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

Solution 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

Solution 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.

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