How do I save a black & white picture to a 1-bit image file?

Question

I need a 1-bit image file to print on a laser engraver.

I used to have quite a few graphics applications that could save 1-bit black & white TIFF files. I've owned Macs since 1987. However most of my old apps won't run on current MacOS versions and I just discovered that my current graphics apps (Affinity Photo, Affinity Designer, Acorn, Pixelmator, Pixelmator Pro) won't save 1-bit files. 8-bit grayscale is as close as they get.

I have no problem editing an image so that it only contains black or white pixels. Pixelmator Pro's Format: Effects: Halftone: Dot Screen or Line Screen effects are great for what I want. It's the underlying file format that I'm having problems getting to with modern software.

I have a workaround. I can save an 8-bit TIFF to a USB thumb drive and open it in Microsoft Paint on the Windows PC attached to the laser cutter and save it as a 1-bit BMP or TIFF file for the laser cutter's software. However the grey haired Mac nerd in me is offended that Windows still has a feature that seems to have vanished from current Mac systems.

Does anyone have suggestions for an app to convert grayscale images to 1-bit TIFF files?

Answer 1

You can use ImageMagick commandline tool for that. If your image is already Black and White you simply do:

convert input_file.tif -depth 1 output_file.tif

If you have to make the file B/W just add -separate -black-threshold 128 (you can adjust the threshold for your needs).

Here's the exiftool output for my sample file:

Bits Per Sample : 1

Answer 2

On macOS you can use GNU Image Manipulation Program (GIMP) to create 1-bit TIFF files.

With the image opened, go to Image > Mode > Indexed > (•) Use black and white (1-bit) palette > Convert

Then File > Export As… > filename.tiff > Export.

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_{GIMP is a free and open source image editor.}