imshow error but imwrite ok in python

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Heading ## The function cv2.imshow() display nothing! But cv2.imwrite() write image right way.

• under ubuntu 12.04 LTS
• Edit by sublime text 2
• coding as follows:

import cv2
import numpy as np

if __name__ == "__main__":
     img = np.zeros((512,512,3), dtype=np.int)
     for j in range(512):
         for i in range(512):
              img.itemset((i,j,0), 255)
              img.itemset((i,j,1), 255)
              img.itemset((i,j,1), 255)

     #print img
     cv2.imshow("test",img)
     cv2.imwrite("test.jpg",img)
     cv2.waitKey(0)
     cv2.destroyAllWindows()

result:

1. cv2.imshow() display an image with all pixels is [0,0,0]
2. cv2.imwrite() write an image with all pixels is [255,255,255]
3. I just test print the data of img, find that all pixels is [255,255,255]