System information (version)

• OpenCV => 3.2.0
• Operating System / Platform => Ubuntu 16.04

Detailed description

If you try to save some monochrome image as bitmap ( .pbm P1/P4) you get a greymap (P2/P5) picture at outcome.

Steps to reproduce

A simple example:

import numpy as np
import cv2
img = np.array([0, 0, 0, 0]).reshape((2,2))
cv2.imwrite('/tmp/out.pbm', img, (32, 0))

And out.pbm looks like this:

P2
2 2
255
   0   0
   0   0