Put optical flow arrows on an image

op_flow.py

import cv2
import numpy as np


def put_optical_flow_arrows_on_image(image, optical_flow_image, threshold=2.0, skip_amount=30):
    # Don't affect original image
    image = image.copy()
    
    # Turn grayscale to rgb if needed
    if len(image.shape) == 2:
        image = np.stack((image,)*3, axis=2)
    
    # Get start and end coordinates of the optical flow
    flow_start = np.stack(np.meshgrid(range(optical_flow_image.shape[1]), range(optical_flow_image.shape[0])), 2)
    flow_end = (optical_flow_image[flow_start[:,:,1],flow_start[:,:,0],:1]*3 + flow_start).astype(np.int32)
    

    # Threshold values
    norm = np.linalg.norm(flow_end - flow_start, axis=2)
    norm[norm < threshold] = 0
    
    # Draw all the nonzero values
    nz = np.nonzero(norm)
    for i in range(0, len(nz[0]), skip_amount):
        y, x = nz[0][i], nz[1][i]
        cv2.arrowedLine(image,
                        pt1=tuple(flow_start[y,x]), 
                        pt2=tuple(flow_end[y,x]),
                        color=(0, 255, 0), 
                        thickness=1, 
                        tipLength=.2)
    return image

Line 15, flow_end = (optical_flow_image[flow_start[:,:,1],flow_start[:,:,0],:1]*3 + flow_start).astype(np.int32), why there is a :1 here? And why *3?