Convert to Monochrome 1bpp images

[brent]

I've been able to convert an image from 8 bit to 4 bit with the following command:

nconvert.exe -o 1bit.bmp -wflag os2 -colors -16 8bit.bmp

But I am not able to convert to a true monochrome image with 1 bit per pixel, even when I set -colors to 2 or 1. How can I convert to a 1 bpp image?

[xnview]

please use -binary