ITU-T T.4 defines 'black' and 'white' (referring to fore- and back-
ground, respectively) pixel sequences, but it doesn't prescribe which
of those is represented by 0 or 1. That would have to be defined by the
BMP specification, but documentation on Huffman BMPs is close to
non-existent.
Current consensus seems to be that 'black' is 1, i.e. indexing the
second color in the palette, and 'white' is 0, i.e. indexing the first
color.
In case that's wrong (in fact it's not even clear if there is a right
and a wrong), bmp_set_huffman_t4black_value() can be used to set the
pixel value of 'black' to either 0 or 1 (and white to the respective
opposite).
Can be used both for reading and writing BMPs.
Changing this value will invert the image colors.
