Working with Black-and-White and Color Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 318

4.1.5 Converting Images into Black-and-White
Graphics
It is possible to convert images into pure black-and-
white graphics with GIMP. Threshold is the quick-and-
dirty function used to convert images and can be found
in the Colors menu. It doesn't matter if the image is a
color or grayscale image, it converts the image into a
pure black-and-white graphical image. The only option
for adjustment is the triangular slide control under the
histogram, marking the point the contrast is inverted.
The GEGL operation threshold ( Tools > GEGL Operation )
works in a similar fashion.
Threshold achieves this effect of black-and-white
inversion by means of a value from which the inversion
from black-and-white occurs but without considering
gradients and shades of brightness.
You obtain a pure black-and-white graphic image by
inverting the image with the Indexed Colors mode
( Image > Mode > Indexed ), using Color dithering . The
result is a simulation of brightness gradients through
dithering (in this case, diffusion of black pixels to
achieve grayscale simulation). In the Convert Image to
Indexed Colors window, first select the option Use black-
and-white (1-bit) palette . Then select Dithering , thereby
selecting the method of diffusion. Experiment a little.
The results depend on how much contrast the image had
beforehand and which dithering you choose. If you want
to dye your image afterward, you must adjust the RGB
mode ( Image > Mode ) again. Otherwise, further editing
isn't possible.
Figure 4.12
Using the Threshold function to achieve graphic effects in
black-and-white