Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!uupsi!halluc!malak
From: malak@halluc.com (Michael Malak)
Newsgroups: comp.graphics
Subject: Rgb -> 16 grayscale
Message-ID: <58.27ECA443@halluc.com>
Date: Sat, 23 Mar 91 12:32:00 EST
Organization: The Hallucination BBS, Fairfax VA (703) 425-5824

-> 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

First, sort the 256 colors on their greyscale equivalent, then tally up 
how many pixels there are in the image for each of the 256 colors.  Then 
break this "histogram" into 16 equal parts (equal number of pixels per "part"; 
the tallies help you do this).  So for each of the 16 parts you have to come up 
with a greyscale value for it; simply do an average weighted by the number of 
pixels associated with each original greyscale.

You may also want to consider F&S dithering, with or without the above 
technique.

--  
Michael Malak -- sysop of Hallucination

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