Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!pasteur!ucbvax!decwrl!mejac!gryphon!pnet02!dsears
From: dsears@pnet02.cts.com (Doug Sears)
Newsgroups: comp.sys.amiga
Subject: RGB to HSV, and RGB to CMY
Message-ID: <4402@gryphon.CTS.COM>
Date: 10 Jun 88 07:20:39 GMT
Sender: root@gryphon.CTS.COM
Organization: People-Net [pnet02], Redondo Beach, CA.
Lines: 65

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kodiak@amiga.UUCP (Robert R. Burns) writes:
>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
 
>  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
 
Not bad for quoting from memory. Actually, it's a hexcone, not a
cylinder, unless there's more than one HSV model.  The article Kodiak was
referring to is probably "Color Gamut Transform Pairs", by Alvy Ray
Smith, SIGGRAPH 78 Proceedings, pp. 12-19 (also reprinted in "Tutorial:
Computer Graphics", 2nd ed, J.C. Beatty & K.S. Booth, eds, pp. 376-383).
They introduced the hexcone model (HSV) and the triangle model (HSL).
Here's their algorithm for the hexcone, RGB to HSV (R, G, B, H, S, and V
all run from 0 to 1):
 
V := max(R, G, B);
 
Let X := min(R, G, B);
 
S := (V - X)/V;   if S=0 return;
 
Let r := (V - R)/(V - X);
    g := (V - G)/(V - X);
    b := (V - B)/(V - X);
 
If R=V then H := (if G=X then 5+b else 1-g);
If G=V then H := (if B=X then 1+r else 3-b);
       else H := (if R=X then 3+g else 5-r);
 
H := H/6;
 
  You have to watch out for when S = 0 (H is undefined).  Another
explanation of the hexcone model is in the September 1984 BYTE, pp.
227-246.  David Pelland's formula looks like the HSL triangle model,
"unbiased case".
 
  Now for a question: any hints on converting RGB to CMY or CMYB
(cyan/magenta/yellow/black: subtractive color) as applied to low-cost
color printers?  C = 1 - R, etc. is too simple. The Neugebauer
equations take into account the properties of real-life colored
inks and variable overlap of dots, but they're real hard to solve
(system of three NONlinear equations).  Has anybody come up with a
practical approach?  I'm afraid I'll have to bypass the printer.device
to make use of this, since it only has the one graphics function,
DumpRPort().

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