gipm-help-2.doap: make schumaml and myself additional maintainers

[gimp-help-2.git] / src / glossary / glossary.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE glossary PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
                          "http://www.docbook.org/xml/4.3/docbookx.dtd">
<glossary id="glossary">
  <title>Glossary</title>

  <indexterm>
    <primary>Glossary</primary>
  </indexterm>

  <glossentry id="glossary-alpha">
    <glossterm>
      <phrase>Alpha</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Alpha</primary>
    </indexterm>
    <glossdef>
      <para>
        An Alpha value indicates the transparency of a pixel. Besides its
        Red, Green and Blue values, a pixel has an alpha value. The smaller
        the alpha value of a pixel, the more visible the colors below it. A
        pixel with an alpha value of 0 is completely transparent. A pixel
        with an alpha value of 255 is fully opaque.
      </para>
      <para>
        With some image
        <link linkend="glossary-fileformat">file formats</link>, you can only
        specify that a pixel is completely transparent or completely opaque.
        Other file formats allow a variable level of transparency.
       </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-alpha-channel">
    <glossterm>
      <phrase>Alpha Channel</phrase>
    </glossterm>
    <glossdef>
      <indexterm>
        <primary>Transparency</primary>
        <secondary>Alpha channel</secondary>
      </indexterm>
      <indexterm>
        <primary>Alpha channel</primary>
      </indexterm>
      <para>
        An alpha <link linkend="glossary-channels">channel</link> of a layer
        is a grayscale image of the same size as the layer representing its
        transparency. For each pixel the gray level (a value between 0 and
        255) represents the pixels's
        <link linkend="glossary-alpha">Alpha</link> value. An alpha channel
        can make areas of the layer to appear partially transparent. That's
        why the background layer has no alpha channel by default.
      </para>
      <para>
        The image alpha channel, which is displayed in the channels dialog,
        can be considered as the alpha channel of the final layer when all
        layers have been merged.
      </para>
      <para>
        See also <xref linkend="alpha-channel-example"/>.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-antialiasing">
    <glossterm>
      <phrase>Antialiasing</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Antialiasing</primary>
      <secondary>Explanation</secondary>
    </indexterm>
    <glossdef>
      <para>
        Antialiasing is the process of reversing an alias, that is,
        reducing the <quote>jaggies</quote>. Antialiasing
        produces smoother curves by adjusting the boundary between the
        background and the pixel region that is being antialiased. Generally,
        pixel intensities or opacities are changed so that a smoother
        transition to the background is achieved. With selections, the
        opacity of the edge of the selection is appropriately reduced.
      </para>
      <para>
        <inlinemediaobject>
          <imageobject>
            <imagedata fileref="images/glossary/alias.png" format="PNG"/>
          </imageobject>
        </inlinemediaobject>
        <inlinemediaobject>
          <imageobject>
            <imagedata fileref="images/glossary/antialias.png" format="PNG"/>
          </imageobject>
        </inlinemediaobject>
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-bezier-curve">
    <glossterm>
      <phrase>B&eacute;zier curve</phrase>
    </glossterm>
    <glossdef>
      <para>
        A spline is a curve which is defined mathematically and has a set of
        control points. A B&eacute;zier spline is a cubic spline which has
        four control points, where the first and last control points (knots or
        anchors) are the endpoints of the curve and the inner two control
        points (handles) determine the direction of the curve at the
        endpoints.
      </para>
      <para>
        In the non-mathematical sense, a spline is a flexible strip of wood or
        metal used for drawing curves. Using this type of spline for drawing
        curves dates back to shipbuilding, where weights were hung on splines
        to bend them. The outer control points of a B&eacute;zier spline are
        similar to the places where the splines are fastened down and the
        inner control points are where weights are attached to modify the
        curve.
      </para>
      <para>
        B&eacute;zier splines are only one way of mathematically representing
        curves. They were developed in the 1960s by Pierre B&eacute;zier, who
        worked for Renault.
      </para>
      <para>
        B&eacute;zier curves are used in <acronym>GIMP</acronym> as component
        parts of <link linkend="glossary-path">Paths</link>.
      </para>
      <para>
        <mediaobject>
          <imageobject>
            <imagedata format="PNG"
              fileref="images/glossary/bezier-curve.png"/>
          </imageobject>
        </mediaobject>
      </para>
      <para>
        The image above shows a B&eacute;zier curve. Points P0 and P3 are
        points on the Path, which are created by clicking with the mouse.
        Points P1 and P2 are handles, which are automatically created by
        <acronym>GIMP</acronym> when you stretch the line.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-bitmap">
    <glossterm>
      <phrase>Bitmap</phrase>
    </glossterm>
    <glossdef>
      <para>
        From
        <emphasis>
          The Free Online Dictionary of Computing (13 Mar 01)
        </emphasis>:
        <blockquote><para>
            bitmap &mdash; A data file or structure which corresponds bit for
            bit with an image displayed on a screen, probably in the same
            format as it would be stored in the display's video memory or
            maybe as a device independent bitmap. A bitmap is characterised by
            the width and height of the image in pixels and the number of bits
            per pixel which determines the number of shades of grey or colors
            it can represent. A bitmap representing a colored image (a
            <quote>pixmap</quote>) will usually have pixels with between one
            and eight bits for each of the red, green, and blue components,
            though other color encodings are also used. The green component
            sometimes has more bits than the other two to cater for the human
            eye's greater discrimination in this component.
          </para></blockquote>
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-bmp">
    <glossterm>
      <anchor id="file-bmp-load" xreflabel="BMP"/>
      <anchor id="file-bmp-save" xreflabel="BMP"/>
      <phrase>BMP</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>BMP</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>Formats</primary>
      <secondary>BMP</secondary>
    </indexterm>
    <glossdef>
      <para>
        BMP is an uncompressed image
        <link linkend="glossary-fileformat">file format</link>
        designed by Microsoft and mainly used in Windows. Colors are
        typically represented in 1, 4 or 8 bits, although the format also
        supports more. Because it is not compressed and the files are large,
        it is not very well suited for use in the internet.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-bumpmapping">
    <glossterm>
      <phrase>Bump mapping</phrase>
    </glossterm>
    <glossdef>
      <para>
        Bump mapping is a technique for displaying extremely detailed objects
        without increasing the geometrical complexity of the objects. It is
        especially used in 3-dimensional visualization programs. The trick is
        to put all the necessary information into a texture, with which
        shadowing is shown on the surface of the object.
      </para>
      <para>
        Bump mapping is only one (very effective) way of simulating surface
        irregularities which are not actually contained in the geometry of the
        model.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-channelmask">
    <glossterm>
      <phrase>Channel Mask</phrase>
    </glossterm>
    <glossdef>
      <para>
        A channel masks is a special type of mask which determines the
        transparency of a selection. See <xref linkend="glossary-masks"/> for
        a detailed description.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-channel-encoding">
    <glossterm>
      <phrase>Channel encoding</phrase>
    </glossterm>
    <indexterm>
      <primary>Channel encoding</primary>
    </indexterm>
    <glossdef>
      <para>
      Channel encoding refers to how fast the intensity (more technically
      correct for grayscale and RGB images, the <ulink url="https://en.wikipedia.org/wiki/Relative_luminance">relative
      Luminance</ulink>) of a channel in a digital image progresses from dark
      to light as the channel values progress from 0.0 to 1.0 floating point
      (0 to 255 for 8-bit integer, 0 to 65535 for 16-bit integer).</para>
      <para>Other ways of referring to "channel encoding" include
      "companding curve", "gamma" (which is technically not correct unless the
      channel encoding is an actual gamma curve), "tone reproduction curve"
      ("TRC" for short), and "tone response curve" (also "TRC" for short).
      </para>
      <para>The linear light channel encoding reflects the way lightwaves
      combine there in the real world. The linear light channel encoding is
      also referred to as "gamma=1.0", "linear gamma" or simply "linear".</para>
      <para>Perceptually uniform channel encodings reflects the way our eyes
      respond to changes in luminance.</para>
      <para>In ICC profile color managed workflows, the following channel
      encodings are commonly used:</para>
      <orderedlist>
        <listitem><para>The LAB companding curve, which is exactly
        perceptually uniform.</para></listitem>
        <listitem><para>The linear light channel encoding, which of course is
        exactly linear.</para></listitem>
        <listitem><para>The sRGB channel encoding and the "gamma=2.2"
        channel encoding, which are both approximately perceptually uniform
        and approximately equal to each other.</para></listitem>
        <listitem><para>The "gamma=1.8" channel encoding, which is neither
        linear nor approximately perceptually uniform, though it's closer to
        being perceptually uniform than it is to being linear.</para></listitem>
      </orderedlist>

      <!--TO TRANSLATORS: a png file without text is in 
https://git.gnome.org/browse/gimp-help-2/tree/docs folder-->
      <mediaobject>
        <imageobject>
          <imagedata fileref="images/glossary/companding-curves-compared.png" format="PNG"/>
          </imageobject>
        <caption>
          <para>The Linear light, sRGB, and LAB channel encodings compared.</para>
        </caption>
      </mediaobject>

      <para>Looking at the above image:</para>
      <orderedlist>
      <listitem><para>The Linear light channel encoding (top row) represents
      how lightwaves combine out there in the real world.</para></listitem>
        <listitem><para>The sRGB channel encoding (middle row) is almost
        perceptually uniform.</para></listitem>
        <listitem><para>The LAB channel encoding (bottom row) is exactly
        perceptually uniform, which means it represents how our eyes respond to
        changes in luminance.</para></listitem>
      </orderedlist>
      <para>In GIMP 2.10 two different channel encodings are used internally
      for various editing operations, these being "Linear light" and
      "Perceptually uniform (sRGB)".</para>
      <para>The companding-curves-compared.png shown above is a slightly
      modified version of an image from <ulink url="http://ninedegreesbelow.com/photography/xyz-rgb.html#Color">
      Completely Painless Programmer's Guide to XYZ, RGB, ICC, xyY, and TRCs
      </ulink>, which is licensed as
      <ulink url="http://creativecommons.org/licenses/by-sa/3.0/deed.en_US">
      Creative Commons Attribution-ShareAlike 3.0 Unported License</ulink>.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-channels">
    <glossterm>
      <phrase>Channel</phrase>
    </glossterm>
    <indexterm>
      <primary>Channel</primary>
    </indexterm>
    <glossdef>
      <!--TRANSLATORS: this is the modified text from concepts.xml, so
      you should check po/LANG/concepts.po for an old translation-->
      <para>
        A channel refers to a certain component of an image. For instance, the
        components of an <link linkend="glossary-rgb">RGB</link> image are the
        three primary colors red, green, blue, and sometimes transparency
        (alpha).
      </para>
      <para>
        Every channel is a grayscale image of exactly the same size as the
        image and, consequently, consists of the same number of pixels. Every
        pixel of this grayscale image can be regarded as a container which can
        be filled with a value ranging from 0 to 255. The exact meaning of
        this value depends on the type of channel, e.g. in the
        <acronym>RGB</acronym> color model the value in the
        <emphasis>R</emphasis>-channel means the amount of red which is added
        to the color of the different pixels; in the selection channel, the
        value denotes how strongly the pixels are selected; and in the alpha
        channel the values denote how opaque the corresponding pixels are.
        See also <xref linkend="gimp-concepts-channels"/>.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-clipboard">
    <glossterm>
      <phrase>Clipboard</phrase>
    </glossterm>
    <glossdef>
      <para>
        The Clipboard is a temporary area of memory which is used to
        transfer data between applications or documents. It is used when you
        Cut, Copy or Paste data in <acronym>GIMP</acronym>.
      </para>
      <para>
        The clipboard is implemented slightly differently under different
        operating systems. Under Linux/XFree, <acronym>GIMP</acronym> uses
        the XFree clipboard for text and the <acronym>GIMP</acronym>
        internal image clipboard for transferring images between image
        documents. Under other operating systems, the clipboard may work
        somewhat differently. See the <acronym>GIMP</acronym> documentation
        for your operating system for further information.
      </para>
      <!--TODO: this para should go to concepts/using-->
      <para>
        The basic operations provided by the clipboard are
        <quote>Cut</quote>, <quote>Copy</quote>, and <quote>Paste</quote>.
        Cut means that the item is removed from the document and copied to
        the clipboard. Copy leaves the item in the document and copies it to
        the clipboard. Paste copies the contents of the clipboard to the
        document. The <acronym>GIMP</acronym> makes an intelligent decision
        about what to paste depending upon the target. If the target is a
        canvas, the Paste operation uses the image clipboard. If the target
        is a text entry box, the paste operation uses the text clipboard.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-color">
    <glossterm>
      <phrase>Color</phrase>
    </glossterm>
    <indexterm>
      <primary>Color</primary>
    </indexterm>
    <glossdef>
      <para>On the one hand, <ulink url="http://en.wikipedia.org/wiki/Light">
      light</ulink> comes from the sun or other radiant sources, and is
      <ulink url="https://en.wikipedia.org/wiki/Atmospheric_refraction">
      refracted</ulink>by mediums (water, the atmosphere, glass) and
      <ulink url="https://en.wikipedia.org/wiki/Diffuse_reflection">diffusely
      </ulink> or
      <ulink url="https://en.wikipedia.org/wiki/Specular_reflection">
      specularly</ulink> reflected by surfaces.</para>

      <para>On the other hand,
      <ulink url="http://en.wikipedia.org/wiki/Color">color</ulink> isn't out
      there in the world in the same tangible way that light is. Rather color
      is part of how we sense the world around us. Light enters the eyes, is
      processed by light receptors
      (<ulink url="http://en.wikipedia.org/wiki/Cone_cell">cones</ulink> and
      <ulink url="http://en.wikipedia.org/wiki/Rod_cell">rods</ulink>), and
      sent via the optic nerves to the brain for further processing and
      interpretation.</para>

      <para>Light varies in
      <ulink url="http://en.wikipedia.org/wiki/Wavelength">wavelengths</ulink>,
      which our eyes and brain interpret as varying hues (reds, blues, greens,
      and so on), and also in <ulink url="http://en.wikipedia.org/wiki/Luminance">intensity (aka "luminance")</ulink>. So our
      <ulink url="http://en.wikipedia.org/wiki/Color_vision">perception of
      color</ulink> is composed of both intensity ("luminance") information and
      chromaticity information.</para>

      <para>The <ulink url="http://www1.icsi.berkeley.edu/wcs/">naming of colors
      </ulink> carries one out of the narrow realm of color perception, and
      into the larger realm of cultural and linguistic interpretation and
      classification of color, and thence into even larger philosophical,
      aesthetic, theological, and metaphysical considerations.</para>

      <para>The above explanation of Color is a slightly modified excerpt from
      the <ulink url="http://ninedegreesbelow.com/photography/xyz-rgb.html#Color">
      Completely Painless Programmer's Guide to XYZ, RGB, ICC, xyY, and TRCs
      </ulink>, which is licensed as
      <ulink url="http://creativecommons.org/licenses/by-sa/3.0/deed.en_US">
      Creative Commons Attribution-ShareAlike 3.0 Unported License</ulink>.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-cmyk">
    <glossterm>
      <phrase>CMY, CMYK</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>CMYK</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>Color</primary>
      <secondary>Subtractive color synthesis</secondary>
    </indexterm>
    <glossdef>
      <para>
        CMYK is a <link linkend="glossary-colormodel">color model</link>
        which has components for Cyan, Magenta, Yellow and Black. It is a
        subtractive color model, and that fact is important when an image
        is printed. It is complementary to the
        <link linkend="glossary-rgb">RGB</link> color model.
      </para>
      <para>
        The values of the individual colors vary between 0% and 100%, where 0%
        corresponds to an unprinted color, and 100% corresponds to a
        completely printed area of color. Colors are formed by mixing the
        three basic colors.
      </para>
      <para>
        The last of these values, K (Black), doesn't contribute to
        the color, but merely serves to darken the other colors.  The
        letter K is used for Black to prevent confusion, since B usually
        stands for Blue.
      </para>
      <figure float="0">
        <title>Subtractive color model</title>
        <mediaobject>
          <imageobject>
            <imagedata format="PNG"
              fileref="images/glossary/color-model-subtractive.png"/>
          </imageobject>
        </mediaobject>
      </figure>
      <para>
        <acronym>GIMP</acronym> does not currently support the CMYK model.
        (An experimental plug-in providing rudimentary CMYK support can be
        found <xref linkend="bibliography-online-plugin-separate"/>.)
      </para>
      <para>
        This is the mode used in printing. These are the colors in the ink
        cartridges in your printer. It is the mode used in painting and in all
        the objects around us, where light is reflected, not emmitted. Objects
        absorb part of the light waves and we see only the reflected part.
        Note that the cones in our eyes see this reflected light in RGB mode.
        An object appears Red because Green and Blue have been absorbed. Since
        the combination of Green and Blue is Cyan, Cyan is absorbed when you
        add Red. Conversely, if you add Cyan, its complementary color, Red, is
        absorbed. This system is <emphasis>subtractive</emphasis>.
        If you add Yellow, you decrease Blue, and if you add Magenta, you
        decrease Green.
      </para>
      <para>
        It would be logical to think that by mixing Cyan, Magenta and Yellow,
        you would subtract Red, Green and Blue, and the eye would see no light
        at all, that is, Black. But the question is more complex. In fact, you
        would see a dark brown. That is why this mode also has a Black value,
        and why your printer has a Black cartridge. It is less expensive that
        way. The printer doesn't have to mix the other three colors to create
        an imperfect Black, it just has to add Black.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-colordepth">
    <glossterm>
      <phrase>Color depth</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Color depth</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>bpp</primary>
    </indexterm>
    <glossdef>
      <para>
        Color depth is simply the number of bits used to represent a color
        (bits per pixel : bpp). There are 3 channels for a pixel (for Red,
        Green and Blue). <acronym>GIMP</acronym> can support 8 bits per
        channel, referred as <emphasis>eight-bit color</emphasis>. So,
        <acronym>GIMP</acronym> color depth is 8&nbsp;*&nbsp;3&nbsp;=&nbsp;24,
        which allows
        256&nbsp;*&nbsp;256&nbsp;*&nbsp;256&nbsp;=&nbsp;16,777,216 possible
        colors (8 bits allow 256 colors).
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-colormodel">
    <glossterm>
      <phrase>Color model</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Color model</primary>
    </indexterm>
    <glossdef>
      <para><!-- Reorganised "color models" to be more like de. There's no need
             to repeat the information again.  scb -->
        A color model is a way of describing and specifying a color. The term
        is often used loosely to refer to both a color space system and the
        color space on which it is based.
      </para>
      <para>
        A color space is a set of colors which can be displayed or
        recognized by an input or output device (such as a scanner, monitor,
        printer, etc.). The colors of a color space are specified as values
        in a color space system, which is a coordinate system in which the
        individual colors are described by coordinate values on various axes.
        Because of the structure of the human eye, there are three axes in
        color spaces which are intended for human observers. The practical
        application of that is that colors are specified with three
        components (with a few exceptions). There are about 30 to 40 color
        space systems in use. Some important examples are:
      </para>
      <itemizedlist>
        <listitem>
          <para>
            <link linkend="glossary-rgb">RGB</link>
          </para>
        </listitem>
        <listitem>
          <para>
            <link linkend="glossary-hsv">HSV</link>
          </para>
        </listitem>
        <listitem>
          <para>
            <link linkend="glossary-cmyk">CMY(K)</link>
          </para>
        </listitem>
        <listitem>
          <para>
            <link linkend="glossary-yuv">YUV</link>
          </para>
        </listitem>
        <listitem>
          <para>
            <link linkend="glossary-ycbcr">YCbCr</link>
          </para>
        </listitem>
      </itemizedlist>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-display-referred">
    <glossterm>
      <phrase>Display-referred</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Display-referred</primary>
    </indexterm>
    <glossdef>
      <para>The phrase "display-referred" refers to images that can be
      displayed (either directly or by means of ICC profile color management)
      on devices. The displaying device might be a monitor, or an image printed
      on paper, or some other display technology.</para>

      <para>Regardless of the technology, when you display an image on a
      device, that device has a maximum and minimum brightness. The maximum
      and minimum brightnesses are referred to as
      <link linkend="glossary-display-referred-white">display-referred white</link>
      and
      <link linkend="glossary-display-referred-black">display-referred black</link>.
      </para>

      <para>The above explanation is a slightly modified excerpt from
      <ulink url="http://ninedegreesbelow.com/photography/display-referred-scene-referred.html">
      Models for image editing: Display-referred and scene-referred</ulink>.
      The modified excerpt was written and quoted by permission of the
      author, who has licensed the modified excerpt under the
      <ulink url="http://creativecommons.org/licenses/by-sa/3.0/deed.en_US">
      Creative Commons Attribution-ShareAlike 3.0 Unported License</ulink>.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-display-referred-white">
    <glossterm>
      <phrase>Display-referred white</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Display-referred white</primary>
    </indexterm>
    <glossdef>
      <para>"Display-referred white" (or for simplicity, "white")
      means the floating point RGB color (1.0, 1.0, 1.0) and the integer
      equivalents (255,255,255),(65535,65535,65535), etc, for 8-bit integer,
      16-bit integer, etc.</para>

      <para>"Display-referred white" has the very special significance that
      in display-referred editing there's no such thing as
      "brighter than white". So in display-referred image editing, all RGB
      channel values are less than or equal to 1.0 and no color is brighter
      than "white", (1.0, 1.0, 1.0).</para>

      <para>The above explanation is a slightly modified excerpt from
      <ulink url="http://ninedegreesbelow.com/photography/display-referred-scene-referred.html">
      Models for image editing: Display-referred and scene-referred</ulink>.
      The modified excerpt was written and quoted by permission of the
      author, who has licensed the modified excerpt under the
      <ulink url="http://creativecommons.org/licenses/by-sa/3.0/deed.en_US">
      Creative Commons Attribution-ShareAlike 3.0 Unported License</ulink>.
      </para>
    </glossdef>

  </glossentry>
    <glossentry id="glossary-display-referred-black">
    <glossterm>
      <phrase>Display-referred black</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Display-referred black </primary>
    </indexterm>
    <glossdef>
     <para>"Display-referred black" (or for simplicity, "black") means the
     floating point RGB color (0.0, 0.0, 0.0) and its integer equivalents. This
     color has the very special significance that there's no such thing as
     "less bright than black". So in display-referred image editing, all RGB
     channel values are greater than or equal to 0.0 and no color is less
     bright than "black", (0.0, 0.0, 0.0).</para>

      <para>The above explanation is a slightly modified excerpt from
      <ulink url="http://ninedegreesbelow.com/photography/display-referred-scene-referred.html">
      Models for image editing: Display-referred and scene-referred</ulink>.
      The modified excerpt was written and quoted by permission of the
      author, who has licensed the modified excerpt under the
      <ulink url="http://creativecommons.org/licenses/by-sa/3.0/deed.en_US">
      Creative Commons Attribution-ShareAlike 3.0 Unported License</ulink>.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-dithering">
    <glossterm>
      <phrase>Dithering</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Dithering</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>Color</primary>
      <secondary>Dithering</secondary>
    </indexterm>
    <glossdef>
      <para>
        Dithering is a technique used in computer graphics to create the
        illusion of more colors when displaying an image which has a low
        <link linkend="glossary-colordepth">color depth</link>. In a
        dithered image, the missing colors are reproduced by a certain
        arrangement of pixels in the available colors. The human eye
        perceives this as a mixture of the individual colors.
      </para>
      <para>
        The <link linkend="gimp-tool-gradient">Gradient tool</link> uses
        dithering. You may also choose to use dithering when you convert an
        image to <link linkend="gimp-image-convert-indexed">Indexed</link>
        format. If you are working on an image with indexed colors, some
        tools (such as the pattern fill tool) may also use dithering, if the
        correct color is not available in the colormap.
      </para>
      <para>
        The <link linkend="plug-in-newsprint">Newsprint</link> filter
        uses dithering as well. You can use the
        <link linkend="plug-in-nlfilt">NL Filter</link> (Non Linear filter)
        to remove unwanted dithering noise from your image.
      </para>
      <para>
        Also note that although <acronym>GIMP</acronym> itself uses 24-bit
        colors, your system may not actually be able to display that many
        colors. If it doesn't, then the software in between
        <acronym>GIMP</acronym> and your system may also dither colors while
        displaying them.
      </para>
      <para>
        See also the glossary entry on
        <link linkend="glossary-floyd-steinberg-dithering">Floyd-Steinberg
        dithering</link>, which is used in <acronym>GIMP</acronym>.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-exif">
    <glossterm>
      <phrase>EXIF</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>EXIF</primary>
    </indexterm>
    <glossdef>
      <para>
        Exchangeable image file format (official abbreviation Exif, not
        EXIF) is a specification for the image file format used by digital
        cameras. It was created by the Japan Electronic Industry Development
        Association (JEIDA). The specification uses the existing JPEG, TIFF
        Rev. 6.0, and RIFF WAVE file formats, with the addition of specific
        metadata tags. It is not supported in JPEG 2000 or PNG. Version 2.1 of
        the specification is dated June 12, 1998 and version 2.2 is dated
        April 2002. The Exif tag structure is taken from that of TIFF files.
        There is a large overlap between the tags defined in the TIFF, Exif,
        TIFF/EP and DCF standards
        <xref linkend="bibliography-online-wkpd-exif"/>.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-feathering">
    <glossterm>
      <phrase>Feathering</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Feathering</primary>
    </indexterm>
    <glossdef>
      <para>
        The process of Feathering makes a smooth transition between a region
        and the background by softly blending the edges of the region.
      </para>
      <para>
        <mediaobject>
          <imageobject>
            <imagedata fileref="images/glossary/feather.png" format="PNG"/>
          </imageobject>
        </mediaobject>
      </para>
      <para>
        In <acronym>GIMP</acronym>, you can feather the edges of a
        selection. Brushes can also have feathered edges.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-fileformat">
    <glossterm>
      <phrase>File Format</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>File format</primary>
    </indexterm>
    <glossdef>
      <para>
        A file format or file type is the form in which computer data is
        stored. Since a file is stored by an operating system as a linear
        series of bytes, which cannot describe many kinds of real data in
        an obvious way, conventions have been developed for interpreting
        the information as representations of complex data. All of the
        conventions for a particular <quote>kind</quote> of file constitute
        a file format.
      </para>
      <para>
        Some typical file formats for saving images are JPEG, TIFF, PNG and
        GIF. The best file format for saving an image depends upon how the
        image is intended to be used. For example, if the image is intended
        for the internet, file size is a very important factor, and if the
        image is intended to be printed, high resolution and quality have
        greater significance. See
        <link linkend="gimp-using-fileformats">Format types</link>.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-floatingselection">
    <glossterm>Floating Selection</glossterm>
    <indexterm>
      <primary>Selection</primary>
      <secondary>Floating selection</secondary>
    </indexterm>
    <glossdef>
      <para>
        A floating selection (sometimes called a <quote>floating
        layer</quote>) is a type of temporary layer which is similar in
        function to a normal layer, except that a floating selection must be
        <link linkend="gimp-layer-anchor">anchored</link> before you can
        resume working on any other layers in the image.
      </para>
      <para>
        In early versions of <acronym>GIMP</acronym>, when
        <acronym>GIMP</acronym> did not use layers, floating selections were
        used for performing operations on a limited part of an image (you can
        do that more easily now with layers). Now floating selections have no
        practical use, but you must know what you have to do with them.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-floyd-steinberg-dithering">
    <glossterm>
      <phrase>Floyd-Steinberg Dithering</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Floyd-Steinberg</primary>
    </indexterm>
    <glossdef>
      <para>
        Floyd-Steinberg dithering is a method of
        <link linkend="glossary-dithering">dithering</link> which was first
        published in 1976 by Robert W. Floyd and Louis Steinberg. The
        dithering process begins in the upper left corner of the image. For
        each pixel, the closest available color in the palette is chosen and
        the difference between that color and the original color is computed
        in each RGB channel. Then specific fractions of these differences
        are dispersed among several adjacent pixels which haven't yet been
        visited (below and to the right of the original pixel). Because of
        the order of processing, the procedure can be done in a single pass
        over the image.
      </para>
      <para>
        When you convert an image to
        <link linkend="gimp-image-convert-indexed">Indexed</link>
        mode, you can choose between two variants of Floyd-Steinberg
        dithering.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-gamma">
    <glossterm>
      <phrase>Gamma</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Gamma</primary>
    </indexterm>
    <glossdef>
      <para>
        Gamma or gamma correction is a non-linear operation which is used to
        encode and decode luminance or color values in video or still image
        systems. It is used in many types of imaging systems to straighten out
        a curved signal-to-light or intensity-to-signal response. For example,
        the light emitted by a CRT is not linear with regard to its input
        voltage, and the voltage from an electric camera is not linear with
        regard to the intensity (power) of the light in the scene. Gamma
        encoding helps to map the data into a perceptually linear domain, so
        that the limited signal range (the limited number of bits in each RGB
        signal) is better optimized perceptually.
      </para>
      <para>
        Gamma is used as an exponent (power) in the correction equation. Gamma
        compression (where gamma &lt; 1) is used to encode linear luminance or
        RGB values into color signals or digital file values, and gamma
        expansion (where gamma &gt; 1) is the decoding process, and usually
        occurs where the current-to-voltage function for a CRT is non-linear.
      </para>
      <para>
        For PC video, images are encoded with a gamma of about 0.45 and
        decoded with a gamma of 2.2. For Mac systems, images are typically
        encoded with a gamma of about 0.55 and decoded with a gamma of 1.8.
        The sRGB color space standard used for most cameras, PCs and printers
        does not use a simple exponential equation, but has a decoding gamma
        value near 2.2 over much of its range.
      </para>
      <para>
        In <acronym>GIMP</acronym>, gamma is an option used in the brush tab
        of the <link linkend="plug-in-gimpressionist">GIMPressionist</link>
        filter and in the <link linkend="plug-in-flame">Flame</link> filter.
        The <link linkend="gimp-display-filter-dialog">display filters</link>
        also include a Gamma filter. Also see the
        <link linkend="gimp-tool-levels">Levels Tool</link>, where you can
        use the middle slider to change the gamma value.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-gamut">
    <glossterm>
      <phrase>Gamut</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Gamut</primary>
    </indexterm>
    <glossdef>
      <para>
        In color reproduction, including computer graphics and photography,
        the gamut, or color gamut (pronounced /ˈgæmət/), is a certain complete
        subset of colors. The most common usage refers to the subset of colors
        which can be accurately represented in a given circumstance, such as
        within a given color space or by a certain output device. Another
        sense, less frequently used but not less correct, refers to the
        complete set of colors found within an image at a given time. In this
        context, digitizing a photograph, converting a digitized image to a
        different color space, or outputting it to a given medium using a
        certain output device generally alters its gamut, in the sense that
        some of the colors in the original are lost in the process.
        <xref linkend="bibliography-online-wkpd-gamut"/>
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-gif">
    <glossterm>
      <phrase>GIF</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>GIF</primary>
    </indexterm>
    <glossdef>
      <para>
        <trademark class="trade">GIF</trademark> stands for Graphics
        Interchange Format.  It is a <link linkend="glossary-fileformat">file
        format</link> with good, lossless compression for images with low
        <link linkend="glossary-colordepth">color depth</link>
        (up to 256 different colors per image). Since GIF was developed, a
        new format called
        <link linkend="file-png-save-defaults">Portable Network Graphics
        (PNG)</link> has been developed, which is better than GIF in all
        respects, with the exception of animations and some rarely-used
        features.
      </para>
      <para>
        GIF was introduced by CompuServe in 1987. It became popular mostly
        because of its efficient, LZW compression. The size of the image files
        required clearly less disk space than other usual graphics formats of
        the time, such as PCX or MacPaint. Even large images could be
        transmitted in a reasonable time, even with slow modems. In addition,
        the open licensing policy of CompuServe made it possible for any
        programmer to implement the GIF format for his own applications free
        of charge, as long as the CompuServe copyright notice was attached to
        them.
      </para>
      <para>
        Colors in GIF are stored in a color table which can hold up to 256
        different entries, chosen from 16.7 million different color values.
        When the image format was introduced, this was not a much of a
        limitation, since only a few people had hardware which could display
        more colors than that. For typical drawings, cartoons, black-and-white
        photographs and similar uses, 256 colors are quite sufficient as a
        rule, even today. For more complex images, such as color photographs,
        however, a huge loss of quality is apparent, which is why the format
        is not considered to be suitable for those purposes.
      </para>
      <para>
        One color entry in the palette can be defined to be transparent.
        With transparency, the GIF image can look like it is non-rectangular
        in shape. However, semi-transparency, as in
        <link linkend="file-png-save-defaults">PNG</link>, is not possible.
        A pixel can only be either entirely visible or completely
        transparent.
      </para>
      <para>
        The first version of GIF was 87a. In 1989, CompuServe published an
        expanded version, called 89a. Among other things, this made it
        possible to save several images in one GIF file, which is especially
        used for simple animation. The version number can be distinguished
        from the first six bytes of a GIF file. Interpreted as ASCII symbols,
        they are <quote>GIF87a</quote> or <quote>GIF89a</quote>.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-gnu">
    <glossterm>
      <phrase>GNU</phrase>
    </glossterm>
    <glossdef>
      <para>
        The GNU project was started in 1983 by Richard Stallman with the
        goal of developing a completely free operating system. It is
        especially well-known from the GNU General Public License (GPL) and
        GNU/Linux, a GNU-variant with a Linux kernel.
      </para>
      <para>
        The name came about from the naming conventions which were in
        practice at MIT, where Stallman worked at the time.
        For programs which were similar to other programs, recursive
        acronyms were chosen as names. Since the new system was to be based
        on the widespread operating system, Unix, Stallman looked for that
        kind of name and came up with GNU, which stands for
        <quote>GNU is not Unix</quote>. In order to avoid confusion, the
        name should be pronounced with the <quote>G</quote>, not like
        <quote>new</quote>. There were several reasons for making GNU
        Unix-compatible. For one thing, Stallman was convinced that most
        companies would refuse a completely new operating system, if the
        programs they used wouldn't run on it. In addition, the architecture
        of Unix made quick, easy and distributed development possible,
        since Unix consists of many small programs that can be developed
        independently of each other, for the most part. Also, many parts of
        a Unix system were freely available to anyone and could therefore
        be directly integrated into GNU, for example, the typesetting
        system, TeX, or the X Window System. The missing parts were newly
        written from the ground up.
      </para>
      <para>
        <acronym>GIMP</acronym> (GNU Image Manipulation Program) is an
        official GNU application
        <xref linkend="bibliography-online-wkpd-gnu"/>.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-grayscale">
    <glossterm>
      <anchor id="glossary-graylevel" xreflabel="Grayscale"/>
      <phrase>Grayscale</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Grayscale</primary>
      <secondary>Overview</secondary>
    </indexterm>
    <glossdef>
      <para>
        Grayscale is a mode for encoding the colors of an image which
        contains only black, white and shades of gray.
      </para>
      <para>
        When you create a new image, you can choose to create it in
        Grayscale mode (which you can colorize later, by changing it to RGB
        mode). You can also change an existing image to grayscale by using
        the <link linkend="gimp-image-convert-grayscale">Grayscale</link>,
        <link linkend="gimp-filter-desaturate">Desaturate</link>,
        <link linkend="plug-in-decompose-registered">Decompose</link>,
        <link linkend="gimp-filter-channel-mixer">Channel
        Mixer</link>, although not all formats will accept these changes.
        Although you can create images in Grayscale mode and convert images
        to it, it is not a color model, in the true sense of the word.
      </para>
      <para>
        As explained in <link linkend="glossary-rgb">RGB mode</link>, 24-bit
        <acronym>GIMP</acronym> images can have up to 256 levels of gray. If
        you change from Grayscale to RGB mode, your image will have an RGB
        structure with three color channels, but of course, it will still be
        gray.
      </para>
      <para>
        Grayscale image files (8-bit) are smaller than RGB files.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-guides">
    <glossterm>
      <phrase>Guides</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Guides</primary>
      <secondary>Using</secondary>
    </indexterm>
    <glossdef>
      <para>
        Guides are lines you can temporarily display on an image while you are
        working on it. You can display as many guides as you would like, in
        either the horizontal or the vertical direction. These lines help you
        position a selection or a layer on the image. They do not appear when
        the image is printed.
      </para>
      <para>
        For more information see
        <xref linkend="gimp-concepts-image-guides"/>.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-high-dynamic-range">
    <glossterm>
      <phrase>High Dynamic Range</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>High Dynamic Range</primary>
    </indexterm>
    <glossdef>
      <para>With
      <link linkend="glossary-display-referred">display-referred</link> data
      you have roughly two and half stops of head room above middle gray and
      maybe six and a half useable stops below middle gray, at which point the
      data is too densely packed into too few tonal steps to accurately display
      differences between solid black and "just barely gray". So at best you
      have 9 stops of dynamic range, compared to the 20 or more stops of
      dynamic range you might find in some (certainly not all!) real world
      scenes.</para>

      <para>The usual solution to the dynamic range limitations of
      display-referred data is to allow channel values to be however high as is
      needed to encode the scene data. This means allowing channel values that
      are above display-referred white.</para>

      <para>Several file formats currently supported by GIMP 2.10 can be used
      to import and export high dynamic range images, including floating point
      tiffs, OpenEXR, and FITS.</para>

      <para>When working with high dynamic range data in GIMP 2.10, the
      <link linkend="glossary-channel-encoding">channel encoding</link> does
      need to be linear to avoid gamma artifacts.</para>

      <para>Editing high dynamic range data requires that there isn't any
      clamping code in editing operations and blend modes. At floating point
      precision:</para>
      <orderedlist>
        <listitem><para>Many (but not all) GIMP 2.10 blend modes are unclamped,
        including Normal, Addition, Subtract, Multiply, Lighten Only,
        Darken Only, Difference, and the LCH and Luminance blend modes.
        Blend modes such as Screen, Soft Light, and Overlay are not unclamped
        as these operations are designed to work with display-referred data.
        </para></listitem>
        <listitem><para>Many (too many to list but certainly not all, as some
        editing operations are designed to work with display-referred data)
        GIMP 2.10 editing operations also are unclamped, including Levels,
        Exposure, transforms such as scaling and rotating, and various filter
        operations such as Gaussian blur.</para></listitem>
      </orderedlist>

      <para>Portions of the above explanation of "high dynamic range" are
      slightly modified excerpts from the <ulink url="http://ninedegreesbelow.com/photography/display-referred-scene-referred.html#scene-referred">Models for image editing: Display-referred and scene-referred</ulink>.
      These excerpts are quoted by permission and the modified excerpts are
      licensed as
      <ulink url="http://creativecommons.org/licenses/by-sa/3.0/deed.en_US">
        Creative Commons Attribution-ShareAlike 3.0 Unported License</ulink>.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-histogram">
    <glossterm>
      <phrase>Histogram</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Histogram</primary>
    </indexterm>
    <glossdef>
      <para>
        In digital image processing, a histogram is a graph representing the
        statistical frequency of the gray values or the color values in an
        image. The histogram of an image tells you about the occurrence of
        gray values or color values, as well as the contrast range and the
        brightness of the image. In a color image, you can create one
        histogram with information about all possible colors, or three
        histograms for the individual color channels. The latter makes the
        most sense, since most procedures are based on grayscale images and
        therefore further processing is immediately possible.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-hsv">
    <glossterm>
      <phrase>HSV</phrase>
    </glossterm>
    <glossdef>
      <para>
        HSV is a <link linkend="glossary-colormodel">color model</link>
        which has components for Hue (the color, such as blue or red),
        Saturation (how strong the color is) and Value (the brightness).
      </para>
      <para>
        The RGB mode is very well suited to computer screens, but it doesn't
        let us describe what we see in everyday life; a light green, a
        pale pink, a dazzling red, etc. The HSV model takes these
        characteristics into account. HSV and RGB are not completely
        independent of each other. You can see that with the Color Picker
        tool; when you change a color in one of the color models, the other
        one also changes. Brave souls can read
        <emphasis>Grokking the GIMP</emphasis>, which explains their
        interrelationship.
      </para>
      <variablelist>
        <para>Brief description of the HSV components:</para>
        <varlistentry>
          <term>Hue</term>
          <listitem>
            <para>
              This is the color itself, which results from the combination of
              primary colors. All shades (except for the gray levels) are
              represented in a <emphasis>chromatic circle</emphasis>: yellow,
              blue, and also purple, orange, etc. The chromatic circle (or
              <quote>color wheel</quote>) values range between 0° and 360°.
              (The term <quote>color</quote> is often used instead of
              <quote>Hue</quote>. The RGB colors are <quote>primary
              colors</quote>.)
            </para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>Saturation</term>
          <listitem>
            <para>
              This value describes how pale the color is. A completely
              unsaturated color is a shade of gray. As the saturation
              increases, the color becomes a pastel shade. A completely
              saturated color is pure. Saturation values go from 0 to 100,
              from white to the purest color.
            </para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>Value</term>
          <listitem>
            <para>
              This value describes the luminosity, the luminous intensity. It
              is the amount of light emitted by a color. You can see a change
              of luminosity when a colored object is moved from being in the
              shadow to being in the sun, or when you increase the luminosity
              of your screen. Values go from 0 to 100. Pixel values in the
              three channels are also luminosities: <quote>Value</quote> in
              the HSV color model is the maximum of these elementary values in
              the RGB space (scaled to 0-100).
            </para>
          </listitem>
        </varlistentry>
      </variablelist>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-html-notation">
    <glossterm>
      <phrase>HTML notation</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>HTML notation</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>Color</primary>
      <secondary>HTML notation</secondary>
    </indexterm>
    <glossdef>
      <para>
        A hex triplet is a way of encoding a color for a computer. The
        <quote>#</quote> symbol indicates that the numbers which follow it
        are encoded in hexadecimal. Each color is specified in two
        hexadecimal digits which make up a triplet (three pairs) of
        hexadecimal values in the form <quote>#rrggbb</quote>, where
        <quote>rr</quote> represents red, <quote>gg</quote> represents green
        and <quote>bb</quote> represents blue.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-imagehose">
    <glossterm>
      <phrase>Image Hose</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Image Hose</primary>
    </indexterm>
    <glossdef>
      <para>
        An image hose in <acronym>GIMP</acronym> is a special type of brush
        which consists of several images. For example, you could have a
        brush with footprints, which consists of two images, one for the
        left footprint and one for the right. While painting with this
        brush, a left footprint would appear first, then a right footprint,
        then a left one, etc. This type of brush is very powerful.
      </para>
      <para>
        An image hose is also sometimes called an <quote>image pipe</quote>
        or <quote>animated brush</quote>. An image hose is indicated in the
        Brushes dialog by a small red triangle in the lower right corner of
        the brush's symbol.
      </para>
      <para>
        For information concerning creating an image hose, please see the
        <xref linkend="gimp-using-animated-brushes"/> and
        <xref linkend="gimp-using-brushes"/>.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-incremental">
    <glossterm>
      <phrase>Incremental, paint mode</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Incremental</primary>
    </indexterm>
    <glossdef>
      <para>
        Incremental mode is a paint mode where each brush stroke is drawn
        directly on the active layer. When it is set, each additional stroke
        of the brush increases the effect of the brush, up to the maximum
        opacity for the brush.
      </para>
      <para>
        If incremental mode is not set, brush strokes are drawn on a canvas
        buffer, which is then combined with the active layer. The maximum
        effect of a brush is then determined by the opacity, and stroking with
        the brush repeatedly does not increase the effect beyond this limit.
      </para>
      <para>
        <inlinemediaobject>
          <imageobject>
            <imagedata format="PNG"
              fileref="images/glossary/tool-opt-increment.png"/>
          </imageobject>
        </inlinemediaobject>
        <inlinemediaobject>
          <imageobject>
            <imagedata format="PNG"
              fileref="images/glossary/tool-opt-nonincrement.png"/>
          </imageobject>
        </inlinemediaobject>
      </para>
      <para>
        The two images above were created using a brush with spacing set to
        60 percent. The image on the left shows non-incremental painting and
        the image on the right shows the difference with incremental painting.
      </para>
      <para>
        Incremental mode is a tool option that is shared by several brush
        tools, except those which have a <quote>rate</quote> control, which
        automatically implies an incremental effect. You can set it by
        checking the <guilabel moreinfo="none">Incremental</guilabel> checkbox in the
        tool option dialog for the tool (Paintbrush, Pencil and Eraser).
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-indexedcolors">
    <glossterm>
      <phrase>Indexed Colors</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Indexed Colors</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>Color</primary>
      <secondary>Indexed colors</secondary>
    </indexterm>
    <glossdef>
      <para>
        Indexed color mode is a mode for encoding colors in an image where
        each pixel in the image is assigned an 8-bit color number. The color
        which corresponds to this number is then put in a table (the palette).
        Changing a color in the palette changes all the pixels which refer
        to this palette color. Although you can create images in
        <emphasis>Indexed Color</emphasis> mode and can transform images to
        it, it is, strictly speaking, not a
        <link linkend="glossary-colormodel">color model</link>.
      </para>
      <para>
        See also the <link linkend="gimp-indexed-palette-dialog">Indexed
        Palette</link> section and the
        <link linkend="gimp-image-convert-indexed">Convert Image to Indexed
        Colors</link> command.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-interpolation">
    <glossterm>
      <phrase>Interpolation</phrase>
    </glossterm>
    <glossdef>
      <para>
        Interpolation means calculating intermediate values. When you
        enlarge (<quote>digitally zoom</quote>) or otherwise transform
        (rotate, shear or give perspective to) a digital image,
        interpolation procedures are used to compute the colors of the
        pixels in the transformed image. <acronym>GIMP</acronym> offers
        three interpolation methods, which differ in quality and speed. In
        general, the better the quality, the more time the interpolation
        takes (see
        <link linkend="gimp-tool-interpolation-methods">Interpolation
        methods</link>).
      </para>
      <para>
        <acronym>GIMP</acronym> uses interpolation when you
        <link linkend="gimp-image-scale">Scale</link> an image,
        <link linkend="gimp-layer-scale">Scale</link> a layer, and when you
        <link linkend="gimp-tools-transform">Transform</link> an image.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-jpeg">
    <glossterm>
      <phrase>JPEG</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>JPEG</primary>
    </indexterm>
    <glossdef>
      <para>
        JPEG is a <link linkend="glossary-fileformat">file format</link>
        which supports compression and works at all color depths. The
        image compression is adjustable, but beware: Too high a compression
        could severely reduce image quality, since JPEG compression is lossy.
      </para>
      <para>
        Use JPEG to create web graphics or if you don't want your
        image to take up a lot of space. JPEG is a good format for
        photographs and for computer-generated images (CGI). It is not well
        suited for:
      </para>
      <itemizedlist>
        <listitem>
          <para>
            digital line drawings (for example, screenshots or vector
            graphics), in which there are many neighboring pixels with the
            same color values, few colors and hard edges,
          </para>
        </listitem>
        <listitem>
          <para>
            Black and white images (only black and white, one bit per pixel)
            or
          </para>
        </listitem>
        <listitem>
          <para>half-toned images (newsprint).</para>
        </listitem>
      </itemizedlist>
      <para>
        Other formats, such as GIF, PNG or JBIG, are far better for these
        kinds of images.
      </para>
      <para>
        In general, JPEG transformations are not reversible. Opening and
        then saving a JPEG file causes a new, lossy compression. Increasing
        the quality factor later will not bring back the image information
        which was lost.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-lab">
    <glossterm>
      <phrase>L*a*b*</phrase>
    </glossterm>
    <glossdef>
      <para>
        The Lab color space (also called the L*a*b* color space) is a
        <link linkend="glossary-colormodel">color model</link>
        developed in the beginning of the 1930s by the Commission
        Internationale d`Eclairage (CIE). It includes all the colors that
        the human eye can perceive. That contains the colors of the
        RGB and the CMYK color spaces, among others. In Lab, a color is
        indicated by three values: L, a and b. Here, the L stands for the
        luminance component — corresponding to the gray value — and a and b
        represent the red-green and blue-yellow parts of the color,
        respectively.
      </para>
      <para>
        In contrast to RGB or CMYK, Lab is not dependent upon the
        various input and output devices. For that reason, it is used as an
        exchange format between devices. Lab is also the internal color
        model of PostScript Level II.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-layer">
    <glossterm>
      <phrase>Layer</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Layer</primary>
    </indexterm>
    <glossdef>
      <para>
        You can think of layers as being a stack of slides which are more or
        less transparent. Each layer represents an aspect of the image and
        the image is the sum of all of these aspects. The layer at the bottom
        of the stack is the background layer. The layers above it are the
        components of the foreground.
      </para>
      <para>
        You can view and manage the layers of the image through the
        <link linkend="gimp-layer-dialog">Layers dialog</link>.
      </para>
      <figure>
        <title>Example image with layers</title>
        <mediaobject>
          <imageobject>
            <imagedata format="PNG"
              fileref="images/dialogs/layers_overview.png"/>
          </imageobject>
          <caption>
            <para>Representation of an image with layers</para>
          </caption>
        </mediaobject>
        <mediaobject>
          <imageobject>
            <imagedata format="PNG"
              fileref="images/dialogs/layers_example.png"/>
          </imageobject>
          <caption><para>The final image</para></caption>
        </mediaobject>
      </figure>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-marching-ants">
    <glossterm>
      <phrase>Marching Ants</phrase>
    </glossterm>
    <glossdef>
      <para>
        Marching ants is a term which describes the dotted line which
        surrounds a selection. The line is animated, so it looks as if
        little ants are running around behind each other.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-masks">
    <glossterm>
      <phrase>Masks</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Masks</primary>
      <secondary>Overview</secondary>
    </indexterm>
    <glossdef>
      <para>
        A mask is like a veil put over a layer (layer mask) or all the layers
        of an image (selection mask). You can remove this mask by painting
        with white color, and you can complete it by painting with black
        color. When the mask is <quote>applied</quote>, non masked pixels
        will remain visible (the others will be transparent) or will be
        selected, according to the type of mask.
      </para>
      <para>There are two types of masks:</para>
      <itemizedlist>
        <listitem>
          <para>
            <emphasis>Layer Mask</emphasis>:
            Every layer can have its own mask. The layer mask represents the
            Alpha channel of the layer and allows you to manage its
            transparency. By painting on the layer mask, you can make parts of
            the layer opaque or transparent: painting with black makes the
            layer transparent, painting with white makes the layer opaque and
            painting with shades of gray makes the layer semi-transparent. You
            can use all paint tools to paint on the mask. You can also apply a
            filter or copy-paste. You can use the Layer mask for transition
            effects, volume effects, merging elements from another image, etc.
            See the <link linkend="gimp-layer-mask">Layer Mask</link>
            section for more details.
          </para>
        </listitem>
        <listitem>
          <para>
            <emphasis>Channel Mask</emphasis>, also called
            <emphasis>Selection Mask</emphasis>:
            Channel Masks determine the transparency of a selection. By
            painting on a Channel Mask with white, you remove the mask and
            increase the selection; with black, you reduce the selection.
            This procedure lets you create a selection very precisely. You
            can also save your selections to a Channel Mask with the
            <link linkend="gimp-selection-to-channel">Save to Channel</link>
            command. You can retrieve it later by using the
            <quote>Channel to selection</quote> command from the
            <link linkend="gimp-channel-menu">Channel menu</link>. Channel
            masks are so important in <acronym>GIMP</acronym> that a
            special type has been implemented: the
            <link linkend="gimp-qmask">Quick&nbsp;mask</link>. See the
            <link linkend="gimp-channel-mask">Selection&nbsp;mask</link>
            section for more details.
          </para>
        </listitem>
      </itemizedlist>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-moire">
    <glossterm>
      <phrase>Moir&eacute; Effect</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Moir&eacute;</primary>
    </indexterm>
    <glossdef>
      <para>
        The moir&eacute; effect (pronounce <quote>Moa-ray</quote>) is an
        unintended pattern which appears when a regular pattern of grids or
        lines interferes with another regular pattern placed over it. This can
        happen, for example, when you are scanning an image with a periodic
        structure (such as a checkered shirt or a half-toned image), scanning
        a digital image, taking a digital photograph of a periodic pattern,
        or even when silkscreening.
       </para>
      <para>
         If you discover the problem in time, the best solution is to move
         the original image a little bit in the scanner or to change the
         camera angle slightly.
       </para>
      <para>
         If you cannot re-create the image file, <acronym>GIMP</acronym>
         offers some filters which may help you with the problem. For more
         information, see the
         <link linkend="plug-in-despeckle">Despeckle</link> and
         <link linkend="plug-in-nlfilt">NL Filter</link> (Non-Linear)
         filters.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-parasite">
    <glossterm>
      <phrase>Parasite</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>XCF</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>.xcf</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>Parasite</primary>
    </indexterm>
    <glossdef>
      <para>
        A Parasite is additional data which may be written to an XCF file. A
        parasite is identified by a name, and can be thought of as an
        extension to the other information in an XCF file.
      </para>
      <para>
        Parasites of an image component may be read by
        <acronym>GIMP</acronym>
        plug-ins. Plug-ins may also define their own parasite names, which are
        ignored by other plug-ins. Examples of parasites are comments, the
        save options for the TIFF, JPEG and PNG file formats, the gamma value
        the image was created with and EXIF data.
      </para>
    </glossdef>
  </glossentry>
        
        <glossentry id="glossary-pass-through">
    <glossterm>
      <phrase>Pass-through</phrase>
    </glossterm>
    <glossdef>
                        <para>
                                Normally, the layers inside a layer group are isolated from the rest of 
                                the image -- the layer group is essentially a separate sub-image, 
                                living inside the bigger image; you can merge the group into a single 
                                layer, replace the original group with it, and the result would be the 
                                same.
                        </para>
                        <para>
                                In following examples, the names of the relevant layers in the images 
                                specify the layer mode, with the composite mode in parentheses where 
                                applicable, and the layer's opacity.
                        </para>
                        <mediaobject>
                                <imageobject>
                                        <imagedata format="PNG" 
fileref="images/dialogs/examples/layer-groups-pass-through-ex1.png"/>
                                </imageobject>
                                <caption>
                                        <para>
                                                In this example, the group uses Normal mode; note that the green 
                                                and blue layers don't affect the red layer: the green layer's color 
                                                isn't added to the the red layer's color, and the blue layer only 
                                                erases the green layer.
                                        </para>
                                </caption>
                        </mediaobject>
                        
                        <para>
                                Layer groups using Pass-through mode are different: the layers inside 
                                them <quote>see</quote> the layers below the group, and interact with 
                                them according to their layer mode. 
                        </para>
                        <mediaobject>
                                <imageobject>
                                        <imagedata format="PNG" 
fileref="images/dialogs/examples/layer-groups-pass-through-ex2.png"/>
                                </imageobject>
                                <caption>
                                        <para>
                                                In this example, the group uses Pass-through mode. Note that the 
                                                green layer's color <emphasis>is</emphasis> added to the red 
                                                layer's color, and the blue layer erases both the green and the red 
                                                layers.
                                        </para>
                                </caption>
                        </mediaobject>
                        
                        <para>
                                In simple cases, pass-through groups behave as though there is no group 
                                involved at all.
                        </para>
                        <mediaobject>
                                <imageobject>
                                        <imagedata format="PNG" 
fileref="images/dialogs/examples/layer-groups-pass-through-ex3.png"/>
                                </imageobject>
                                <caption>
                                        <para>
                                                The green and blue layers are not inside a group, and the result is 
                                                the same as in the preceding example.
                                        </para>
                                </caption>
                        </mediaobject>
                        <para>
                                In these cases, the group is primarily an organizational tool: it 
                                allows you to group together several layers, achieving some desired 
                                effect, and handle them as a unit.
                        </para>

                        <para>
                                However, in general, pass-through groups are not equivalent to having 
                                no group at all.  For example, when the group's opacity is less than 
                                100%, pass-through groups still behave as a single unit, applying the 
                                opacity to the group as a whole (like a normal group would) rather than 
                                to the individual layers, while still letting the group layers interact 
                                with the background layers.
                        </para>
                        <figure><title>Three images</title>
                        <mediaobject>
                                <imageobject>
                                        <imagedata format="PNG" 
fileref="images/dialogs/examples/layer-groups-pass-through-ex4.png"/>
                                </imageobject>
                        </mediaobject>
                        <mediaobject>
                                <imageobject>
            <imagedata format="PNG" 
fileref="images/dialogs/examples/layer-groups-pass-through-ex5.png"/>
                                </imageobject>
                        </mediaobject>
                        <mediaobject>
                                <imageobject>
            <imagedata format="PNG" 
fileref="images/dialogs/examples/layer-groups-pass-through-ex6.png"/>
                                </imageobject>
                        </mediaobject>
                        </figure>
                        <para>
                                Compare these three images, which demonstrate the same compositions as 
                                above, with the group (or the individual layers, in the last example) 
                                having an opacity of 50%. When using pass-through groups to group 
                                together several layers achieving a collective effect, the group's 
                                opacity essentially lets you control the <quote>strength</quote> of the 
                                effect, which can't be achieved using either normal groups, or 
                                individual layers.
                        </para>
                </glossdef>
  </glossentry>         

  <glossentry id="glossary-path">
    <glossterm>
      <phrase>Path</phrase>
    </glossterm>
    <glossdef>
      <para>
        A Path is a contour composed of straight lines, curves, or both. In
        <acronym>GIMP</acronym>, it is used to form the boundary of a
        selection, or to be <emphasis>stroked</emphasis> to create visible
        marks on an image. Unless a path is stroked, it is not visible when
        the image is printed and it is not saved when the image is written
        to a file (unless you use XCF format).
      </para>
      <para>
        See the <link linkend="gimp-concepts-paths">Paths Concepts</link>
        and <link linkend="gimp-using-paths">Using Paths</link> sections for
        basic information on paths, and the
        <link linkend="gimp-tool-path">Path Tool</link> section for
        information on how to create and edit paths. You can manage the
        paths in your image with the
        <link linkend="gimp-path-dialog">Paths dialog</link>.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-pdb">
    <glossterm>
      <phrase>PDB</phrase>
    </glossterm>
    <glossdef>
      <para>
        All of the functions which <acronym>GIMP</acronym> and its
        extensions make available are registered in the Procedure Database
        (PDB). Developers can look up useful programming information about
        these functions in the PDB by using the
        <link linkend="plug-in-dbbrowser">Procedure Browser</link>.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-pdf">
    <glossterm>
      <phrase>PDF</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>PDF</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>Formats</primary>
      <secondary>PDF</secondary>
    </indexterm>
    <glossdef>
      <para>
        PDF (Portable Document Format) is a
        <link linkend="glossary-fileformat">file format</link> which was
        developed by Adobe to address some of the deficiencies of
        PostScript. Most importantly, PDF files tend to be much smaller than
        equivalent PostScript files. As with PostScript,
        <acronym>GIMP</acronym>'s support of the PDF format is through the
        free Ghostscript libraries.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-pixel">
    <glossterm>
      <phrase>Pixel</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Pixel</primary>
    </indexterm>
    <glossdef>
      <para>
        A pixel is a single dot, or <quote>picture element</quote>, of an
        image. A rectangular image may be composed of thousands of pixels,
        each representing the color of the image at a given location. The
        value of a pixel typically consists of several
        <link linkend="glossary-channels">Channels</link>, such as the Red,
        Green and Blue components of its color, and sometimes its Alpha
        (transparency).
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-plug-in">
    <glossterm>
      <anchor id="glossary-plugin" xreflabel="Plugin"/>
      <phrase>Plugin</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Plugins</primary>
      <secondary>Definition</secondary>
    </indexterm>
    <glossdef>
      <para>
        Optional extensions for the <acronym>GIMP</acronym>. Plugins are
        external programs that run under the control of the main GIMP
        application and provide specific functions on-demand. See
        <xref linkend="gimp-concepts-plugins"/> for further information.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-png">
    <glossterm>
      <phrase>PNG</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>PNG</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>.png</primary>
    </indexterm>
    <glossdef>
      <para>
        PNG is the acronym of <quote>Portable Network Graphic</quote>
        (pronounce <quote>ping</quote>. This recent format offers many
        advantages and a few drawbacks: it is not lossy and gives files
        more heavy than the JPEG format, but it is perfect for saving your
        images because you can save them several times without losing
        data each time (it is used for this Help). It supports True Colors
        (several millions of colors), indexed images (256 colors like GIF),
        and 256 transparency levels (while GIF supports only two levels).
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-postscript">
    <glossterm>PostScript</glossterm>
    <indexterm>
      <primary>Formats</primary>
      <secondary>PostScript</secondary>
    </indexterm>
    <glossdef>
      <para>
        Created by Adobe, PostScript is a page description language mainly
        used by printers and other output devices. It's also an excellent way
        to distribute documents. <acronym>GIMP</acronym> does not support
        PostScript directly: it depends on a powerful free software program
        called Ghostscript.
      </para>
      <para>
        The great power of PostScript is its ability to represent vector
        graphics—lines, curves, text, paths, etc.—in a resolution-independent
        way. PostScript is not very efficient, though, when it comes to
        representing pixel-based raster graphics. For this reason, PostScript
        is not a good format to use for saving images that are later going to
        be edited using <acronym>GIMP</acronym> or another graphics program.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-psd">
    <glossterm>
      <phrase>PSD</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>PSD</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>.psd</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>Formats</primary>
      <secondary>PSD</secondary>
    </indexterm>
    <glossdef>
      <para>
        PSD is Adobe Photoshop's native
        <link linkend="glossary-fileformat">file format</link>, and it is
        therefore comparable to <link linkend="glossary-xcf">XCF</link>
        in complexity. <acronym>GIMP</acronym>'s ability to handle PSD files
        is sophisticated but limited: some
        features of PSD files are not loaded, and only older versions of PSD
        are supported. Unfortunately, Adobe has now made the Photoshop
        Software Development Kit — which includes their file format
        specifications — proprietary, and only available to a limited set of
        developers approved by Adobe. This does not include the
        <acronym>GIMP</acronym>
        development team, and the lack of information makes it very difficult
        to maintain up-to-date support for PSD files.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-quantization">
    <glossterm>
      <phrase>Quantization</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Quantization</primary>
    </indexterm>
    <glossdef>
      <para>
        Quantization is the process of reducing the color of a pixel into one
        of a number of fixed values by matching the color to the nearest color
        in the colormap. Actual pixel values may have far more precision than
        the discrete levels which can be displayed by a digital display. If
        the display range is too small, then abrupt changes in colors (false
        contours, or banding) may appear where the color intensity changes
        from one level to another. This is especially noticeable in Indexed
        images, which have 256 or fewer discrete colors.
      </para>
      <para>
        One way to reduce quantization effects is to use
        <link linkend="glossary-dithering">Dithering</link>. The
        operations in <acronym>GIMP</acronym> which perform
        dithering are the
        <link linkend="gimp-tool-gradient">Gradient tool</link>
        (if you have enabled the dithering option) and the
        <link linkend="gimp-image-convert-indexed">Convert to Indexed</link>
        command. However, they only work on RGB images and not on Indexed
        images.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-rendering-intent">
    <glossterm>
      <phrase>Rendering Intent</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Color Management</primary>
    </indexterm>
    <glossdef>
      <para>
        Rendering intents are ways of dealing with colors that are
        out-of-<xref linkend="glossary-gamut"/> colors present in the source
        space that the destination space is incapable of producing. There are
        four rendering intents defined by the ICC:
      </para>
      <variablelist>
        <varlistentry>
          <term>Perceptual</term>
          <listitem>
            <para>
              This rendering intent is typically used for photographic
              content. It scales one gamut to fit into the other while
              maintaining the relative position of colors.
            </para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>Relative colorimetric</term>
          <listitem>
            <para>
              This rendering intent is typically used for spot colors. Colors
              that are not out of gamut are left unchanged. Colors outside the
              gamut are converted to colors with the same lightness, but
              different saturation, at the edge of the gamut.
            </para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>Saturation</term>
          <listitem>
            <para>
              This method is typically used for business graphics. The
              relative saturation of colors is mostly maintained, but
              lightning is usually changed.
            </para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>Absolute colorimetric</term>
          <listitem>
            <para>
              This rendering intent is most often used in proofing. It
              preserves the native device white point of the source image.
            </para>
          </listitem>
        </varlistentry>
      </variablelist>
    </glossdef>
  </glossentry>
  <glossentry id="glossary-rgb">
    <glossterm>
      <phrase>RGB</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>RGB</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>Color</primary>
      <secondary>Additive color model</secondary>
    </indexterm>
    <glossdef>
      <figure float="0">
        <title>
          <phrase>Additive color model</phrase>
        </title>
        <mediaobject>
          <imageobject>
            <imagedata format="PNG"
              fileref="images/glossary/color-model-additive.png"/>
          </imageobject>
        </mediaobject>
      </figure>
      <para>
        RGB is a <link linkend="glossary-colormodel">color model</link>
        which has components for Red, Green and Blue. These colors are
        emitted by screen elements and not reflected as they are with paint.
        The resulting color is a combination of the three primary RGB colors,
        with different degrees of lightness. If you look closely at your
        television screen, whose pitch is less than that of a computer
        screen, you can see the red, green and blue elements lit with
        different intensities. The RGB color model is
        <emphasis>additive</emphasis>.
      </para>
      <para><acronym>GIMP</acronym> uses eight bits per channel for each primary
        color. That means there are 256 intensities (Values) available,
        resulting in 256×256×256 = 16,777,216 colors.
      </para>
      <para>
        It is not obvious why a given combination of primary colors produces a
        particular color. Why, for instance, does 229R+205G+229B give a shade
        of pink? This depends upon the human eye and brain. There is no color
        in nature, only a continuous spectrum of wavelengths of light. There
        are three kinds of cones in the retina. The same wavelength of light
        acting upon the three types of cones stimulates each of them
        differently, and the mind has learned, after several million years of
        evolution, how to recognize a color from these differences.
      </para>
      <para>
        It is easy to see that no light (0R+0G+0B) produces complete darkness,
        black, and that full light (255R+255G+255B) produces white. Equal
        intensity on all color channels produces a level of gray. That is why
        there can only be 256 gray levels in <acronym>GIMP</acronym>.
      </para>
      <para>
        Mixing two <emphasis>Primary colors</emphasis> in RGB mode
        gives a <emphasis>Secondary color</emphasis>, that is, a
        color in the CMY model. Thus combining Red and Green gives
        Yellow, Green and Blue give Cyan, Blue and Red give Magenta.
        Don't confuse secondary colors with
        <emphasis>Complementary colors</emphasis> which are
        directly opposite a primary color in the chromatic
        circle:
      </para>
      <figure float="0">
        <title>Colorcircle</title>
        <mediaobject>
          <imageobject>
            <imagedata format="PNG"
              fileref="images/glossary/colorcircle.png"/>
          </imageobject>
          <caption>
            <para>
              Mixing a primary color with its complementary color gives gray
              (a neutral color).
            </para>
          </caption>
        </mediaobject>
      </figure>
      <para>
        It is important to know what happens when you are dealing with colors
        in <acronym>GIMP</acronym>.
        The most important rule to remember is that decreasing the intensity
        of a primary color results in increasing the intensity of the
        complementary color (and vice versa). This is because when you
        decrease the value of a channel, for instance Green, you automatically
        increase the relative importance of the other two, here Red and Blue.
        The combination of these two channels gives the secondary color,
        Magenta, which is the complementary color of Green.
      </para>
      <!-- this is definitely off-topic in a glossary!
           TODO: move this para to somewhere else
      <para>
        <emphasis>Exercise</emphasis>:
        You can check this out. Create a new image with only a white
        background (255R+255G+255B). Open the
        <menuchoice>
          <guimenu>Tools</guimenu>
          <guisubmenu>Color Tools</guisubmenu>
          <guimenuitem>Levels</guimenuitem>
        </menuchoice>
        dialog and select the Red channel. If necessary, check the preview
        box. Move the white slider to the left to decrease the Red value. You
        will notice that the background of your image gets closer and closer
        to Cyan. Now, decrease the Blue channel: only the Green will remain.
        For practice, go backwards, add a color and try to guess what hue will
        appear.
      </para>
      -->
      <!-- probably this is off-topic too... -->
      <para>
        The <link linkend="gimp-tool-color-picker">Color Picker</link>
        tool lets you find out the RGB values of a pixel and the
        <link linkend="glossary-html-notation">hextriplet</link>
        for the color.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-samplemerge">
    <glossterm>
      <phrase>Sample Merge</phrase>
    </glossterm>
    <glossdef>
      <para>
        Sample Merged is an option you can set when you use the
        <link linkend="gimp-tool-bucket-fill">Bucket Fill</link>
        tool, the <link linkend="gimp-tool-color-picker">Color Picker</link>
        tool and various selection tools. It is useful when you are working on
        an image with several layers and the active layer is either
        semi-transparent or has a
        <link linkend="gimp-concepts-layer-modes">Layer Mode</link>
        which is not set to Normal. When you check the Sample Merged option,
        the color which is used for the operation is the composite color of
        all the visible layers. When the Sample Merged option is not checked,
        the color used is the color of the active layer itself.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-saturation">
    <glossterm>
      <phrase>Saturation</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Saturation</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>Color</primary>
      <secondary>Saturation</secondary>
    </indexterm>
    <glossdef>
      <para>
        This term refers to color purity. Imagine you add pigment to white
        paint. Saturation varies from 0 (white, fully toned down, fully
        diluted) to 100 (pure color).
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-scene-referred">
    <glossterm>
      <phrase>Scene-referred</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Scene-referred</primary>
    </indexterm>
    <glossdef>
      <para>When speaking of images captured by a camera, scene-referred means
      that the intensities in the image RGB channels are proportional to the
      intensities in the scene that was photographed.</para>

      <para>"Scene-referred" is not the same as
      <link linkend="glossary-high-dynamic-range">high dynamic range</link>,
      as the camera might have been aimed at a low dynamic range scene such as
      a foggy early morning view. However, adding a light source to the
      captured frame (eg the moon breaking through the clouds or a street lamp)
      will turn even a foggy morning into a high dynamic range scene.</para>

      <para>As lightwaves do combine linearly, by definition a scene-referred
      image (whether real or imaginary) must be encoded linearly to preserve
      the scene-referred nature of the data.</para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-supersampling">
    <glossterm>
      <phrase>Supersampling</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Supersampling</primary>
    </indexterm>
    <glossdef>
      <para>
      Supersampling is a more sophisticated antialiasing technique, that
      is, a method of reducing jagged and stair-stepped edges along a
      slanted or curved line. Samples are taken at several locations
      <emphasis>within</emphasis> each pixel, not just at the center, and
      an average color is calculated. This is done by rendering the image
      at a much higher resolution than the one being displayed and then
      shrinking it to the desired size, using the extra pixels for
      calculation. The result is a smoother transition from one line of
      pixels to another along the edges of objects.
    </para>
      <para>
      The quality of the result depends on the number of samples.
      Supersampling is often performed at a range of 2× to 16× the original
      size. It greatly increases the amount of time needed to draw the image
      and also the amount of space needed to store the image in memory.
    </para>
      <para>
      One way to reduce the space and time requirement is to use Adaptive
      Supersampling. This method takes advantage of the fact that very few
      pixels are actually on an object boundary, so only those pixels need to
      be supersampled. At first, only a few samples are taken within a pixel.
      If the colors are very similar to each other, only those samples are
      used to calculate the final color. If not, more samples are used. This
      means that the higher number of samples is calculated only where
      necessary, which improves performance.
    </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-svg">
    <glossterm>
      <phrase>SVG</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>SVG</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>Formats</primary>
      <secondary>SVG</secondary>
    </indexterm>
    <glossdef>
      <para>
        SVG stands for Scalable Vector Graphics. It is a format for
        two-dimensional vector graphics, both static and animated. You can
        export GIMP paths to SVG and you can import SVG documents into GIMP
        from a vector graphic software. See
        <xref linkend="bibliography-online-wkpd-svg"/> for more details.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-tga">
    <glossterm><phrase>TGA</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>TGA</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>TARGA</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>Formats</primary>
      <secondary>TGA</secondary>
    </indexterm>
    <glossdef>
      <para>
        TGA (TARGA Image File) is a
        <link linkend="glossary-fileformat">file format</link> which
        supports 8, 16, 24 or 32 bits per pixel and optional RLE compression.
        It was originally developed by the Truevision company.
        <quote>TGA</quote> stands for Truevision Graphics Adapter and
        <quote>TARGA</quote> stands for Truevision Advanced Raster Graphics
        Adapter.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-tiff">
    <glossterm>
      <phrase>TIFF</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>TIFF</primary>
    </indexterm>
    <glossdef>
      <para>
        TIFF (Tagged Image File Format) is a
        <link linkend="glossary-fileformat">file format</link> which was
        developed primarily for scanned
        raster graphics for color separation. Six different encoding routines
        are supported, each with one of three different image modes: black and
        white, grayscale and color. Uncompressed TIFF images may be 1, 4, 8 or
        24 bits per pixel. TIFF images compressed using the LZW algorithm may
        be 6, 8 or 24 bits per pixel. Besides PostScript format, TIFF is one
        of the most important formats for preliminary stages of printing. It
        is a high quality file format, which is perfect for images you want to
        import to other programs like FrameMaker or CorelDRAW.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-tile">
    <glossterm>
      <phrase>Tile</phrase>
    </glossterm>
    <glossdef>
      <para>
        A Tile is a part of an image which <acronym>GIMP</acronym>
        currently has open. In order to avoid having to store an
        entire image in memory at the same time,
        <acronym>GIMP</acronym> divides it into smaller pieces.
        A tile is usually a square of 64 x 64 pixels, although tiles at
        the edges of an image may be smaller than that.
      </para>
      <para>
        At any time, a tile may be in main memory, in the tile cache
        in RAM, or on disk. Tiles which are currently being worked on are
        in main memory. Tiles which have been used recently are in RAM.
        When the tile cache in RAM is full, tiles which have been used
        least recently are written to disk. <acronym>GIMP</acronym>
        can retrieve the tiles from RAM or disk when they are needed.
      </para>
      <para>
        Do not confuse these tiles with those in the
        <link linkend="plug-in-tile">Tile Filter</link>
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-uri">
    <glossterm>
      <phrase>URI</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>URI</primary>
    </indexterm>
    <glossdef>
      <para>
        A Uniform Resource Identifier (URI) is a string of characters that
        serves to identify an abstract or a physical resource. URIs are used
        for the identification of resources in the Internet (such as web
        pages, miscellaneous files, calling up web services, and for receivers
        of e-mail) and they are especially used in the Worldwide Web.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-url">
    <glossterm>
      <phrase>URL</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>URL</primary>
    </indexterm>
    <glossdef>
      <para>
        URLs (Uniform Resource Locators) are one type of Uniform Resource
        Identifiers (URIs). URLs identify a resource by its primary access
        mechanism (commonly http or ftp) and the location of the resource in
        the computer network. The name of the URI scheme is therefore
        generally derived from the network protocol used for it. Examples of
        network protocols are http, ftp and mailto.
      </para>
      <para>
        Since URLs are the first and most common kinds of URIs, the terms are
        often used synonymously.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-value">
    <glossterm>
      <phrase>Value</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>Value</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>Color</primary>
      <secondary>Value</secondary>
    </indexterm>
    <glossdef>
      <para>
        This term often refers to the light intensity, the luminosity of
        a color. It varies from 0 (black) to 100 (full light).
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-xcf">
    <glossterm>
      <phrase>
      XCF
    </phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>XCF</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>.xcf</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>.xcf.gz</primary>
    </indexterm>
    <indexterm significance="normal">
      <primary>Formats</primary>
      <secondary>XCF</secondary>
    </indexterm>
    <glossdef>
      <para>
        XCF is a <link linkend="glossary-fileformat">file format</link>
        which is special because it is <acronym>GIMP</acronym>'s
        native file format: that is, it was designed specifically to store all
        of the data that goes to make up a <acronym>GIMP</acronym> image.
        Because of this, XCF files may be quite complicated, and there are
        few programs other than <acronym>GIMP</acronym> that can read them.
      </para>
      <para>
        When an image is stored as an XCF file, the file encodes nearly
        everything there is to know about the image: the pixel data for each
        of the layers, the current selection, additional channels if there are
        any, paths if there are any, and guides. The most important thing that
        is <emphasis>not</emphasis> saved in an XCF file is the undo history.
      </para>
      <para>
        The pixel data in an XCF file is represented
        in a lossless compressed form: the image byte blocks are compressed
        using the lossless RLE algorithm. This means that no matter how many
        times you load and save an image using this format, not a single
        pixel or other image data is lost or modified because of this format.
        XCF files can become very large, however <acronym>GIMP</acronym>
        allows you to compress the files themselves, using either the gzip
        or bzip2 compression methods, both of which are fast, efficient, and
        freely available. Compressing an XCF file will often shrink it by a
        factor of 10 or more.
      </para>
      <para>
        The <acronym>GIMP</acronym> developers have made a great effort to
        keep the XCF file format compatible across versions. If you create a
        file using <acronym>GIMP</acronym> 2.0, it ought to be possible to
        open the file in <acronym>GIMP</acronym> 1.2. However, some of the
        information in the file may not be usable: for example,
        <acronym>GIMP</acronym> 2.0 has a much more sophisticated way of
        handling text than <acronym>GIMP</acronym> 1.2, so a text layer
        from a <acronym>GIMP</acronym> 2.0 XCF file will appear as an
        ordinary image layer if the file is opened in
        <acronym>GIMP</acronym> 1.2.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-ycbcr">
    <glossterm>
      <phrase>YCbCr</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>YCbCr</primary>
    </indexterm>
    <glossdef>
      <para>
        YCbCr is a <link linkend="glossary-colormodel">color model</link>
        which was developed for the PAL television standard as a simple
        modification to the YUV color model. In the
        meantime, it has become the CCIR-601 standard for image and video
        recording. For example, it is used for JPEG pictures and MPEG videos,
        and therefore also on DVDs, video CDs and for most other widespread
        digital video standards. Note that a color model is still not a color
        space, since it doesn't determine which colors are actually meant by
        <quote>red</quote>, <quote>green</quote> and <quote>blue</quote>.
        For a color space, there must still be a reference to a specific
        absolute color value.
      </para>
      <para>
        There are color models which do not express a color by the additive
        basic colors, red, green and blue (RGB), but by other properties, for
        example, the brightness-color model. Here, the criteria are the basic
        brightness of the colors (from black, through gray, to white), the
        colors with the largest portion (red, orange, yellow, green, blue,
        violet, or other pure colors that lie between them) and the saturation
        of the colors (<quote>gaudy</quote> to pale). This color model is
        based on the
        ability of the eye to recognize small differences in luminosity
        better than small color differences, and to recognize those better
        than small differences in saturation. That makes gray text written on
        a black background easy to read, but blue text on a red background
        very hard to read, even with the same basic brightness. Such color
        models are called brightness-color models.
      </para>
      <para>
        The YCbCr model is a slight adaptation of such a brightness-color
        model. An RGB color value is divided into a basic brightness, Y, and
        two components, Cb and Cr, where Cb is a measurement of the deviation
        from gray in the blue direction, or if it is less than 0.5, in the
        direction of yellow. Cr is the corresponding measurement for the
        difference in the direction of red or turquoise. This representation
        uses the peculiarity of the eye of being especially sensitive to green
        light. That is why most of the information about the proportion of
        green is in the basic brightness, Y, an only the deviations for the
        red and blue portions need to be represented. The Y values have twice
        the resolution of the other two values, Cb and Cr, in most practical
        applications, such as on DVDs.
      </para>
    </glossdef>
  </glossentry>

  <glossentry id="glossary-yuv">
    <glossterm>
      <phrase>YUV</phrase>
    </glossterm>
    <indexterm significance="normal">
      <primary>YUV</primary>
    </indexterm>
    <glossdef>
      <para>
        YUV is a <link linkend="glossary-colormodel">color model</link>
        which uses two components to represent the color
        information, luma (the strength of the light per area) and the
        chrominance, or proportion of color (chroma), where the chrominance
        again consists of two components. The development of the YUV color
        model also goes back to the development of color television (PAL),
        where ways were sought for transmitting the color information
        along with the black-and-white signal, in order to achieve backwards
        compatibility with old black and white televisions without having to
        increase the available transmission bandwidth. From the YUV color
        model of the analog television techiques, the YCrCb color model was
        developed, which is used for most kinds of digital image and video
        compression. Erroneously, the YUV color model is also often spoken
        about in those fields, although the YCbCr model is actually used.
        This often causes confusion.
      </para>
      <para>
        For the calculation of the luma signals, the underlying RGB data is
        first adjusted with the <link linkend="glossary-gamma">gamma</link>
        value of the output device, and an R'G'B' signal is obtained. The
        three individual components are added together with different
        weights, to form the brightness information, which also functions as
        the VBS signal (Video Baseband Signal, the black-and-white signal)
        for the old black and white televisions.
      </para>
      <para>Y=R+G+B</para>
      <para>
        The exact calculation is more complicated, however, since some aspects
        of the color perception of the human eye have to be taken into
        account. For example, green is perceived to be lighter than red, and
        this is perceived to be lighter than blue. Furthermore, in some
        systems gamma correction of the basic color is first performed.
      </para>
      <para>
        The chrominance signals, and the color difference signals also,
        contain the color information. They are formed by the difference of
        blue minus luma or red minus luma.
      </para>
      <para>U=B-Y</para>
      <para>V=R-Y</para>
      <para>
        From the three generated components, Y, U and V, the individual
        color proportions of the basic color can be calculated again later:
      </para>
      <para>Y + U = Y + ( B - Y ) = Y - Y + B = B</para>
      <para>Y + V = Y + ( R - Y ) = Y - Y + R = R</para>
      <para>Y - B - R = ( R + G + B ) - B - R = G</para>
      <para>
        Furthermore, because of the structure of the retina of the human
        eye, it turns out that the brightness information is perceived at a
        higher resolution than the color, so that many formats based on
        the YUV color model compress the chrominance to save bandwidth
        during transmission.
      </para>
    </glossdef>
  </glossentry>
</glossary>