The amount of divergence from the target palette which will be
Currently the only palette that is supported is RGB black and
white. If an RGB image contains only grayscale components, and
those grayscale pixels fit into the black and white palette with
less than the amount of divergence specified, then it will be
converted to one bit grayscale.
The tolerance is measured in terms of the average distance
between the colors of pixels and the color they would be converted
to in the palette. Because the maximum distance varies depending on
the number of components in the color space, this distance is then
rescaled to a value between zero and one. Zero means that the
pixels and the palette need to match exactly. One means all pixels
will always match.