#!/usr/bin/perl -I/usr/lib/perl5/site_perl -w
# 如果需要完整的顏色轉換程式, 請至 CPAN 下載 Image::Colorimetry 模組.

use strict;

sub HSBtoRGB {
# "Computer Graphics: Principles and Practice" Foley, Van Dam, Feiner, Hughes
    my ($h, $s, $b) = @_;
    my ($i, $f);
    $h = $h - int($h);
    $h = $h * 6;
    $i = int($h);
    $f = $h - $i;
    my ($p, $q, $t) = ($b*(1-$s), $b*(1-$s*$f), $b*(1-$s*(1-$f)));
    return
	$i == 0 ? ($b, $t, $p) :
	$i == 1 ? ($q, $b, $p) :
	$i == 2 ? ($p, $b, $t) :
	$i == 3 ? ($p, $q, $b) :
	$i == 4 ? ($t, $p, $b) :
	$i == 5 ? ($b, $p, $q) :
	die("strange?");
}

# use vars qw($PI $sin60);
# 
# BEGIN {
#     $PI = atan2(1,1) * 4;
#     $sin60 = sin($PI/3);
# }
# 
# # http://www.prip.tuwien.ac.at/~hanbury/CVWW02.pdf
# sub RGBtoHSB {
#     my ($r, $g, $b) = @_;
#     my ($y, $c1, $c2, $chrome, $h, $hstar);
#     $y  = 0.2125*$r + 0.7154*$g + 0.0721*$b;
#     $c1 =        $r -    0.5*$g -    0.5*$b;
#     $c2 =           - 0.8860*$g + 0.8860*$b;
#     $chrome = sqrt($c1*$c1 + $c2*$c2);
#     $h = atan2($c2, $c1);
#     $h /= 2 * $PI;
#     $h += 1 if $h < 0;
#     $hstar = $h * 6;
#     $hstar -= int($hstar);
#     my ($s) = $chrome*sin($PI/3*(2 - $hstar))/$sin60;
#     return ($h, $s, $y);
# }
# 
# # Inverse is difficult to calculate.
# # First use hue and brightness to find a ray.
# # Intersect this ray with one of the 6 faces of the cube.
# # Somewhere between the achromatic axis to the intersection
# # point lies the answer. The distance is determined by saturation.
# # The following code is too simplistic and is incorrect.
# sub HSBtoRGB {
#     my ($h, $s, $y) = @_;
#     my ($hstar, $chrome, $c1, $c2);
#     $hstar = $h * 6;
#     $hstar -= int($hstar);
#     $chrome = $s * $sin60 / sin($PI/3*(2 - $hstar));
#     $h *= 2 * $PI;
#     $c1 = $chrome * cos($h); $c2 = $chrome * sin($h);
#     my ($r, $g, $b) = (
# 	$y+0.7875*$c1+0.371409*$c2,
# 	$y-0.2125*$c1-0.205941*$c2,
# 	$y-0.2125*$c1+0.94876*$c2
#     );
#     return ($r, $g, $b);
# }
# 
# if (0) {
#     my ($i, @ans);
#     for ($i=0; $i<6; ++$i) {
# 	printf "hsb(%f,1,1) ==> ", $i/6.0;
# 	@ans = HSBtoRGB($i/6.0,1,1);
# 	printf "rgb(%f,%f,%f) ==> ", @ans;
# 	printf "hsb(%f,%f,%f)\n", RGBtoHSB(@ans);
#     }
# }

1;