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  6.     <title>6.┬áHistogram dialog</title>
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  11.     <link rel="up" href="ch04.html" title="Chapter┬á4.┬áDialogs" />
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  18.         <tr>
  19.           <th colspan="3" align="center" id="chaptername">Chapter┬á4.┬áDialogs</th>
  20.         </tr>
  21.         <tr>
  22.           <td width="20%" align="left"><a accesskey="p" href="ch04s05.html">Prev</a>┬á</td>
  23.           <th width="60%" align="center" id="sectionname">6.┬áHistogram dialog</th>
  24.           <td width="20%" align="right">┬á<a accesskey="n" href="ch04s07.html">Next</a></td>
  25.         </tr>
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  29.     <div class="sect1" lang="en" xml:lang="en">
  30.       <div class="titlepage">
  31.         <div>
  32.           <div>
  33.             <h2 class="title" style="clear: both"><a id="gimp-histogram-dialog"></a>6.┬áHistogram dialog</h2>
  34.           </div>
  35.         </div>
  36.       </div>
  37.       <a id="id3451729" class="indexterm"></a>
  38.       <div class="informalfigure">
  39.         <div class="mediaobject">
  40.           <img src="../images/dialogs/dialogs-histogram-basic.png" />
  41.           <div class="caption">
  42.             <p>
  43.       The Histogram dialog
  44.     </p>
  45.           </div>
  46.         </div>
  47.       </div>
  48.       <p>
  49.     The Histogram dialog shows you information about the statistical
  50.     distribution of color values in the image that is currently
  51.     active.  This information is often useful when you are trying to
  52.     <span class="emphasis"><em>color balance</em></span> an image.  However, the
  53.     Histogram dialog is purely informational:  nothing you do with it
  54.     will cause any change to the image.  If you want to perform a
  55.     histogram-based color correction, use the <a href="ch03s05s06.html" title="5.6.┬áLevels tool">Levels</a> tool.
  56.   </p>
  57.       <p>
  58.     This is a dockable dialog; see the section on <a href="ch02s03s04.html" title="3.4.┬áDialogs and Docking">Dialogs and Docking</a> for help on
  59.     manipulating it.  It can be activated in two ways:
  60.   </p>
  61.       <div class="itemizedlist">
  62.         <ul type="disc">
  63.           <li>
  64.             <p>   
  65.         From the Toolbox menu:
  66.         <span class="guimenu">File</span>-><span class="guisubmenu">Dialogs</span>-><span class="guimenuitem">Histogram</span>
  67.       </p>
  68.           </li>
  69.           <li>
  70.             <p>   
  71.         From the image menu:
  72.         <span class="guimenu">Dialogs</span>-><span class="guimenuitem">Histogram</span>
  73.       </p>
  74.           </li>
  75.         </ul>
  76.       </div>
  77.       <div class="simplesect" lang="en" xml:lang="en">
  78.         <div class="titlepage">
  79.           <div>
  80.             <div>
  81.               <h3 class="title"><a id="id3451852"></a>About Histograms</h3>
  82.             </div>
  83.           </div>
  84.         </div>
  85.         <p>
  86.       In GIMP, each layer of an image can be decomposed into one or
  87.       more color channels:  for an RGB image, into R, G, and B
  88.       channels; for a grayscale image, into a single Value channel.
  89.       Layers that support transparency have an additional channel, the
  90.       alpha channel.  Each channel supports a range of intensity
  91.       levels from 0 to 255 (integer valued).  Thus, a black pixel is
  92.       encoded by 0 on all color channels; a white pixel by 255 on all
  93.       color channels.  A transparent pixel is encoded by 0 on the
  94.       alpha channel; an opaque pixel by 255.
  95.     </p>
  96.         <p>
  97.       For RGB images, it is convenient to define a Value "pseudochannel".
  98.       This is not a real color channel:  it does not reflect
  99.       any information stored directly in the image.  Instead, the
  100.       Value at a pixel is given by the equation 
  101.       <tt class="code">
  102.         V = (R + G + B)/3
  103.       </tt>,
  104.       rounded to the nearest integer.  Essentially, the Value is what
  105.       you would get at that pixel if you converted the image to
  106.       Grayscale mode.
  107.     </p>
  108.         <p>
  109.       For more information on channels, please consult the <a href="ch02s04.html" title="4.┬áWorking with Images">Working with Images</a> section.
  110.     </p>
  111.         <div class="variablelist">
  112.           <p class="title">
  113.             <b>Using the Histogram dialog</b>
  114.           </p>
  115.           <dl>
  116.             <dt>
  117.               <span class="term">Channel</span>
  118.             </dt>
  119.             <dd>
  120.               <div class="informalfigure">
  121.                 <div class="mediaobject">
  122.                   <img src="../images/dialogs/dialogs-histogram-menu.png" />
  123.                   <div class="caption">
  124.                     <p>
  125.           Channel options for an RGB layer with alpha channel.
  126.         </p>
  127.                   </div>
  128.                 </div>
  129.               </div>
  130.               <p>
  131.             This allows you to select which channel to use.  The
  132.             possibilities depend on the layer type of the active
  133.             layer.  Here are the entries you might see, and what they
  134.             mean: 
  135.           </p>
  136.               <div class="variablelist">
  137.                 <dl>
  138.                   <dt>
  139.                     <span class="term">Value</span>
  140.                   </dt>
  141.                   <dd>
  142.                     <p>
  143.                   For RGB and Grayscale images, this shows the
  144.                   distribution of brightness values 
  145.                   across the layer.  For a grayscale image, these are
  146.                   read directly from the image data.  For an RGB
  147.                   image, they are taken from the Value pseudochannel.  
  148.                 </p>
  149.                     <p>
  150.                   For an indexed image, the "Value" channel actually
  151.                   shows the distribution of frequencies for each
  152.                   colormap index:  thus, it is a "pseudocolor"
  153.                   histogram rather than a true color histogram.  
  154.                 </p>
  155.                   </dd>
  156.                   <dt>
  157.                     <span class="term">Red, Green, Blue</span>
  158.                   </dt>
  159.                   <dd>
  160.                     <p>
  161.                   These only appear for layers from RGB images.  They
  162.                   show the distribution of intensity levels for the
  163.                   Red, Green, or Blue channels respectively.
  164.                 </p>
  165.                   </dd>
  166.                   <dt>
  167.                     <span class="term">Alpha</span>
  168.                   </dt>
  169.                   <dd>
  170.                     <p>
  171.                   This shows the distribution of opacity levels.  If
  172.                   the layer is completely opaque or completely
  173.                   transparent, the histogram will consist of a single
  174.                   bar on the left or right edge.
  175.                 </p>
  176.                   </dd>
  177.                   <dt>
  178.                     <span class="term">RGB</span>
  179.                   </dt>
  180.                   <dd>
  181.                     <div class="informalfigure">
  182.                       <div class="mediaobject">
  183.                         <img src="../images/dialogs/dialogs-histogram-rgb.png" />
  184.                         <div class="caption">
  185.                           <p>
  186.             Combined histograms of R, G, and B channels.
  187.               </p>
  188.                         </div>
  189.                       </div>
  190.                     </div>
  191.                     <p>
  192.                   This entry, only available for RGB layers, shows the
  193.                   R, G, and B histograms superimposed, so that you can
  194.                   see all of the color distribution information in a
  195.                   single view.
  196.                 </p>
  197.                   </dd>
  198.                 </dl>
  199.               </div>
  200.             </dd>
  201.             <dt>
  202.               <span class="term">Linear/Logarithmic buttons</span>
  203.             </dt>
  204.             <dd>
  205.               <div class="informalfigure">
  206.                 <div class="mediaobject">
  207.                   <img src="../images/dialogs/dialogs-histogram-log.png" />
  208.                   <div class="caption">
  209.                     <p>
  210.           The histogram shown at the top, changed to logarithmic
  211.           mode. 
  212.         </p>
  213.                   </div>
  214.                 </div>
  215.               </div>
  216.               <p>
  217.             These buttons determine whether the histogram will be
  218.             displayed using a linear or logarithmic Y axis.  For
  219.             images taken from photographs, the linear mode is most
  220.             commonly useful.  For images that contain substantial
  221.             areas of constant color, though, a linear histogram will
  222.             often be dominated by a single bar, and a logarthmic
  223.             histogram will often be more useful.  
  224.           </p>
  225.             </dd>
  226.             <dt>
  227.               <span class="term">Range Setting</span>
  228.             </dt>
  229.             <dd>
  230.               <div class="informalfigure">
  231.                 <div class="mediaobject">
  232.                   <img src="../images/dialogs/dialogs-histogram-range.png" />
  233.                   <div class="caption">
  234.                     <p>
  235.                   Dialog aspect after range fixing.
  236.                     </p>
  237.                   </div>
  238.                 </div>
  239.               </div>
  240.               <p>
  241.             You can restrict the analysis, for the statistics shown at
  242.             the bottom of the dialog, to a limited range of values if
  243.             you wish.  You can set the range in one of three ways:
  244.           </p>
  245.               <div class="itemizedlist">
  246.                 <ul type="disc">
  247.                   <li>
  248.                     <p>
  249.                 Click and drag the pointer across the histogram
  250.                 display area, from the lowest level to the highest
  251.                 level of the range you want.
  252.               </p>
  253.                   </li>
  254.                   <li>
  255.                     <p>
  256.                 Click and drag the black or white triangles on the
  257.                 slider below the histogram.
  258.               </p>
  259.                   </li>
  260.                   <li>
  261.                     <p>
  262.                 Use the spinbutton entries below the slider (left
  263.                 entry: bottom of range; right entry: top of range).
  264.               </p>
  265.                   </li>
  266.                 </ul>
  267.               </div>
  268.             </dd>
  269.             <dt>
  270.               <span class="term">Statistics</span>
  271.             </dt>
  272.             <dd>
  273.               <p>
  274.             At the bottom of the dialog are shown some basic
  275.             statistics describing the distribution of channel values,
  276.             restricted to the selected range.  These are the mean,
  277.             standard deviation, and median of the selected histogram
  278.             portion; the number of pixels in the image; the number
  279.             whose values fall within the selected range; and the
  280.             percentage whose values fall within the selected range.
  281.           </p>
  282.             </dd>
  283.           </dl>
  284.         </div>
  285.       </div>
  286.     </div>
  287.     <div class="navfooter">
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  290.         <tr>
  291.           <td width="40%" align="left"><a accesskey="p" href="ch04s05.html">Prev</a>┬á</td>
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  297.         <tr>
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