def is_almost_white(im_array,threshold = 200):
    """
    Tell if pixels of an image are white or not.
    im_array : the image represented as a numpy array in RGB (width*height*3)
    threshold: the threshold above which a pixel is considered as white
    Return : a numpy mask of size width*height
    """
    return (im_array[:,:,0] > threshold) & (im_array[:,:,1] > threshold) & (im_array[:,:,2] > threshold)