SYNOPSIS

use Graphics::ColorObject;

# rgb to hsv
$color = Graphics::ColorObject->new_RGB([$r, $g, $b]);
($h, $s, $v) = @{ $color->as_HSV() };

# one rgb space to another (NTSC to PAL)
$color = Graphics::ColorObject->new_RGB([$r, $g, $b], space=>'NTSC');
($r, $g, $b) = @{ $color->as_RGB(space=>'PAL') };

ABSTRACT

Use this module to convert between all the common color spaces. As a pure Perl module, it is not very fast, and so it you want to convert entire images quickly, this is probably not what you want. The emphasis is on completeness and accurate conversion.

Supported color spaces are: RGB (including sRGB, Apple, Adobe, CIE Rec 601, CIE Rec 709, CIE ITU, and about a dozen other RGB spaces), CMY, CMYK, HSL, HSV, XYZ, xyY, Lab, LCHab, Luv, LCHuv, YPbPr, YCbCr, YUV, YIQ, PhotoYCC.

Conversion between different RGB working spaces, and between different white-points, is fully supported.

DESCRIPTION

For any supported color space XXX, there is one constructor new_XXX that creates a color using data in that color space, and one method as_XXX that returns the current color as expressed in that color space. For example, for RGB there is new_RGB and as_RGB. The color data is always passed as an array reference to a three-element array (four-element in the case of CMYK). Thus, to convert from RGB to HSL, you can use:

  $color = Graphics::ColorObject->new_RGB([$r, $g, $b]);
  ($h, $s, $l) = @{ $color->as_HSL() };

The constructor can always take a hash of optional arguments in addition to the color value, namely the working RGB space and the white point. For example:

  $color = Graphics::ColorObject->new_RGB([$r, $g, $b], space=>'Adobe', white_point=>'D65');

For a list of all supported color spaces, call Graphics::ColorObject->list_colorspaces(). For a list of all RGB working spaces and of all white points that this module supports, call Graphics::ColorObject->list_rgb_spaces() and Graphics::ColorObject->list_white_points().

If not specified, the working RGB space will be sRGB. Many non-RGB conversions also rely on an implicit RGB space, and passing an RGB space as an option (either to the constructor or later) will have an effect on the values.

VARIOUS NOTES AND GOTCHAS

Most conversions will return out-of-gamut values if necessary, because that way they are lossless and can be chained in calculations, or reversed to produce the original values. Many conversion methods will take an optional boolean ``clip'' parameter to restrict the returned values to be within gamut:

  ($r, $g, $b) = @{ $color->as_RGB(space=>'sRGB', clip=>1) };

Currently clipping is supported in RGB, RGB-derived (HSL, CMY) and chroma-luma separated (YUV, etc) spaces, but not in XYZ-derived spaces. The only way to check whether a value is within gamut is to convert it with and without the clip option and compare the two results. An RGB value is within gamut simply if R, G and B are between 0 and 1, but other spaces can be much harder to check.

RGB values are non-linear (gamma-adjusted) floating-point values scaled in the range from 0 to 1. If you want integer values in the range 0..255, use the new_RGB255/as_RGB255 functions instead. If you want linear RGB (not gamma-adjusted) use RGB_to_linear_RGB([$r, $g, $b]).

Functions that use an angle value always express it in degrees from 0 to 360. That includes the hue H in HSL, HSV, LCHab and LCHuv. Use rad2deg and deg2rad from Math::Trig to convert to/from degrees if necessary.

There is some confusion in the naming of YUV and related (Y-something-something) colorspaces. Most of the time when ``YUV'' or ``YCC'' is used in software, for example in JPEG and MPEG2, that is actually YCbCr, a chroma-luma separated space with integer values of Y in the range [16..235], Cb and Cr in [16..240]. JPEG uses a modified YCbCr with values in [0..255] (which is not implemented in this module). As used here, YUV is a floating-point representation of the analog signal in PAL TV, YIQ is the same for NTSC TV, YPbPr is component analog video, and PhotoYCC or YCC is the Kodak PhotoCD standard.

The set_white_point() function can take arbitrary temperatures as well as the predefined standard illuminants. The valid range of temperatures is from 4000K to 25000K.

RECOMMENDATIONS

Aside from converting from one space to another, what colorspace is the best one to use for a particular task? This section attempts to answer that question.

For ``generic'' RGB values, use sRGB (which is the default).

For 2D effects filters, use Lab (or LCHab).

For adjustment of brightness, saturation and hue, use LCHab or LSHab.

For compression, use YCbCr, or use YPbPr and convert to integer values in a way that makes sense in your application.

For representing data as colors, use Lab (straight lines between points in Lab are more-or-less uniform gradients, unlike straight lines in RGB, for example).

UPGRADING FROM 0.3a2 AND OLDER VERSIONS

Version 0.4 and later are a complete rewrite from the previous major version, 0.3a2. The API is completely changed. The old API should be emulated exactly in all cases. Please test any code that uses this module when upgrading. If you encounter any strange behavior, please downgrade to 0.3a2 and email me a bug report. Additionally, the exact values returned by some functions may be slightly different, this is not a bug - the new values are (more) correct.

METHODS

$color = Graphics::ColorObject->new_XYZ([$X, $Y, $Z])

$color = Graphics::ColorObject->new_xyY([$x, $y, $Y])

$color = Graphics::ColorObject->new_RGB([$R, $G, $B])

$color = Graphics::ColorObject->new_RGB255([$R, $G, $B])

$color = Graphics::ColorObject->new_RGBhex($rgbhex)

$color = Graphics::ColorObject->new_Lab([$L, $a, $b])

$color = Graphics::ColorObject->new_LCHab([$L, $C, $H])

$color = Graphics::ColorObject->new_Luv([$L, $u, $v])

$color = Graphics::ColorObject->new_LCHuv([$L, $C, $H])

$color = Graphics::ColorObject->new_HSL([$H, $S, $L])

$color = Graphics::ColorObject->new_HSV([$H, $S, $V])

$color = Graphics::ColorObject->new_CMY([$C, $M, $Y])

$color = Graphics::ColorObject->new_CMYK([$C, $M, $Y])

$color = Graphics::ColorObject->new_YPbPr([$Y, $Pb, $Pr])

$color = Graphics::ColorObject->new_YCbCr([$Y, $Cb, $Cr])

$color = Graphics::ColorObject->new_YUV([$Y, $Cb, $Cr])

$color = Graphics::ColorObject->new_YIQ([$Y, $I, $Q])

$color = Graphics::ColorObject->new_PhotoYCC([$Y, $C1, $C2])

($X, $Y, $Z) = @{ $color->as_XYZ() }

($R, $G, $B) = @{ $color->as_RGB() }

($R, $G, $B) = @{ $color->as_RGB255() }

$hex = $color->as_RGBhex()

($x, $y, $Y) = @{ $color->as_xyY() }

($L, $a, $b) = @{ $color->as_Lab() }

($L, $C, $H) = @{ $color->as_LCHab() }

($L, $u, $v) = @{ $color->as_Luv() }

($L, $C, $H) = @{ $color->as_LCHuv() }

($H, $S, $L) = @{ $color->as_HSL() }

($H, $S, $V) = @{ $color->as_HSV() }

($C, $M, $Y) = @{ $color->as_CMY() }

($C, $M, $Y, $K) = @{ $color->as_CMYK() }

($Y, $Pb, $Pr) = @{ $color->as_YPbPr() }

($Y, $Cb, $Cr) = @{ $color->as_YCbCr() }

($Y, $U, $V) = @{ $color->as_YUV() }

($Y, $I, $Q) = @{ $color->as_YIQ() }

($Y, $C1, $C2) = @{ $color->as_PhotoYCC() }

$white_point = $color->get_white_point() Returns the name of the current white point. Value is one of the entries returned from list_white_points, such as D65, or a color temperature.

$color->set_white_point(D65) Sets the current white point by name. Argument is one of the entries returned from list_white_points, or a temperature value like 6800K. This changes the current color slightly since white-point adaptation is not completely reversible. This does not affect the current RGB space, thus it is possible to use RGB spaces at whitepoints other than those they were defined at.

$rgb_space = $color->get_rgb_space() Returns the name of the current RGB color space. Value is one of the entries returned from list_rgb_spaces, such as NTSC.

$color->set_rgb_space(NTSC) Sets the current RGB color space by name. Argument is one of the entries returned from list_rgb_spaces. This may change the current color if the old and new spaces have different white points.

$color2 = $color->copy() Creates an exact duplicate of the current color.

if ($color->equals($color2)) { ... } Checks if another color is the same as this one. Optionally takes an accuracy parameter which is the distance between the two colors as measured by the city-block metric in XYZ space (default accuracy is 0.01%).

$d = $color->difference($color2) Calculates the difference between this color and another one. The difference measure is (approximately) perceptually uniform.

@colorspaces = &Graphics::ColorObject->list_colorspaces() Returns a list of all supported colorspaces.

@rgb_spaces = &Graphics::ColorObject->list_rgb_spaces() Returns a list of all supported RGB spaces. Some items are aliases, so the same space may be listed more than once under different names.

@white_points = &Graphics::ColorObject->list_white_points() Returns a list of all supported white points.

EXPORT

None by default. The 'all' tag causes the non-object-oriented interface to be exported, and you get all the XXX_to_YYY functions, for example RGB_to_XYZ. Please note that some of these functions need extra arguments in addition to the color value to be converted.

TODO

Add clipping to gamut for every color space.

Choose between several clipping algorithms (nearest, luminance-preserving, hue-preserving).

Add a simpler way to check whether something is within gamut.

Add user-defined RGB spaces.

Calculate RGB matrices from chromaticity coordinates.

Only load non-RGB matrices once at startup.

Add colorspaces: uvw, YOZ, RYB, others?

Add RGB spaces: ROMM, others?

Convert arrays of colors efficiently (maybe someday in C).

AUTHOR

Alex Izvorski, <[email protected]>

Alfred Reibenschuh <[email protected]> was the original author for versions up to 0.3a2.

Many thanks to:

Alfred Reibenschuh <[email protected]> for the previous versions of Graphics::ColorObject, and for the HSL/HSV/CMYK code.

Bruce Lindbloom <[email protected]> for providing a wealth of information on color space conversion and color adaptation algorithms, and for the precalculated RGB conversion matrices.

Charles Poynton <[email protected]> for the Color FAQ.

Timo Autiokari <[email protected]> for information on white points.

COPYRIGHT AND LICENSE

Copyright 2003-2005 by Alex Izvorski

Portions Copyright 2001-2003 by Alfred Reibenschuh

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.