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From: raveling@isi.edu (Paul Raveling)
Newsgroups: comp.graphics
Subject: Re: How to map 24-bit RGB to best EGA/VGA palette?
Keywords: RGB EGA VGA color
Message-ID: <9473@venera.isi.edu>
Date: 31 Aug 89 18:23:35 GMT
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Reply-To: raveling@isi.edu (Paul Raveling)
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> 
> Now *this* is interesting! A geometric look at color space. How do
> the shapes of the plumes vary as one
>  + changes the ambient lighting
>  + changes the basis for color space (RGB to HSV, etc.) ?

	I haven't looked at enough yet to have a good feel for lighting
	changes.  Since most of the images are digitized photos, it
	requires a little intuition.  A further refinement that I hope
	to try today is to add another input knob to control a
	popularity threshold.  By twiddling this knob it should be
	possible to get something like a time-multiplexed histogram
	(pixel frequency over the RGB domain).

	I haven't tried HSV yet.  Maybe next week... this is still
	"extracurricular" work.

> On interesting, realistic images, do well-formed clusters appear in
> the 3D color space?

	For the most part, yes.  Most are much more well-formed than
	I'd expected them to be.  Some images also show fields with
	a scatter of points that are clearly related but not very
	dense in RGB space.  Synthetic images, such as one of Cristy's
	molecules or a Bill The Cat cartoon, look like a very sparse
	set of points that sometimes allow a "connect the dots" approach
	to make sense of them -- sort of like drawing constellations
	among the stars.

>  If so, one *could* attempt to reduce the pallette
> through the application of a traditional clustering algorithm (there
> was an article in ACM TOMS on this last year; ...

	Good question.  I'll look for the article & read it when
	time allows.  (BTW, right now I'm officially testing changes
	to xrn before releasing it to our users.)

	It might be possible that a Monte Carlo color selection algorithm
	would work well for some images.  I may try that as an experiment
	too.

> ... and I
> seriously doubt that the `clusters' in color space (if any) are
> (say) multivariate Gaussian; I also doubt that people are willing to wait
> hours (!) for a clustering algorithm to reduce their pallette.

	Caramba!  Maybe I'll only skim that article.


----------------
Paul Raveling
Raveling@isi.edu