Re: [GM-help] "Posterize" Color Reduction

[Bob F. <bfr...@si...>]

On Tue, 24 Feb 2004, Kevin Myers wrote:
>
> Does either ImageMagick or GraphicsMagick have a color reduction option that
> can produce results similar to "Posterize" in the GIMP?
>
> I'm trying to do color reductions on 24 bit scanned tiff images of some data
> plots that only contain 5 original colors (white, black, red, green, blue)
> on the hardcopy documents.  Upon scanning, noise and intra-pixel smearing of
> colors at edges makes the resulting image files contain many more than the 5
> original colors.  I attempted to use convert's -colors option to reduce the
> colors back to the original 5, but that resulted in images with essentially
> varying levels of gray.  It appears that convert considered all of the
> various gray levels in the scanned image that were produced at black/white
> edge boundaries to be more significant than the relatively small (but much
> more important) red, green, and blue areas of the image.  On the other hand,
> if I use the GIMP's Posterize option, then I get essentially the desired
> result.  However, I need to do this processing in batch on a large number of
> images.  Therefore, I need to use something like ImageMagick or
> GraphicsMagick for this task in general, rather than the GIMP.

While it is not strictly a color reduction option, you can try the
image segmentation algorithm which coalesces regions of similar colors
For example

  gm convert input.tiff -segment 1.0x1.5 -colors 5 output.tiff

Color reduction is normally based on evaluating the number of uses of
a color while minimizing the maximum error.  Segmentation pools
regions of similar color ("posterization"), which effectively reduces
the number of colors used.

Unfortunately, the segmentation algorithm can be a bit slow, and it
seems to produce poor results for some images.  It seems to work best
for images which have already been color reduced to some extent.

It is possible that the -treedepth option may also have some useful
effect when quantizing the image.  A deeper treedepth (maximum 8) is
less likely to discard infrequently used colors.

Bob
======================================
Bob Friesenhahn
bfr...@si...
http://www.simplesystems.org/users/bfriesen