ImageMagick convert -monochrome
The -monochrome option uses some smart dithering, and makes the output much more visible than -threshold if you intend it for human consumption:
convert -monochrome signature.png out.png 
Does not make much difference for such a simple image, but for larger ones, it is striking:
- https://stackoverflow.com/questions/15861025/how-to-convert-an-image-to-1bit-px-binary-bitmap-with-imagemagick-or-rmagick
- https://askubuntu.com/questions/9868/convert-an-image-from-grayscale-to-binary
- https://superuser.com/questions/405686/how-to-convert-a-photo-to-a-black-and-white-image-by-imagemagick
- https://superuser.com/questions/75373/convert-color-photos-of-documents-to-good-black-and-white-bitonal-images