Xref: utzoo comp.graphics:4121 comp.windows.x:7305
Path: utzoo!attcan!uunet!seismo!sundc!pitstop!sun!falk
From: falk@sun.uucp (Ed Falk)
Newsgroups: comp.graphics,comp.windows.x
Subject: Re: Luminance from RGB
Message-ID: <85366@sun.uucp>
Date: 14 Jan 89 19:52:17 GMT
References: <23105@apple.Apple.COM> <2263@eos.UUCP> <83604@sun.uucp> <7559@watcgl.waterloo.edu>
Organization: Sun Microsystems, Inc. - Mtn View, CA
Lines: 24

> >	BTW, this is an example of a function that couldn't easily
> >	be accelerated by table lookup unless R, G, and B have very
> >	few bits.  Even then 3D subscript computation puts the table
> >	lookup in the same speed range as the faster 2 of these
> >	alternatives.
> 
> Huh?  Table look up can be used to replace each of the three multiplies
> in aR+bG+cB so that the code becomes something like
> 
> 	a[R]+b[G]+c[B]
> 

I'm embarrassed.  I had been doing (r*77 + g*150 + 29*b)/256 all along
thinking that all multiplies took the same amount of time (the compiler,
it turns out, optimizes constant multiplies in interesting ways).

I've switched all my code to use look-up tables now.  I've gained a new
respect for look-up tables.


-- 
		-ed falk, sun microsystems
		 sun!falk, falk@sun.com
		 card-carrying ACLU member.