Path: utzoo!attcan!uunet!cs.utexas.edu!sun-barr!newstop!sun!falk
From: falk@sun.Eng.Sun.COM (Ed Falk)
Newsgroups: comp.graphics
Subject: Re: Color quantization: flesh tones
Message-ID: <125134@sun.Eng.Sun.COM>
Date: 22 Sep 89 06:52:04 GMT
References: <6087@pt.cs.cmu.edu> <390037@hpfcdq.HP.COM>
Organization: Sun Microsystems, Inc. - Mtn View, CA
Lines: 61

In article <390037@hpfcdq.HP.COM>, jlg@hpfcdq.HP.COM (Jeff Gerckens) writes:
> > > 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.

All true.  Mea Culpa.  Here are the equations from the FCC regs:

	Y = .30R + .59G + .11B
	I = -.27(B-Y) + .74(R-Y)
	Q =  .41(B-Y) + .48(R-Y)

In matrix form, this is:

	|Y|   | .300  .590  .110 | |R|
	|I| = | .599 -.277 -.322 | |G|
	|Q|   | .213 -.525  .312 | |B|

	|R|   | 1.   .947  .624 | |Y|
	|G| = | 1.  -.275 -.636 | |I|
	|B|   | 1. -1.108 1.709 | |Q|

The I axis was chosen to be the fleshtone axis, and about three times as
much power is transmitted on this axis as on the Q axis.  This way, a weak
signal will not degrade flesh tones as much as other colors.

-- 
		-ed falk, sun microsystems, sun!falk, falk@sun.com

  "If you wrapped yourself in the flag like George Bush does, you'd
  be worried about flag-burning too"