NAME¶
Graphics::ColorObject - convert between color spaces
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%).¶
@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.
BUGS¶
Backwards compatibility with versions before 0.4 is not very well tested.
This module will produce results that are, in some cases, different from other
software. Most of the time that is not a bug in this module, but rather a case
where the other software uses an approximate (trading accuracy for speed)
algorithm. That is particularly true for YUV and related conversions which are
often implemented using integer-math approximations. As far as possible, this
module produces results which are exact according to the definitions in the
relevant CIE/ITU or other standards.
Some color transformations are not exactly reversible. In particular,
conversions between different white points are almost but not exactly
reversible. This is not a bug.
There is no way to choose any other color-adaptation algorithm than the Bradford
algorithm. That is probably ok since the Bradford algorithm is better than
other algorithms (such as Von Kries or simple scaling).
There is no way to choose a RGB space other than the built-in ones.
Support for CMYK is very basic, it relies on assumptions that completely do not
work in the physical world of subtractive pigment mixtures. If you tried to
convert an image to CMYK directly for printing using these functions, the
results will not be very good, to say the least.
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).
SEE ALSO¶
The Color FAQ by Charles Poynton is one of the definitive references on the
subject:
http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.txt
Bruce Lindbloom's web site contains a tremendous amount of information on color:
http://www.brucelindbloom.com/index.html?Math.html
AUTHOR¶
Alex Izvorski, <izv@dslextreme.com>
Alfred Reibenschuh <alfredreibenschuh@yahoo.com> was the original author
for versions up to 0.3a2.
Many thanks to:
Alfred Reibenschuh <alfredreibenschuh@yahoo.com> for the previous versions
of Graphics::ColorObject, and for the HSL/HSV/CMYK code.
Bruce Lindbloom <info@brucelindbloom.com> for providing a wealth of
information on color space conversion and color adaptation algorithms, and for
the precalculated RGB conversion matrices.
Charles Poynton <colorfaq@poynton.com> for the Color FAQ.
Timo Autiokari <timo.autiokari@aim-dtp.net> 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.