From: ellman AT xs4all DOT nl ()
Newsgroups: comp.os.msdos.djgpp
Subject: Re: RGB HSV functions that "actually work"
Date: 25 Oct 1997 15:25:28 GMT
Organization: XS4ALL
Message-ID: <62t318$269$1@news2.xs4all.nl>
References: <199710202348 DOT TAA00340 AT delorie DOT com> <62pai0$8p3 AT freenet-news DOT carleton DOT ca>
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In article <62pai0$8p3 AT freenet-news DOT carleton DOT ca>,
Paul Derbyshire <ao950 AT FreeNet DOT Carleton DOT CA> wrote:
>
>"Andrei 'old boy' Ellman" (ellman AT xs4all DOT nl) writes:
>> + RGB <-> HSV functions that actually work, as well as RGB <-> HLS
>> functions. 
>
>Is something wrong with the ones in Allegro??

Allegro's RGB <-> HSV functions often don't procuce the correct results.
This is because whoever wrote them forgot to properly convert floats in the
range [0...1] to ints in the range [0...63]. Instead, they just cast the
float directly into an int. When Shawn added this to Allegro 2.2, he did not
test it, and the result was a buggy set of RGB <-> HLS functions. (I think
there may have been another bug as well)>

The ones in AllegroPak are a slight modification from the ones in
Principals of Computer Graphics by Foley & VanDam. The difference is that
they are less strict about the inputs (eg. they don't return an error if
saturation is 0.0 and hue is not UNDEFINED). Also, the RGB <-> HLS functions
which are not in Allegro at all, have had a bug fixed that appeared in
Foley & VanDam (the transition of L values from 0 to 1 is smoother).

Allegro 3.0 WIP includes corrected RGB <-> HSV functions, but I'm not sure
if Shawn used my fix or not.

AE.


Andrei Ellman -- URL: http://www.xs4all.nl/~ellman/ae-a -- ae1 AT york DOT ac DOT uk
"All I wanna do is have some fun     :-)      ||  ae-a AT minster DOT york DOT ac DOT uk
 I've got the feeling I'm not the only one"   ||  mailto:ellman AT xs4all DOT nl
     -- Sheryl Crow      :-)    ||       It's what you make of it.