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@mpeven
Created April 6, 2018 18:19
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Put optical flow arrows on an image
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
@wangchuan
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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?

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