title: color detection using opencv python
body: <p>I am trying to run a script written using opencv in python which uses webcam to track colored objects (here the object is blue colored), which is also mentioned in opencv's documentation <a href="https://opencv-python-tutroals.readthedocs.org/en/latest/py_tutorials/py_imgproc/py_colorspaces/py_colorspaces.html#converting-colorspaces" rel="noreferrer">here</a></p>
  
  <pre><code>import cv2
  import numpy as np
  
  cap = cv2.VideoCapture(0)
  
  while(1):
  
      # Take each frame
      _, frame = cap.read()
  
      # Convert BGR to HSV
      hsv = cv2.cvtColor(frame, cv2.COLOR_BGR2HSV)
  
      # define range of blue color in HSV
      lower_blue = np.array([110,50,50])
      upper_blue = np.array([130,255,255])
  
      # Threshold the HSV image to get only blue colors
      mask = cv2.inRange(hsv, lower_blue, upper_blue)
  
      # Bitwise-AND mask and original image
      res = cv2.bitwise_and(frame,frame, mask= mask)
  
      cv2.imshow('frame',frame)
      cv2.imshow('mask',mask)
      cv2.imshow('res',res)
      k = cv2.waitKey(5) &amp; 0xFF
      if k == 27:
          break
  
  cv2.destroyAllWindows()
  </code></pre>
  
  <p>But this code produces error :</p>
  
  <pre><code>OpenCV Error: Sizes of input arguments do not match (The lower bounary is neither an      array of the same size and same type as src, nor a scalar) in inRange, file     /build/buildd/opencv-2.4.2+dfsg/modules/core/src/arithm.cpp, line 2527
  Traceback (most recent call last):
  File "blue.py", line 19, in &lt;module&gt;
  mask = cv2.inRange(hsv, lower_blue, upper_blue)
  cv2.error: /build/buildd/opencv-2.4.2+dfsg/modules/core/src/arithm.cpp:2527: error: (     (-209) The lower bounary is neither an array of the same size and same type as src, nor a scalar in function inRange
  </code></pre>
  
  <p>I've tried solutions provided in related stackoverflow questions, but none of them helped.
  What is the problem with the code ? why this error arises ?</p>
  
  <p>I'm using opencv 2.4.2 &amp; python 2.7 on ubuntu </p>