Web colors
==========
A simple library for working with the color names and color codes
defined by the HTML and CSS specifications.
An overview of HTML and CSS colors
----------------------------------
Colors on the Web are specified in `the sRGB color space`_, where each
color is made up of a red component, a green component and a blue
component. This is useful because it maps (fairly) cleanly to the red,
green and blue components of pixels on a computer display, and to the
cone cells of a human eye, which come in three sets roughly
corresponding to the wavelengths of light associated with red, green
and blue.
`The HTML 4 standard`_ defines two ways to specify sRGB colors:
* A hash mark ('#') followed by three pairs of hexdecimal digits,
specifying values for red, green and blue components in that order;
for example, ``#0099cc``. Since each pair of hexadecimal digits can
express 256 different values, this allows up to 256**3 or 16,777,216
unique colors to be specified (though, due to differences in display
technology, not all of these colors may be clearly distinguished on
any given physical display).
* A set of predefined color names which correspond to specific
hexadecimal values; for example, ``white``. HTML 4 defines sixteen
such colors.
`The CSS 2 standard`_ allows any valid HTML 4 color specification, and
adds three new ways to specify sRGB colors:
* A hash mark followed by three hexadecimal digits, which is expanded
into three hexadecimal pairs by repeating each digit; thus ``#09c``
is equivalent to ``#0099cc``.
* The string 'rgb', followed by parentheses, between which are three
numeric values each between 0 and 255, inclusive, which are taken to
be the values of the red, green and blue components in that order;
for example, ``rgb(0, 153, 204)``.
* The same as above, except using percentages instead of numeric
values; for example, ``rgb(0%, 60%, 80%)``.
`The CSS 2.1 revision`_ does not add any new methods of specifying
sRGB colors, but does add one additional named color.
`The CSS 3 color module`_ (currently a W3C Candidate Recommendation)
adds one new way to specify sRGB colors:
* A hue-saturation-lightness triple (HSL), using the construct
``hsl()``.
It also adds support for variable opacity of colors, by allowing the
specification of alpha-channel information, through the ``rgba()`` and
``hsla()`` constructs, which are identical to ``rgb()`` and ``hsl()``
with one exception: a fourth value is supplied, indicating the level
of opacity from ``0.0`` (completely transparent) to ``1.0``
(completely opaque). Though not technically a color, the keyword
``transparent`` is also made available in lieu of a color value, and
corresponds to ``rgba(0,0,0,0)``.
Additionally, CSS3 defines a new set of color names; this set is taken
directly from the named colors defined for SVG (Scalable Vector
Graphics) markup, and is a proper superset of the named colors defined
in CSS 2.1. This set also has significant overlap with traditional X11
color sets as defined by the ``rgb.txt`` file on many Unix and
Unix-like operating systems, though the correspondence is not exact;
the set of X11 colors is not standardized, and the set of CSS3 colors
contains some definitions which diverge significantly from customary
X11 definitions (for example, CSS3's ``green`` is not equivalent to
X11's ``green``; the value which X11 designates ``green`` is
designated ``lime`` in CSS3).
.. _the sRGB color space: http://www.w3.org/Graphics/Color/sRGB
.. _The HTML 4 standard: http://www.w3.org/TR/html401/types.html#h-6.5
.. _The CSS 2 standard: http://www.w3.org/TR/REC-CSS2/syndata.html#value-def-color
.. _The CSS 2.1 revision: http://www.w3.org/TR/CSS21/
.. _The CSS 3 color module: http://www.w3.org/TR/css3-color/
What this module supports
-------------------------
The mappings and functions within this module support the following
methods of specifying sRGB colors, and conversions between them:
* Six-digit hexadecimal.
* Three-digit hexadecimal.
* Integer ``rgb()`` triplet.
* Percentage ``rgb()`` triplet.
* Varying selections of predefined color names (see below).
This module does not support ``hsl()`` triplets, nor does it support
opacity/alpha-channel information via ``rgba()`` or ``hsla()``.
If you need to convert between RGB-specified colors and HSL-specified
colors, or colors specified via other means, consult `the colorsys
module`_ in the Python standard library, which can perform conversions
amongst several common color spaces.
.. _the colorsys module: http://docs.python.org/library/colorsys.html
Normalization
-------------
For colors specified via hexadecimal values, this module will accept
input in the following formats:
* A hash mark (#) followed by three hexadecimal digits, where letters
may be upper- or lower-case.
* A hash mark (#) followed by six hexadecimal digits, where letters
may be upper- or lower-case.
For output which consists of a color specified via hexadecimal values,
and for functions which perform intermediate conversion to hexadecimal
before returning a result in another format, this module always
normalizes such values to the following format:
* A hash mark (#) followed by six hexadecimal digits, with letters
forced to lower-case.
The function ``normalize_hex()`` in this module can be used to perform
this normalization manually if desired; see its documentation for an
explanation of the normalization process.
For colors specified via predefined names, this module will accept
input in the following formats:
* An entirely lower-case name, such as ``aliceblue``.
* A name using initial capitals, such as ``AliceBlue``.
For output which consists of a color specified via a predefined name,
and for functions which perform intermediate conversion to a
predefined name before returning a result in another format, this
module always normalizes such values to be entirely lower-case.
Mappings of color names
-----------------------
For each set of defined color names -- HTML 4, CSS 2, CSS 2.1 and CSS
3 -- this module exports two mappings: one of normalized color names
to normalized hexadecimal values, and one of normalized hexadecimal
values to normalized color names. These eight mappings are as follows:
``html4_names_to_hex``
Mapping of normalized HTML 4 color names to normalized hexadecimal
values.
``html4_hex_to_names``
Mapping of normalized hexadecimal values to normalized HTML 4
color names.
``css2_names_to_hex``
Mapping of normalized CSS 2 color names to normalized hexadecimal
values. Because CSS 2 defines the same set of named colors as HTML
4, this is merely an alias for ``html4_names_to_hex``.
``css2_hex_to_names``
Mapping of normalized hexadecimal values to normalized CSS 2 color
nams. For the reasons described above, this is merely an alias for
``html4_hex_to_names``.
``css21_names_to_hex``
Mapping of normalized CSS 2.1 color names to normalized
hexadecimal values. This is identical to ``html4_names_to_hex``,
except for one addition: ``orange``.
``css21_hex_to_names``
Mapping of normalized hexadecimal values to normalized CSS 2.1
color names. As above, this is identical to ``html4_hex_to_names``
except for the addition of ``orange``.
``css3_names_to_hex``
Mapping of normalized CSS3 color names to normalized hexadecimal
values.
``css3_hex_to_names``
Mapping of normalized hexadecimal values to normalized CSS3 color
names.
Normalization functions
-----------------------
``normalize_hex(hex_value)``
Normalize a hexadecimal color value to the following form and
return the result::
#[a-f0-9]{6}
In other words, the following transformations are applied as
needed:
* If the value contains only three hexadecimal digits, it is
expanded to six.
* The value is normalized to lower-case.
If the supplied value cannot be interpreted as a hexadecimal color
value, ``ValueError`` is raised.
Example:
>>> normalize_hex('#09c')
'#0099cc'
Conversions from named colors
-----------------------------
``name_to_hex(name, spec='css3')``
Convert a color name to a normalized hexadecimal color value.
The optional keyword argument ``spec`` determines which
specification's list of color names will be used; valid values are
``html4``, ``css2``, ``css21`` and ``css3``, and the default is
``css3``.
The color name will be normalized to lower-case before being
looked up, and when no color of that name exists in the given
specification, ``ValueError`` is raised.
Example:
>>> name_to_hex('deepskyblue')
'#00bfff'
``name_to_rgb(name, spec='css3')``
Convert a color name to a 3-tuple of integers suitable for use in
an ``rgb()`` triplet specifying that color.
The optional keyword argument ``spec`` determines which
specification's list of color names will be used; valid values are
``html4``, ``css2``, ``css21`` and ``css3``, and the default is
``css3``.
The color name will be normalized to lower-case before being
looked up, and when no color of that name exists in the given
specification, ``ValueError`` is raised.
Example:
>>> name_to_rgb('navy')
(0, 0, 128)
``name_to_rgb_percent(name, spec='css3')``
Convert a color name to a 3-tuple of percentages suitable for use
in an ``rgb()`` triplet specifying that color.
The optional keyword argument ``spec`` determines which
specification's list of color names will be used; valid values are
``html4``, ``css2``, ``css21`` and ``css3``, and the default is
``css3``.
The color name will be normalized to lower-case before being
looked up, and when no color of that name exists in the given
specification, ``ValueError`` is raised.
Example:
>>> name_to_rgb_percent('navy')
('0%', '0%', '50%')
Conversions from hexadecimal values
-----------------------------------
``hex_to_name(hex_value, spec='css3')``
Convert a hexadecimal color value to its corresponding normalized
color name, if any such name exists.
The optional keyword argument ``spec`` determines which
specification's list of color names will be used; valid values are
``html4``, ``css2``, ``css21`` and ``css3``, and the default is
``css3``.
The hexadecimal value will be normalized before being looked up,
and when no color name for the value is found in the given
specification, ``ValueError`` is raised.
Example:
>>> hex_to_name('#000080')
'navy'
``hex_to_rgb(hex_value)``
Convert a hexadecimal color value to a 3-tuple of integers
suitable for use in an ``rgb()`` triplet specifying that color.
The hexadecimal value will be normalized before being converted.
Example:
>>> hex_to_rgb('#000080')
(0, 0, 128)
``hex_to_rgb_percent(hex_value)``
Convert a hexadecimal color value to a 3-tuple of percentages
suitable for use in an ``rgb()`` triplet representing that color.
The hexadecimal value will be normalized before converting.
Example:
>>> hex_to_rgb_percent('#ffffff')
('100%', '100%', '100%')
Conversions from integer rgb() triplets
---------------------------------------
``rgb_to_name(rgb_triplet, spec='css3')``
Convert a 3-tuple of integers, suitable for use in an ``rgb()``
color triplet, to its corresponding normalized color name, if any
such name exists.
The optional keyword argument ``spec`` determines which
specification's list of color names will be used; valid values are
``html4``, ``css2``, ``css21`` and ``css3``, and the default is
``css3``.
If there is no matching name, ``ValueError`` is raised.
Example:
>>> rgb_to_name((0, 0, 0))
'black'
``rgb_to_hex(rgb_triplet)``
Convert a 3-tuple of integers, suitable for use in an ``rgb()``
color triplet, to a normalized hexadecimal value for that color.
Example:
>>> rgb_to_hex((255, 255, 255))
'#ffffff'
``rgb_to_rgb_percent(rgb_triplet)``
Convert a 3-tuple of integers, suitable for use in an ``rgb()``
color triplet, to a 3-tuple of percentages suitable for use in
representing that color.
This function makes some trade-offs in terms of the accuracy of
the final representation; for some common integer values,
special-case logic is used to ensure a precise result (e.g.,
integer 128 will always convert to '50%', integer 32 will always
convert to '12.5%'), but for all other values a standard Python
``float`` is used and rounded to two decimal places, which may
result in a loss of precision for some values.
Examples:
>>> rgb_to_rgb_percent((255, 255, 255))
('100%', '100%', '100%')
>>> rgb_to_rgb_percent((0, 0, 128))
('0%', '0%', '50%')
>>> rgb_to_rgb_percent((33, 56, 192))
('12.94%', '21.96%', '75.29%')
>>> rgb_to_rgb_percent((64, 32, 16))
('25%', '12.5%', '6.25%')
Conversions from percentage rgb() triplets
------------------------------------------
``rgb_percent_to_name(rgb_percent_triplet, spec='css3')``
Convert a 3-tuple of percentages, suitable for use in an ``rgb()``
color triplet, to its corresponding normalized color name, if any
such name exists.
The optional keyword argument ``spec`` determines which
specification's list of color names will be used; valid values are
``html4``, ``css2``, ``css21`` and ``css3``, and the default is
``css3``.
If there is no matching name, ``ValueError`` is raised.
Example:
>>> rgb_percent_to_name(('0%', '0%', '50%'))
'navy'
``rgb_percent_to_hex(rgb_percent_triplet)``
Convert a 3-tuple of percentages, suitable for use in an ``rgb()``
color triplet, to a normalized hexadecimal color value for that
color.
Example:
>>> rgb_percent_to_hex(('100%', '100%', '0%'))
'#ffff00'
``rgb_percent_to_rgb(rgb_percent_triplet)``
Convert a 3-tuple of percentages, suitable for use in an ``rgb()``
color triplet, to a 3-tuple of integers suitable for use in
representing that color.
Some precision may be lost in this conversion. See the note
regarding precision for ``rgb_to_rgb_percent()`` for details;
generally speaking, the following is true for any 3-tuple ``t`` of
integers in the range 0...255 inclusive::
t == rgb_percent_to_rgb(rgb_to_rgb_percent(t))
Examples:
>>> rgb_percent_to_rgb(('100%', '100%', '100%'))
(255, 255, 255)
>>> rgb_percent_to_rgb(('0%', '0%', '50%'))
(0, 0, 128)
>>> rgb_percent_to_rgb(('25%', '12.5%', '6.25%'))
(64, 32, 16)
>>> rgb_percent_to_rgb(('12.94%', '21.96%', '75.29%'))
(33, 56, 192)
