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From: kempkec@mist.CS.ORST.EDU (Christopher Kempke)
Newsgroups: comp.graphics
Subject: Re: RGB -> 16 grayscale
Message-ID: <1991Mar19.115516.21440@lynx.CS.ORST.EDU>
Date: 19 Mar 91 11:55:16 GMT
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In article <5083.669373541@ics.uci.edu> wong@ICS.UCI.EDU (Wang-chan Wong) writes:
>Hi,  I am working on a project that requires to display 256 colors with
>a 16 levels grayscale (as with a gas plasma screen.)  I understand
>that the following formula will compute the corresponding of graylevel
>given the rgb intensity:
>
>    grayscale= .299*red+ .587*green + .114*blue
>
>However, given 256 colors, this formula still maps onto more than 16
>distinct grayscales.  My question is: how to map exactly onto 16
>grayscales?  Are there any algorithms that map onto 16 grayscales
>while minimizing color collison at the same time? (i.e. I can still
>view a 256 pictures in the 16 grayscales gas plasma screen.)
>
>Thanks in advance.   -- wang


	Yes, easily.  Assuming that those (red, green, blue) values are
	0-255, you just divide the resultant value by 16, and you get 16
	possible outcomes.  Now just devide your available greys into
	16 possibilities 0 = black, 15 = white.

	The fundamental "secret" here is that 256 colors will always map
	to (roughly) 256 greyscales, so you will HAVE to have multiple
	colors represented by the same grey (intensity) value.  Just
	throw away the least significant bits (as many as necessary) to
	leave yourself with the number of intensity values you want.

	You ARE losing information this way, but there's no way to reflect
	the 256 colors with only 16 greys unless you halftone, dither, or
	use some other method which actually changes the pixel pattern.

		--Chris