title: reading pgm images with cv2 in python
body: <p>I want to read pgm image in python. I use cv2.imread('a.pgm') but it returns wrong results. In Matlab, I use imread and get the right result which is a single channel 16-bit image. But cv2.imread in python returns a 3-channel image and the pixel values are also wrong.
  Why it happens?
  How should I read the 16-bit pgm images in python?
  And what libraries?</p>
  
  <p>Thanks in advance.</p>