[magick-users] performance of colour reduction algorithm

[Stephan Hennig]

Hi,

Task:
Convert a image to grey scale with a limited number of colours.


Approach:
  convert bluebells_darker.jpg -colorspace Gray -colors 64 c.png

Checking the number of colours in c.png with

  identify -format "%k unique colors" c.png
  16 unique colors

shows that it has /far/ less than 64 colours.

For different arguments to '-colors <col>' I get the following results:

 <col>      colours in image
  1             1
  2             2
  3             3
  4 -  15       4
 16 -  63       8
 64 - 254      16
255            64

For different images there are other values.

Although, this is in accordance to the description of option -colors,
which says

> The actual number of colors in the image may be less than your 
> request, but never more.

I think the colour reduction algorithm performs really poor.  Is that a bug?

Best regards,
Stephan Hennig

> convert -version
> Version: ImageMagick 6.3.7 11/29/07 Q8 http://www.imagemagick.org
> Copyright: Copyright (C) 1999-2007 ImageMagick Studio LLC