours:
        M = cv2.moments(c)

        if M["m00"] != 0:
            cX = int(M["m10"] / M["m00"])
            cY = int(M["m01"] / M["m00"])
        else:
            cX, cY = 0, 0
        tempStr = str(cX) + ", " + str(cY)
        cv2.circle(frame, (cX, cY), 1, (0, 0, 0), -1) #make a dot at the center of the object 
        cv2.putText(frame, tempStr, (cX - 25, cY - 25),cv2.FONT_HERSHEY_TRIPLEX, 0.4, (0, 0, 0), 1) #print the coordinates on the image

#get the region of interest
def find_ROI(frame):
    image = frame.copy()

    #change to grayscale
    gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)

    #Get binary image 
    _, thresh = cv2.threshold(gray, 127, 255, cv2.THRESH_BINARY)

    #create structural element
    struc_ele = np.ones((5, 5), np.uint8)

    #Use Open Morphology
    img_open = cv2.morphologyEx(thresh, cv2.MORPH_OPEN, struc_ele, iterations = 1)

    #find contours
    _, ctrs, _ = cv2.findContours(img_open.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

    # sort contours
    sorted_ctrs = sorted(ctrs, key=lambda ctr: cv2.boundingRect(ctr)[1])

    for i, ctr in enumerate(sorted_ctrs):
        # Get bounding box
        x, y, w, h = cv2.boundingRect(ctr)

        # Getting ROI
        roi = image[y:y + h, x:x + w]

        # show ROI
        cv2.imshow('Region of Interest',roi)
        cv2.rectangle(image, (x, y), (x + w, y + h), (0, 255, 0), 2)
    cv2.imshow('marked areas', image)


cap = cv2.VideoCapture(0)

if __name__ == "__main__":
    while True:
        _, frame = cap.read()
        hsv = cv2.cvtColor(frame, cv2.COLOR_BGR2HSV)

        find_ROI(frame)
        # l_h = cv2.getTrackbarPos("L - H", "Trackbars")
        # l_s = cv2.getTrackbarPos("L - S", "Trackbars")
        # l_v = cv2.getTrackbarPos("L - V", "Trackbars")
        # u_h = cv2.getTrackbarPos("U - H", "Trackbars")
        # u_s = cv2.getTrackbarPos("U - S", "Trackbars")
        # u_v = cv2.getTrackbarPos("U - V", "Trackbars")

        #Create kernel to use in filter
        kernel = np.ones((5, 5), np.uint8)

        #Create filter for yellow
        lower_yellow = np.array([15, 100, 100]) #Lower boundary values for HSV
        upper_yellow = np.array([30, 255, 255]) #Upper boundary values for HSV

        yellow_mask = cv2.inRange(hsv, lower_yellow, upper_yellow) # Threshold the HSV image to get only yellow colors
        opening_y = cv2.morphologyEx(yellow_mask, cv2.MORPH_OPEN, kernel, iterations = 2) #Use morphology open to rid ...