Distance between 2 points in opencv (Pixel to cm)

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Hello, I asked a month about a code that can measure the distance betweet two points, but I think the result is in pixels, so would like to get the distance, but in cm. I'm using webcam Microsoft lifecam hd-3000, and I really don't know what to do at this point in the code.

import cv2   
    import numpy as np

    #Captura de video a traves de la webcam
    cap=cv2.VideoCapture(0)

    D = []
    max_samples = 10000 

    outfile = open('new.txt', 'w')

    while(1):
            d=0.1
            centers=[]
            _, img = cap.read()

            hsv=cv2.cvtColor(img,cv2.COLOR_BGR2HSV) #Se obtiene un histograma basada en las saturaciones de colores.

            blue_lower=np.array([95,150,100],np.uint8)
            blue_upper=np.array([150,255,255],np.uint8)

            blue=cv2.inRange(hsv,blue_lower,blue_upper) #Se crea una mascara utilizando intervalos de color azul.

            kernel = np.ones((5 ,5), "uint8") #Crea una matriz de 5x5 la cual recorrera el video,
            blue = cv2.morphologyEx(blue,cv2.MORPH_OPEN,kernel)
            blue = cv2.morphologyEx(blue,cv2.MORPH_CLOSE,kernel)    

            blue=cv2.erode(blue,kernel, iterations=1) #Se erosiona utilizando el kernel sobre la mascara.
            res1=cv2.bitwise_and(img, img, mask = blue) #La nueva imagen reemplazara a blue.


            (_,contours,hierarchy)=cv2.findContours(blue,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE) #Encuentra los contornos de los objetos que se ven en el filtro

            for pic, contour in enumerate(contours):
                area = cv2.contourArea(contour) #funcion de opencv que obtiene los contornos
                mayor_contorno = max(contours, key = cv2.contourArea)

                if(area>2000):
                    x,y,w,h = cv2.boundingRect(contour) #Encuentra coordenadas de los contornos.
                    img = cv2.rectangle(img,(x,y),(x+w,y+h),(255,0,0),2)
                    cv2.putText(img,"Marcador",(x,y),cv2.FONT_HERSHEY_SIMPLEX, 0.7, (255,0,0))
                    M = cv2.moments(contour) #Se obtiene el centro de masa de los marcadores enconrados.
                    cx = int(M['m10'] /M['m00'])
                    cy = int(M['m01'] /M['m00'])
                    centers.append([cx,cy])
                    cv2.circle(img, (cx, cy), 7, (255, 255, 255), -1)

                if len(centers)==2:
                    D = (np.linalg.norm(cx-cy)) #Se aplica distancia euclidiana para encontrar la distancia entre los centros de masa.
                    print(D)
                    outfile.write("{0} \n".format(D))


            cv2.imshow("Color Tracking",img)
            if cv2.waitKey(10) & 0xFF == ord('q'):
                outfile.close()
                print('Los datos han sido enviados al doctor')
                cap.release()
                cv2.destroyAllWindows()
                break

I read about extrinsic and intrinsic parameters, but how do I get them with the webcam? Greetings