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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: <572@midgard.Midgard.MN.ORG>
Date: 9 Jan 89 21:20:06 GMT
References: <23105@apple.Apple.COM> <2263@eos.UUCP> <83604@sun.uucp>
Reply-To: dal@syntel.UUCP (Dale Schumacher)
Organization: The Midgard Realm, St Paul MN
Lines: 23

In article <83604@sun.uucp> falk@sun.uucp (Ed Falk) writes:
|In article <2263@eos.UUCP>, jbm@eos.UUCP (Jeffrey Mulligan) writes:
|> From article <23105@apple.Apple.COM>, by turk@Apple.COM (Ken "Turk" Turkowski):
|> > Y = (R + 2G + B) / 4
|> Y = 2R + 5G + B
|    out = (77*r + 151*g + 28*b)/256 ;	/* NTSC weights (.3,.59,.11)*/
|
|The results are correct to four decimal places and the divide is replaced
|by a right-shift in a decent compiler and a byte-move in a good compiler.

I don't know where you got your numbers.  The values I have for the Y
component of YIQ (luminance) from RGB are: R=.299 G=.587 B=.144
The following formula is the best approximation with 8-bit values:
  Y = (R*77 + G*150 + B*29) / 256
Which gives the weights: R=.3008 G=.5859 B=.1133,   total error=.0036
Your values give the weights: R=.3008 G=.5898 B=.1094,   total error=.0092
Even MY numbers don't have 4 places of accuracy, but they are a better
approximation to the 3 place target values I have.  Someone mentioned that
the NTSC weight may have been changed recently, is that so?

PS.  I fully agree with the idea that more accurate values should be used
if you're going to use integer, and do 3 multiplies and a 'divide' (which
can be optimized if it's a power of 2) anyway.