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