Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!bionet!agate!ucbvax!hplabs!hpfcso!hpfcdq!jlg
From: jlg@hpfcdq.HP.COM (Jeff Gerckens)
Newsgroups: comp.graphics
Subject: Re: Color quantization: flesh tones
Message-ID: <390037@hpfcdq.HP.COM>
Date: 19 Sep 89 00:32:23 GMT
References: <6087@pt.cs.cmu.edu>
Organization: Hewlett-Packard - Fort Collins, CO
Lines: 38

> > Has anyone experimented with psychological color preferences in
> > quantizing using Heckbert's median cut?  Here's an example problem:
> > 
> >     In some images containing people's faces, where the face is
> >     only a small part of the image, very few colors are assigned
> >     to "flesh" color.  The result is banding/loss of resolution in
> >     an area of the image that is interesting to the viewer out of
> >     proportion to its relative size.  The problem is most severe
> >     when quantizing to 32 or fewer colors.
> > 
> 
> Here's a thought; try converting RGB to the NTSC IYQ coordinates
> and quantize in IYQ space.  I suggest this because NTSC chose
> the Y axis to be biased towards flesh tones and TV pictures transmit
> more power along that axis than along the Q axis (I is intensity).
> 
> I'm sorry, but I don't have the transformation matrix from RGB to IYQ handy.
> 
> -- 
> 		-ed falk, sun microsystems, sun!falk, falk@sun.com

Almost....

The Y axis in the NTSC encoding (IYQ) is the intensity, which was selected to
match the CIE-1931 XYZ intensity for the NTSC standard phosphors.  Both the I
axis and the Q axis are named after the signal encoding technique used, being
In-phase and Quadrature, respectively. The I and Q axis carry the chromaticity
information, and are selected for encoding the most information on the I and
less on the Q.

Use of this space will only affect your results if you weight the different 
axis differently, since the linear transform to/from YIQ and RGB yields the 
same results for any linear interpolation regardless of which space the 
interpolation takes place in.

- Jeff Gerckens, Graphics Technology Division, Hewlett-Packard Company.
  ...!hplabs!hpfcla!jlg
  "What color is a white horse in a dark room?"