Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!unmvax!ogccse!cvedc!nosun!fpssun.fps.com!celit!hutch
From: hutch@fps.com (Jim Hutchison)
Newsgroups: comp.graphics
Subject: Re: Color quantization: flesh tones (really RGB->YIQ)
Keywords: color rgb yiq quantization
Message-ID: <643@celit.fps.com>
Date: 19 Sep 89 16:56:02 GMT
References: <6087@pt.cs.cmu.edu> <124742@sun.Eng.Sun.COM> <1212@midgard.Midgard.MN.ORG>
Sender: daemon@fps.com
Reply-To: hutch@fps.com (Jim Hutchison)
Organization: FPS Computing
Lines: 35

In <1212@midgard.Midgard.MN.ORG> dal@midgard.Midgard.MN.ORG (Dale Schumacher):
[... In NTSC ...]
>I thought Y was the intensity (luminance) component... and the most
>bandwidth is used for the luminance, with less for the color components.
>Here are the integer [0..255] pixel value formulae that I use:

>	Y = (((77 * R) + (150 * G) + (29 * B)) / 256);
>	I = (((153 * R) + (-70 * G) + (-82 * B)) / 256);
>	Q = (((54 * R) + (-134 * G) + (80 * B)) / 256);

>	R = (((256 * Y) + (245 * I) + (159 * Q)) / 256);
>	G = (((256 * Y) + (-70 * I) + (-167 * Q)) / 256);
>	B = (((256 * Y) + (-283 * I) + (436 * Q)) / 256);

>The above forms the heart of a utility I wrote to set the luminance (Y)
>of a color image from a monochrome image.  I use this in convolutions,
>particularly for edge sharpening, such that I do the convolution only
>on the luminance component, then recombine the output with the original
>color image.

It would seem that by using these equations, you might end up with a fair
amount of error in the process of remapping the colors.  Atleast you will
want to round-up in order to halve the error.  E.g.

	Y = ((77 * R) + (150 * G) + (29 * B) + 128) / 256;

Or, you could use the scaled numbers for the convolution and only rescale
them when you reconvert to RGB.  This might make your convolution algorithm
to messy, perhaps you might want to just save the error from the original
RGB->YIQ conversion and add that back into the RGB at the end.  Have you
noticed the error in your output?  Is it significant enough to cause
darkening of image or loss of shadow detail?

/*    Jim Hutchison   		{dcdwest,ucbvax}!ucsd!celerity!hutch  */
/*    Disclaimer:  I am not an official spokesman for FPS computing   */