How to convert an image to 1bit/px binary bitmap with ImageMagick or RMagick?

Question

I'm looking for the command and options to convert images (png, jpeg, whatever…) to 1bit/px bitmap image (similar to what happen when in photoshop when you convert it to bitmap). See example below

I'd like to do it programatically from within a rails app (e.g. RMagick or through ImageMagick)

The following works :

convert yo.jpg -remap pattern:gray30 mono.gif

but I' like to understand better what's happening, compared to what phosotsohp would do in that case; considering the many possibilities explained here (@roger_rowland thx for the link again)

In the end what I want to do is to have this "effect", and to enlarge the image without ending up with a blurry effect

Answer 1

-monochrome

This option uses some smart dithering and generates very visible output:

convert -monochrome in.png out.png

Documentation: http://www.imagemagick.org/Usage/quantize/#monochrome

Compare that to a simpler -threshold 50 transform:

convert -threshold 50 in.png out.png

which loses most of the image. We could likely have improved that with a better -threshold value, but the beauty of -monochrome is that it is smarter and often works without us having to guess anything.

Concrete example from: https://www.nasa.gov/mission_pages/galex/pia15416.html

wget -O orig.jpg http://www.nasa.gov/images/content/650137main_pia15416b-43_full.jpg
# Downsize to 400 height to have a reasonable file size for upload here.
convert orig.jpg -resize x400 in.jpg
convert -monochrome in.jpg out.jpg
convert -threshold 50 in.jpg threshold-50.jpg

in.jpg

out.jpg

threshold-50.jpg

Related questions:

• https://askubuntu.com/questions/9868/convert-an-image-from-grayscale-to-binary
• https://superuser.com/questions/75373/convert-color-photos-of-documents-to-good-black-and-white-bitonal-images
• https://unix.stackexchange.com/questions/108613/how-do-you-binarize-a-colored-image

Tested in Ubuntu 19.10, ImageMagick 6.9.10.