Path: utzoo!attcan!uunet!convex!killer!ames!oliveb!amiga!kodiak
From: kodiak@amiga.UUCP (Robert R. Burns)
Newsgroups: comp.sys.amiga
Subject: Re: RGB to HSV Colors
Summary: H-S-What?
Message-ID: <2245@amiga.UUCP>
Date: 30 May 88 18:27:49 GMT
References: <8805271453.AA13503@jade.berkeley.edu> <7271@swan.ulowell.edu>
Reply-To: kodiak@tooter.UUCP (Robert Burns)
Organization: Commodore-Amiga Inc, Los Gatos CA
Lines: 33

In article <7271@swan.ulowell.edu> dpelland@hawk.ulowell.edu (David Pelland) writes:
>In article <8805271453.AA13503@jade.berkeley.edu> BBOURBIN@UMDD.BITNET (Brett S Bourbin) writes:
>>Does anyone out there know of a formula for converting a RGB (red green blue)
>>color value, into a HSV (hue saturation value) one?
>
>H = cos^-1((1/2((R-G)+(R-G)))/(((R-G)^2+(R-B)(G-B))^(1/2)))
>S = 1 - ((3*(min(RGB)))/I)
>I = (R+B+G)/3

I'm jumping in in the middle of this, so I don't know the original question,
but this "I" seems to be that associated w/ the Tektronix double-ended cone
color model.  Most Amiga stuff I've seen uses the HSV cylinder, w/ black on
the bottom, white in the middle at the top, and fully saturated colors around
the outside of the cylinder.  My favorite reference is the 1978 (?) SIGGRAPH
proceedings (but I don't have them).  I think of HSV as...

H = [0 == 1 == red; .33 == green; .67 == blue]	// that may be the one above?
S = 1 - min(R, G, B);				// distance from "grey"
V = max(R, G, B);				// non-blackness

>((good) (luck) (with) (the) (parenthesis))

Different color models have different origins for the hue (e.g. 0 is red
vs. blue).  You can also skip the arccos and do linear interpolation in
the 6 sextants (?) of the hue -- remember you've only got to get it right
to 4 bits.  I admit I didn't decode those parantheses :->

- Kodiak
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