can not dither posterize quantilization

[Auguste]

according to http://www.imagemagick.org/Usage/quantize/#posterize , we can apply dither to the posterize operation.

however, withouth +dither, i get the same image that is not dithered.
and either -dither floydsteinberg or -dither riemersma failed to dither the image.

reproduction procedure:
1. download the logo.png from http://www.imagemagick.org/Usage/quantize/logo.png
2. convert logo.png +dither -posterize 2 miff:- | display -
3. convert logo.png -dither riemersma -posterize 2 miff:- | display -
4. convert logo.png -dither floydsteinberg -posterize 2 miff:- | display -
2,3,4 all produce the exactly same image.

[anthony]

I reported this when it first appeared myself. My understanding was that the change was caused by a change in the way the 'posterization' was performed. I would prefer a dithered result myself.