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  6.     <title>2.5.¬† Histogram dialog</title>
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  14.     <link rel="prev" href="gimp-indexed-palette-dialog.html" title="2.4.¬† Colormap Dialog" />
  15.     <link rel="next" href="gimp-navigation-dialog.html" title="2.6.¬† Navigation Dialog" />
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  17.   <body>
  18.     <div class="navheader">
  19.       <table width="100%" summary="Navigation header">
  20.         <tr>
  21.           <th colspan="3" align="center">2.5.¬†
  22.     <span lang="en" xml:lang="en">Histogram dialog</span>
  23.   </th>
  24.         </tr>
  25.         <tr>
  26.           <td width="20%" align="left"><a accesskey="p" href="gimp-indexed-palette-dialog.html"><img src="../images/prev.png" alt="Prev" /></a>¬†</td>
  27.           <th width="60%" align="center">2.¬†
  28.     <span lang="en" xml:lang="en">Image Structure Related Dialogs</span>
  29.   </th>
  30.           <td width="20%" align="right">¬†<a accesskey="n" href="gimp-navigation-dialog.html"><img src="../images/next.png" alt="Next" /></a></td>
  31.         </tr>
  32.       </table>
  33.       <hr />
  34.     </div>
  35.     <div class="sect2" lang="en" xml:lang="en">
  36.       <div class="titlepage">
  37.         <div>
  38.           <div>
  39.             <h3 class="title"><a id="gimp-histogram-dialog"></a>2.5.¬†
  40.     <span lang="en" xml:lang="en">Histogram dialog</span>
  41.   </h3>
  42.           </div>
  43.           <div>
  44.             <div class="revhistory">
  45.               <table border="1" width="100%" summary="Revision history">
  46.                 <tr>
  47.                   <th align="left" valign="top" colspan="3">
  48.                     <b>Revision History</b>
  49.                   </th>
  50.                 </tr>
  51.                 <tr>
  52.                   <td align="left">Revision $Revision: 2416 $</td>
  53.                   <td align="left">2007-09-30</td>
  54.                   <td align="left">j.h</td>
  55.                 </tr>
  56.               </table>
  57.             </div>
  58.           </div>
  59.         </div>
  60.       </div>
  61.       <a id="id2658915" class="indexterm"></a>
  62.       <a id="id2658929" class="indexterm"></a>
  63.       <div class="figure">
  64.         <a id="id2658940"></a>
  65.         <p class="title">
  66.           <b>Figure¬†14.17.¬†
  67.       <span lang="en" xml:lang="en">The Histogram dialog</span>
  68.     </b>
  69.         </p>
  70.         <div class="figure-contents">
  71.           <div class="mediaobject">
  72.             <img src="../images/dialogs/dialogs-histogram-basic.png" alt="The Histogram dialog" />
  73.           </div>
  74.         </div>
  75.       </div>
  76.       <br class="figure-break" />
  77.       <p> 
  78.     The Histogram dialog shows you information about the statistical
  79.     distribution of color values in the active layer or selection. This
  80.     information is often useful when you are trying to
  81.     <span class="emphasis"><em>color balance</em></span>
  82.     an image. However, the Histogram dialog is purely informational: nothing
  83.     you do with it will cause any change to the image. If you want to perform
  84.     a histogram-based color correction, use the
  85.     <a class="link" href="gimp-tool-levels.html" title="5.6.¬† Levels">Levels</a> tool.
  86.   </p>
  87.       <div class="sect3" lang="en" xml:lang="en">
  88.         <div class="titlepage">
  89.           <div>
  90.             <div>
  91.               <h4 class="title"><a id="id2658993"></a>2.5.1.¬†
  92.       <span lang="en" xml:lang="en">Dialog call</span>
  93.     </h4>
  94.             </div>
  95.           </div>
  96.         </div>
  97.         <p>
  98.       This is a dockable dialog; see the section on
  99.       <a class="link" href="gimp-concepts-docks.html" title="2.3.¬† Dialogs and Docking">Dialogs and Docking</a>
  100.       for help on manipulating it. It can be activated in two ways:
  101.     </p>
  102.         <div class="itemizedlist">
  103.           <ul type="disc">
  104.             <li>
  105.               <p>
  106.           From the Toolbox menu:
  107.           <span class="guimenu">File</span> ‚Üí <span class="guisubmenu">Dialogs</span> ‚Üí <span class="guimenuitem">Histogram</span>.
  108.         </p>
  109.             </li>
  110.             <li>
  111.               <p>
  112.           From the image menu:
  113.           <span class="guimenu">Dialogs</span> ‚Üí <span class="guimenuitem">Histogram</span>.
  114.         </p>
  115.             </li>
  116.             <li>
  117.               <p>
  118.           From the image menu:
  119.           <span class="guimenu">Colors</span> ‚Üí <span class="guisubmenu">Info</span> ‚Üí <span class="guimenuitem">Histogram</span>.
  120.         </p>
  121.             </li>
  122.           </ul>
  123.         </div>
  124.       </div>
  125.       <div class="sect3" lang="en" xml:lang="en">
  126.         <div class="titlepage">
  127.           <div>
  128.             <div>
  129.               <h4 class="title"><a id="id2659115"></a>2.5.2.¬†
  130.       <span lang="en" xml:lang="en">About Histograms</span>
  131.     </h4>
  132.             </div>
  133.           </div>
  134.         </div>
  135.         <p>
  136.       In GIMP, each layer of an image can be decomposed into one or more color
  137.       channels: for an RGB image, into R, G, and B channels; for a grayscale
  138.       image, into a single Value channel. Layers that support transparency
  139.       have an additional channel, the alpha channel. Each channel supports a
  140.       range of intensity levels from 0 to 255 (integer valued). Thus, a black
  141.       pixel is encoded by 0 on all color channels; a white pixel by 255 on all
  142.       color channels. A transparent pixel is encoded by 0 on the alpha
  143.       channel; an opaque pixel by 255.
  144.     </p>
  145.         <p>
  146.       For RGB images, it is convenient to define a Value "pseudochannel". This
  147.       is not a real color channel: it does not reflect any information stored
  148.       directly in the image. Instead, the Value at a pixel is given by the
  149.       equation <code class="code">V = max(R,G,B)</code>.
  150.       Essentially, the Value is what you would get at that pixel if you
  151.       converted the image to Grayscale mode.
  152.     </p>
  153.         <p>
  154.       For more information on channels, please consult the
  155.       <a class="xref" href="gimp-images-in.html#gimp-concepts-image-types" title="1.¬† Image Types">Section¬†1, ‚Äú
  156.     <span lang="en" xml:lang="en">Image Types</span>
  157.   ‚Äù</a>.
  158.     </p>
  159.       </div>
  160.       <div class="sect3" lang="en" xml:lang="en">
  161.         <div class="titlepage">
  162.           <div>
  163.             <div>
  164.               <h4 class="title"><a id="id2659170"></a>2.5.3.¬†
  165.       <span lang="en" xml:lang="en">Using the Histogram dialog</span>
  166.     </h4>
  167.             </div>
  168.           </div>
  169.         </div>
  170.         <p> 
  171.       The active layer name is shown at the top of the dialog.
  172.     </p>
  173.         <div class="variablelist">
  174.           <dl>
  175.             <dt>
  176.               <span class="term">Channel</span>
  177.             </dt>
  178.             <dd>
  179.               <div class="figure">
  180.                 <a id="id2659199"></a>
  181.                 <p class="title">
  182.                   <b>Figure¬†14.18.¬†
  183.               <span lang="en" xml:lang="en">
  184.                 Channel options for an RGB layer with alpha channel.
  185.               </span>
  186.             </b>
  187.                 </p>
  188.                 <div class="figure-contents">
  189.                   <div class="mediaobject">
  190.                     <img src="../images/dialogs/dialogs-histogram-menu.png" alt="Channel options for an RGB layer with alpha channel." />
  191.                   </div>
  192.                 </div>
  193.               </div>
  194.               <br class="figure-break" />
  195.               <p>
  196.             This allows you to select which channel to use. The possibilities
  197.             depend on the layer type of the active layer. Here are the entries
  198.             you might see, and what they mean:
  199.           </p>
  200.               <div class="variablelist">
  201.                 <dl>
  202.                   <dt>
  203.                     <span class="term">Value</span>
  204.                   </dt>
  205.                   <dd>
  206.                     <p>
  207.                   For RGB and Grayscale images, this shows the distribution of
  208.                   brightness values across the layer. For a grayscale image,
  209.                   these are read directly from the image data. For an RGB
  210.                   image, they are taken from the Value pseudochannel.
  211.                 </p>
  212.                     <p>
  213.                   For an indexed image, the "Value" channel actually shows the
  214.                   distribution of frequencies for each colormap index: thus,
  215.                   it is a "pseudocolor" histogram rather than a true color
  216.                   histogram.
  217.                 </p>
  218.                   </dd>
  219.                   <dt>
  220.                     <span class="term">Red, Green, Blue</span>
  221.                   </dt>
  222.                   <dd>
  223.                     <p>
  224.                   These only appear for layers from RGB images. They show the
  225.                   distribution of intensity levels for the Red, Green, or Blue
  226.                   channels respectively.
  227.                 </p>
  228.                   </dd>
  229.                   <dt>
  230.                     <span class="term">Alpha</span>
  231.                   </dt>
  232.                   <dd>
  233.                     <p>
  234.                   This shows the distribution of opacity levels. If the layer
  235.                   is completely opaque or completely transparent, the
  236.                   histogram will consist of a single bar on the left or right
  237.                   edge.
  238.                 </p>
  239.                   </dd>
  240.                   <dt>
  241.                     <span class="term">RGB</span>
  242.                   </dt>
  243.                   <dd>
  244.                     <div class="figure">
  245.                       <a id="id2659325"></a>
  246.                       <p class="title">
  247.                         <b>Figure¬†14.19.¬†
  248.                     <span lang="en" xml:lang="en">
  249.                       Combined histograms of R, G, and B channels.
  250.                     </span>
  251.                   </b>
  252.                       </p>
  253.                       <div class="figure-contents">
  254.                         <div class="mediaobject">
  255.                           <img src="../images/dialogs/dialogs-histogram-rgb.png" alt="Combined histograms of R, G, and B channels." />
  256.                         </div>
  257.                       </div>
  258.                     </div>
  259.                     <br class="figure-break" />
  260.                     <p>
  261.                   This entry, only available for RGB layers, shows the R, G,
  262.                   and B histograms superimposed, so that you can see all of
  263.                   the color distribution information in a single view.
  264.                 </p>
  265.                   </dd>
  266.                 </dl>
  267.               </div>
  268.             </dd>
  269.             <dt>
  270.               <span class="term">Linear/Logarithmic buttons</span>
  271.             </dt>
  272.             <dd>
  273.               <div class="figure">
  274.                 <a id="id2659380"></a>
  275.                 <p class="title">
  276.                   <b>Figure¬†14.20.¬†
  277.               <span lang="en" xml:lang="en">
  278.                 The histogram shown at the top, changed to logarithmic mode.
  279.               </span>
  280.             </b>
  281.                 </p>
  282.                 <div class="figure-contents">
  283.                   <div class="mediaobject">
  284.                     <img src="../images/dialogs/dialogs-histogram-log.png" alt="The histogram shown at the top, changed to logarithmic mode." />
  285.                   </div>
  286.                 </div>
  287.               </div>
  288.               <br class="figure-break" />
  289.               <p>
  290.             These buttons determine whether the histogram will be displayed
  291.             using a linear or logarithmic Y axis. For images taken from
  292.             photographs, the linear mode is most commonly useful. For images
  293.             that contain substantial areas of constant color, though, a linear
  294.             histogram will often be dominated by a single bar, and a
  295.             logarithmic histogram will often be more useful.
  296.           </p>
  297.             </dd>
  298.             <dt>
  299.               <span class="term">Range Setting</span>
  300.             </dt>
  301.             <dd>
  302.               <div class="figure">
  303.                 <a id="id2659436"></a>
  304.                 <p class="title">
  305.                   <b>Figure¬†14.21.¬†
  306.               <span lang="en" xml:lang="en">Dialog aspect after range fixing.</span>
  307.             </b>
  308.                 </p>
  309.                 <div class="figure-contents">
  310.                   <div class="mediaobject">
  311.                     <img src="../images/dialogs/dialogs-histogram-range.png" alt="Dialog aspect after range fixing." />
  312.                   </div>
  313.                 </div>
  314.               </div>
  315.               <br class="figure-break" />
  316.               <p>
  317.             You can restrict the analysis, for the statistics shown at the
  318.             bottom of the dialog, to a limited range of values if you wish.
  319.             You can set the range in one of three ways:
  320.           </p>
  321.               <div class="itemizedlist">
  322.                 <ul type="disc">
  323.                   <li>
  324.                     <p>
  325.                 Click and drag the pointer across the histogram display area,
  326.                 from the lowest level to the highest level of the range you
  327.                 want.
  328.               </p>
  329.                   </li>
  330.                   <li>
  331.                     <p>
  332.                 Click and drag the black or white triangles on the slider
  333.                 below the histogram.
  334.               </p>
  335.                   </li>
  336.                   <li>
  337.                     <p>
  338.                 Use the spinbutton entries below the slider (left entry:
  339.                 bottom of range; right entry: top of range).
  340.               </p>
  341.                   </li>
  342.                 </ul>
  343.               </div>
  344.             </dd>
  345.             <dt>
  346.               <span class="term">Statistics</span>
  347.             </dt>
  348.             <dd>
  349.               <p>
  350.             At the bottom of the dialog some basic statistics are shown
  351.             describing the distribution of channel values, restricted to the
  352.             selected range:
  353.           </p>
  354.               <div class="itemizedlist">
  355.                 <ul type="disc">
  356.                   <li>
  357.                     <p>
  358.                  <span class="guilabel">Mean</span> : the mean value of the interval
  359.                  in the selected channel.
  360.                </p>
  361.                   </li>
  362.                   <li>
  363.                     <p>
  364.                  <span class="guilabel">Std Dev</span> : Standard deviation. Gives an
  365.                  idea about how homogeneous the distribution of values in the
  366.                  interval is.
  367.                </p>
  368.                   </li>
  369.                   <li>
  370.                     <p>
  371.                  <span class="guilabel">Median</span> : For example, the value of the
  372.                  fiftieth peak in a 100 peaks interval.
  373.                </p>
  374.                   </li>
  375.                   <li>
  376.                     <p>
  377.                  <span class="guilabel">Pixels</span> : The number of pixels in the
  378.                  active layer or selection.
  379.                </p>
  380.                   </li>
  381.                   <li>
  382.                     <p>
  383.                  <span class="guilabel">Count</span> : The number of pixels in a peak
  384.                  (when you click on the histogram) or in the interval.
  385.                </p>
  386.                   </li>
  387.                   <li>
  388.                     <p>
  389.                  <span class="guilabel">Percentile</span> : The ratio between the
  390.                  number of pixels in the interval and the total number of
  391.                  pixels in the active layer or selection.
  392.                </p>
  393.                   </li>
  394.                 </ul>
  395.               </div>
  396.             </dd>
  397.           </dl>
  398.         </div>
  399.       </div>
  400.     </div>
  401.     <div class="navfooter">
  402.       <hr />
  403.       <table width="100%" summary="Navigation footer">
  404.         <tr>
  405.           <td width="40%" align="left"><a accesskey="p" href="gimp-indexed-palette-dialog.html"><img src="../images/prev.png" alt="Prev" /></a>¬†</td>
  406.           <td width="20%" align="center">
  407.             <a accesskey="u" href="gimp-dialogs-structure.html">
  408.               <img src="../images/up.png" alt="Up" />
  409.             </a>
  410.           </td>
  411.           <td width="40%" align="right">¬†<a accesskey="n" href="gimp-navigation-dialog.html"><img src="../images/next.png" alt="Next" /></a></td>
  412.         </tr>
  413.         <tr>
  414.           <td width="40%" align="left" valign="top"><a accesskey="p" href="gimp-indexed-palette-dialog.html">2.4.¬†
  415.     <span lang="en" xml:lang="en">Colormap Dialog</span>
  416.   </a>¬†</td>
  417.           <td width="20%" align="center">
  418.             <a accesskey="h" href="index.html">
  419.               <img src="../images/home.png" alt="Home" />
  420.             </a>
  421.           </td>
  422.           <td width="40%" align="right" valign="top">¬†<a accesskey="n" href="gimp-navigation-dialog.html">2.6.¬†
  423.     <span lang="en" xml:lang="en">Navigation Dialog</span>
  424.   </a></td>
  425.         </tr>
  426.       </table>
  427.     </div>
  428.   </body>
  429. </html>
  430.