ORB algorithm in python-openCV
<p>I use python 2.7.2 and opencv  3.1.0, when I type below sentences in to spyder(an Editor for python)</p>

<pre><code>import numpy as np
import cv2
import  matplotlib.pyplot as plt
brief = cv2.xfeatures2d.BriefDescriptorExtractor_create()
img = cv2.imread("home.jpg",0)
img2 = np.zeros((img.shape[:2]))
print img2.shape
orb = cv2.ORB_create()
kp = orb.detect(img,None)
kp,des = orb.compute(img,kp)
img2 = cv2.drawKeypoints(img,kp,img2,color = (0,255,0),flags=0)
plt.imshow(img2),plt.show()
</code></pre>

<p>I get an error that says </p>

<p><strong>OpenCV Error: Assertion failed (The data should normally be NULL!) in NumpyAllocator::allocate, file ......\opencv-3.1.0\modules\python\src2\cv2.cpp, line 163
Traceback (most recent call last):
  File "E:\program\orbTest.py", line 17, in 
    kp,des = orb.compute(img,kp)
cv2.error: ......\opencv-3.1.0\modules\python\src2\cv2.cpp:163: error: (-215) The data should normally be NULL! in function NumpyAllocator::allocate</strong></p>

<p>that means there is error in line 17, I try to find out what's the matter with it , but failed. I am a fresh man to opencv, what should I do to deal with this problem</p>