Python opencv drawContours does not show anything

Question

I followed the tutorial at this page but nothing seems to happen when the line cv2.drawContours(im,contours,-1,(0,255,0),3) is executed. I was expecting to see star.jpg with a green outline, as shown in the tutorial. Here is my code:

import numpy as np
import cv2

im = cv2.imread('C:\Temp\ip\star.jpg')
print im.shape #check if the image is loaded correctly
imgray = cv2.cvtColor(im,cv2.COLOR_BGR2GRAY)
ret,thresh = cv2.threshold(imgray,127,255,0)
contours, hierarchy = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
cv2.drawContours(im,contours,-1,(0,255,0),3)
pass

There are no error messages. star.jpg is the star from the above mentioned webpage. I am using opencv version 2.4.8 and Python 2.7.

Is drawContours supposed to show an image on my screen? If so, what did I do wrong? If not, how do I show the image?

Thanks

Edit:

Adding the following lines will show the image:

cv2.imshow("window title", im)
cv2.waitKey()

waitKey() is needed otherwise the window will just show a gray background. According to this post, that's because waitKey() tells it to start handling the WM_PAINT event.

Answer 1

I had the same issue. I believe the issue is that the underlying image is 1-channel rather than 3-channel. Therefore, you need to set the color so it's nonzero in the first element (e.g. (255,0,0)).

Answer 2

i too had the same problem. The thing is it shows, but too dark for our eyes to see. Solution: change the colour from (0,255,0) (for some weird reason, i too had give exactly the same color!) to (128,255,0) (or some better brighter colour)