You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
150 lines
5.4 KiB
150 lines
5.4 KiB
<chapter id="colorspaces">
|
|
<title>Colorspaces</title>
|
|
|
|
<para>
|
|
This chapter gives information on what colorspaces are, which colorspaces
|
|
&chalk; offers, and what you should keep in mind when using them.
|
|
</para>
|
|
|
|
<sect1 id="colorspaces-intro">
|
|
<title>Introduction to colorspaces</title>
|
|
|
|
<sect2 id="colorspaces-intro-whatis">
|
|
<title>What is a colorspace?</title>
|
|
|
|
<para>
|
|
In short, a colorspace is a way to represent colors by specifying a number of
|
|
parameters. As parameters, one can choose for example the amounts of red,
|
|
green and blue light needed for the color. This results in the commonly known
|
|
RGB colorspace. One can visualize this as a three-dimensional space, with each
|
|
of the red, green, and blue light components being an axis in the colorspace.
|
|
A color then corresponds to a certain point in this colorspace, defined by its
|
|
coordinates on the three axes.
|
|
</para>
|
|
<note><para>
|
|
To be more precise, a colorspace is a combination of a color model (indicating
|
|
which axes are present) and a mapping function (indicating which values
|
|
correspond to which colors).
|
|
</para></note>
|
|
<para>
|
|
Not every color can be represented in every colorspace. Some colorspaces
|
|
define more, or different, colors than others. The set of colors that can be
|
|
represented in a certain colorspace is called its gamut. Because gamuts
|
|
can differ widely, it is not guaranteed that images in a certain colorspace
|
|
can be converted to another colorspace without having to substitute certain
|
|
colors for others, even if they are based on the same color model.
|
|
</para>
|
|
|
|
</sect2>
|
|
|
|
</sect1>
|
|
|
|
<sect1 id="colorspaces-list">
|
|
<title>Available colorspaces</title>
|
|
|
|
<para>
|
|
&chalk; offers colorspaces based on RGB, CMYK, Lab, LMS, YCbCr, and Gray
|
|
color models. These are shortly discussed in this section.
|
|
</para>
|
|
|
|
<sect2 id="colorspaces-list-rgb">
|
|
<title>The RGB color models</title>
|
|
|
|
<para>
|
|
The abbreviation RGB stands for Red, Green, Blue, and the color model with
|
|
this name refers to the three light components that are emitted in displays
|
|
(televisions, computer monitors, etcetera) to create a certain color. This
|
|
color model is used by default in virtually any standard painting application.
|
|
</para><para>
|
|
When defining a color in the RGB model, its red, green and blue components are
|
|
specified. If all components are absent (each component is emitted at 0
|
|
percent intensity, so no light at all), the color is pure black. If all
|
|
components are fully present (100 percent intensity), the color is pure white.
|
|
If one component is present at full intensity and the other two are absent,
|
|
the pure respective color is obtained.
|
|
</para><para>
|
|
Two more examples: if both red and green are emitted at 100 percent and blue
|
|
is not emitted, pure yellow is obtained. A color with all three components at
|
|
the same intensity is a shade of gray.
|
|
</para><para>
|
|
There are various colorspaces that implement the RGB model. For example, the
|
|
so-called RGB8 colorspace represents each color with 8 bits per component.
|
|
Since 8 bits allow for 256 distinct values, the total number of different
|
|
colors that can be specified in this colorspace is 256 (red) * 256 (green) *
|
|
256 (blue), or about 16.7 million colors. In &chalk;, a couple of RGB
|
|
colorspaces are available, for example RGB32, which is able to distinguish
|
|
between 4.2 billion values per component.
|
|
</para>
|
|
|
|
</sect2>
|
|
|
|
<sect2 id="colorspaces-list-cmyk">
|
|
<title>The CMYK color model</title>
|
|
|
|
<para>
|
|
CMYK is the abbreviation for Cyan, Magenta, Yellow, blacK (although officially
|
|
the K stands for Key, black is much more commonly used). This color model is
|
|
based on ink: a color is specified by the amount of ink needed for a point
|
|
to be perceived as having that color.
|
|
</para><para>
|
|
Since CMYK colors are used by printers while RGB colors are used on-screen,
|
|
one often wants to convert RGB colors to CMYK colors. As this cannot always be
|
|
done correctly, printed images may turn out to look quite different than what
|
|
is perceived on-screen.
|
|
</para>
|
|
|
|
</sect2>
|
|
|
|
<sect2 id="colorspaces-list-lab">
|
|
<title>The L*a*b* color model</title>
|
|
|
|
<para>
|
|
This color model uses three parameters for a color: its
|
|
luminance or lightness (L*, which lies between 0 for black and
|
|
100 for white), its position between absolute red and absolute green (a*,
|
|
which is negative for colors closer to green and positive for colors closer to
|
|
red), and its position between yellow and blue (b*, which is negative for
|
|
colors closer to blue and positive for colors closer to yellow).
|
|
</para>
|
|
|
|
</sect2>
|
|
|
|
<sect2 id="colorspaces-list-LMS">
|
|
<title>The LMS color model</title>
|
|
|
|
<para>
|
|
This model is based on the contribution of actual light wave lengths to the
|
|
color. The human eye is sensitive to three types of light waves, distinguished
|
|
by their wave lengths: long (L), middle (M) and short (S) waves. The eye's
|
|
sensitivity for a certain color on these three wavelengths can be expressed in
|
|
L, M and S coordinates.
|
|
</para>
|
|
|
|
</sect2>
|
|
|
|
<sect2 id="colorspaces-list-YCbCr">
|
|
<title>The YCbCr color model</title>
|
|
|
|
<para>
|
|
The YCbCr model is often used for video systems. The Y parameter indicates the
|
|
luminance or lightness of the color (which can be seen as a gray-tone), the Cb
|
|
and Cr parameters indicate the chrominance (color tone): Cb places the color
|
|
on a scale between blue and yellow, Cr indicates the place of the color
|
|
between red and green.
|
|
</para>
|
|
|
|
</sect2>
|
|
|
|
<sect2 id="colorspaces-list-Gray"><title>The Gray color model</title>
|
|
|
|
<para>
|
|
The Gray color model simply represents colors as shades of gray (with black
|
|
and white being the extremes).
|
|
</para>
|
|
|
|
</sect2>
|
|
|
|
</sect1>
|
|
|
|
</chapter>
|