轨迹栏回调python openCV

Question

I have been trying to have a simple window with a trackbar setting the threshold for the binarisation.
I have seen several example using the getTrackbarPos method of the trackbar in a while loop, but not many using a callback function, which I would like to use (or at least understand why getTrackbarPos is better)
I actually see the thresholded image when I move the slider, but it is almost instantaneously replaced by the original image. I also tried using a global ImageBin in the callback function but it does not help. Someone has a suggestion or can reroute me to some similar issue ? Thanks

import cv2

# callback function
def Update(value):
    print value
    ret,ImageBin = cv2.threshold(Image,value,255,cv2.THRESH_BINARY)
    cv2.imshow('Fenetre',ImageBin)

# window
cv2.namedWindow('Fenetre',cv2.WINDOW_GUI_NORMAL)

# Trackbar 
Slider = cv2.createTrackbar('Threshold','Fenetre',0,255,Update)

# Open image
Home = r'C:\Users\Laurent Thomas\Documents\DataSet\170922110941_BISCHOFF_DORSAL_2ndGO'
Image = cv2.imread(Home + '\WE00001---A001--PO01--LO001--CO6--SL001--PX32500--PW0040--IN0020--TM245--X014262--Y011163--Z216816--T1373979007.tif',0)

# Initialise first view as the normal image
ImageBin = Image[:]


while(1):
    cv2.imshow('Fenetre',ImageBin)
    k = cv2.waitKey(1) & 0xFF
    if k == 27:
        break


cv2.destroyAllWindows()

Answer 1

Modify the Update function as follows:

# callback function
def Update(value):
    global ImageBin
    print (value)
    ret,ImageBin = cv2.threshold(Image,value,255,cv2.THRESH_BINARY)