Porównywanie obrazów za pomocą cam cv

Witam,
mam problem z porównywaniem obrazów z wykorzystaniem pakietu cv w pythonie. Problem jest w tym, że chce sprawdzić skale podobieństwa pomiędzy dwoma obrazami, wykorzystuje tutaj metodę threshold. Problem w tym, że uzyskuje bardzo małą miarę podobieństwa pomiędzy obrazem temp a obrazem z kamery. W momencie, kiedy obraz z kamery przedstawia białą kartkę i miara podobieństwa jest znacznie większa niż jak mam obiekt porównywany.
Tutaj jest kod.

def f_TM_CCOEFF():

   # intialize the webcam and pass a constant which is 0
    cam = cv2.VideoCapture(0)

    # title of the app
    cv2.namedWindow('python webcam screenshot app')

    # let's assume the number of images gotten is 0
    img_counter = 0

    # while loop
    while True:
        # intializing the frame, ret
        ret, frame = cam.read()
        # if statement
        if not ret:
            print('failed to grab frame')
            break
        # the frame will show with the title of test
        cv2.imshow('test', frame)
        #to get continuous live video feed from my laptops webcam
        k  = cv2.waitKey(1)
        # if the escape key is been pressed, the app will stop
        if k%256 == 27:
            print('escape hit, closing the app')
            break
        # if the spacebar key is been pressed
        # screenshots will be taken
        elif k%256  == 32:
            # the format for storing the images scrreenshotted
            img_name = "ORGINAL/opencv_frame_{}.png".format(img_counter)
            # saves the image as a png file
            cv2.imwrite(img_name, frame)
            print('screenshot taken')
            # the number of images automaticallly increases by 1
            img_counter += 1
    # release the camera
    cam.release()


    #img = cv2.imread(img_name,0)
    
    #sql = "SELECT photo (photo, userid) FROM photos VALUES (%s, (SELECT Uid FROM `users` WHERE `username` = '"+ username_entry.get().strip() + "'))"
    #sql = "SELECT photo, userid FROM photos VALUES (%s, (SELECT Uid FROM `users` WHERE `username` = '"+ username_entry.get().strip() + "'))" 

    sql = "SELECT photo FROM photos WHERE userid = (SELECT Uid FROM users WHERE username = '"+ username_entry.get().strip() + "')"

    cursor= db.cursor(buffered=True)
    cursor.execute(sql, )
    data2 = cursor.fetchall()
    db.commit()
    # data2 wyciaga nam z bazy danych sciezke (b'PHOTO/opencv_frame_109.png',) a nie sam obrazek
    # 1. ze sciezki wyciagnac plik
    # 2. inaczej zapisac w bazie danych obrazek
    template = cv2.imread(sql)
    print(data2)
    w, h = template.shape[:-1]
    method = cv2.TM_CCOEFF

       # img = img2.copy()
     
    # Apply template Matching
    res = cv2.matchTemplate(img,template,method)
    min_val, max_val, min_loc, max_loc = cv2.minMaxLoc(res)
    # If the method is TM_SQDIFF or TM_SQDIFF_NORMED, take minimum
    top_left = max_loc
    bottom_right = (top_left[0] + w, top_left[1] + h)
        
    cv2.rectangle (img, top_left, bottom_right, 255, 2)

        #bottom_right = (top_left[0] + w, top_left[1] + h)
        #cv.rectangle(img,top_left, bottom_right, 255, 2)
    plt.subplot(121),plt.imshow(res,cmap = 'gray')
    plt.title('Matching Result'), plt.xticks([]), plt.yticks([])
    plt.subplot(122),plt.imshow(img,cmap = 'gray')
    plt.title('Detected Point'), plt.xticks([]), plt.yticks([])
    plt.show()

        
       # if cv2.waitKey(1) & 0xFF == ord('q'):
        #    break
                #frame.close()
        #cap.release()
cv2.destroyAllWindows()