Re: [Gimp-developer] Floyd-Steinberg/GIF color optimization

[Robert Hanson <hansonr stolaf edu>]

Thank you, Mukund, ...

On Tue, Oct 21, 2014 at 10:47 AM, Mukund Sivaraman <muks banu com> wrote:

> Hi Robert
>
> On Tue, Oct 21, 2014 at 10:04:14AM -0500, Robert Hanson wrote:
> > http://chemapps.stolaf.edu/jmol/test/dither.htm .  The solution was to
>
> This page seems to use JavaScript and WebGL, and so I'm not able to view
> it. It would have been better if you had sent a link to static images of
> the source and result.

It's not WebGL -- just HTML5. But that is a fine idea:
http://chemapps.stolaf.edu/jmol/test/dither2.htm


> > partition red, green, and blue channels into a 6 x 6 x 6 color space,
> > allowing 216 colors in total. Not perfect, but amazingly good, I think.
>
> So you are doing FS dithering with a fixed uniform palette?

That sounds right. I'm not familiar with the lingo here. Equal partitions
along R G B coordinates.


> > The problem comes when I have a small molecule with not too many colors.
> If
> > I go just over 256, then the FS algorithm cuts this down to something
> like
> > 64 colors. I'm wondering if I could get some advise as to how to
> optimize a
> > set of colors down to 255 colors without undershooting in such a dramatic
> > way while still using FS.
>
> You are looking for color quantization methods. You can find description
> of an octree method in Graphics Gems I. GIMP uses median cut.
>
> See https://www.cs.cmu.edu/~ph/ciq_thesis for Heckbert's thesis on
> median cut.
That's helpful. So we have n colors, each with p_i pixels. This median cut
idea just
considers the n colors, not p_i, correct? (Ignores frequency?)

How is this then also implemented with Floyd-Steinberg dithering? Is the
idea that
the averages of the median cut boxes form our fixed points for the
dithering? That
a nearest-point calculation is done to generate the error function?

Can you point me to the place in the GIMP code this is accomplished?


Bob


-- 
Robert M. Hanson
Larson-Anderson Professor of Chemistry
Chair, Department of Chemistry
St. Olaf College
Northfield, MN
http://www.stolaf.edu/people/hansonr


If nature does not answer first what we want,
it is better to take what answer we get.

-- Josiah Willard Gibbs, Lecture XXX, Monday, February 5, 1900