Convert image.png to a farbfeld, run it through a filter and write the result to image-filtered.png:

    $ png2ff < image.png | filter | ff2png > image-filtered.png

invert.c is an example for such a filter which inverts the colors. Notice that there are no dependencies on external libraries. A hypothetical farbfeld-library would hardly exceed invert.c's size.

Store a png as a compressed farbfeld:

    $ png2ff < image.png | bzip2 > image.ff.bz2

Access a compressed farbfeld as a png:

    $ bunzip2 < image.ff.bz2 | ff2png {> image.png, | feh -, ...}

Handle arbitrary image data using 2ff(1), which falls back to imagemagick's convert(1) for unknown image types:

    $ 2ff < image | filter | ff2png > image-filtered.png

Refer to the manpages for more information. farbfeld(5) is a good start.