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From: dal@midgard.Midgard.MN.ORG (Dale Schumacher)
Newsgroups: comp.graphics,comp.windows.x
Subject: Re: Luminance from RGB
Message-ID: <518@midgard.Midgard.MN.ORG>
Date: 13 Dec 88 23:21:38 GMT
References: <8241@pasteur.Berkeley.EDU>
Reply-To: dal@midgard.Midgard.MN.ORG (Dale Schumacher)
Organization: The Midgard Realm, St Paul MN
Lines: 20

In article <8241@pasteur.Berkeley.EDU> ph@miro.Berkeley.EDU (Paul Heckbert) writes:
|
|Paraphrasing, Dale is convering the 3 bit number abc, where each of a, b,
|and c are 0 or 1, into the 8 bit number abcabcab.
|
|This is very close to the "correct" formula, but you've found a somewhat
|roundabout way to compute it.  The formula you want will map black (000)
|to black (00000000) and white (111) to white (11111111) and map everything
|inbetween linearly.  In other words, you want to multiply by 255/7.
|Your formula actually multiplies by 255.9375/7.

The point of my "round-about" method is performance.  It's much easier to
replicate bits and do shifts than to divide by 7.  I believe that this
approximation will yield the "correct" value (to 8-bit int precision)
for all cases, right?

|Dale also asked about algorithms for selecting the 16 colors out of a
|palette of 512 that best represent an image.  This is called "color image
|quantization".  I wrote about it in a paper:

Thank you for the references.  I'll check into them.