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  6.     <title>4.2.¬† Difference of Gaussians</title>
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  15.     <link rel="next" href="plug-in-edge.html" title="4.3.¬† Edge" />
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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">4.2.¬†
  22.     <span lang="en" xml:lang="en">Difference of Gaussians</span>
  23.   </th>
  24.         </tr>
  25.         <tr>
  26.           <td width="20%" align="left"><a accesskey="p" href="filters-edge.html"><img src="../images/prev.png" alt="Prev" /></a>¬†</td>
  27.           <th width="60%" align="center">4.¬†
  28.     <span lang="en" xml:lang="en">Edge-Detect Filters</span>
  29.   </th>
  30.           <td width="20%" align="right">¬†<a accesskey="n" href="plug-in-edge.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="plug-in-dog"></a>4.2.¬†
  40.     <span lang="en" xml:lang="en">Difference of Gaussians</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: 2440 $</td>
  53.                   <td align="left">2006-11-19</td>
  54.                   <td align="left">j.h</td>
  55.                 </tr>
  56.               </table>
  57.             </div>
  58.           </div>
  59.         </div>
  60.       </div>
  61.       <a id="id2735269" class="indexterm"></a>
  62.       <a id="id2735287" class="indexterm"></a>
  63.       <div class="sect3" lang="en" xml:lang="en">
  64.         <div class="titlepage">
  65.           <div>
  66.             <div>
  67.               <h4 class="title"><a id="id2735296"></a>4.2.1.¬†
  68.       <span lang="en" xml:lang="en">Overview</span>
  69.     </h4>
  70.             </div>
  71.           </div>
  72.         </div>
  73.         <div class="figure">
  74.           <a id="id2735306"></a>
  75.           <p class="title">
  76.             <b>Figure¬†16.31.¬†
  77.         <span lang="en" xml:lang="en">
  78.           Applying example for the ‚Äú<span class="quote">Difference of Gaussians</span>‚Äù
  79.           filter
  80.         </span>
  81.       </b>
  82.           </p>
  83.           <div class="figure-contents">
  84.             <div class="mediaobject">
  85.               <img src="../images/filters/examples/taj_orig.jpg" alt="Applying example for the Difference of Gaussians filter" />
  86.               <div class="caption">
  87.                 <p>Original image</p>
  88.               </div>
  89.             </div>
  90.             <div class="mediaobject">
  91.               <img src="../images/filters/examples/edge-taj-dog.jpg" alt="Applying example for the Difference of Gaussians filter" />
  92.               <div class="caption">
  93.                 <p>Filter ‚Äú<span class="quote">Difference of Gaussians</span>‚Äù
  94.           applied</p>
  95.               </div>
  96.             </div>
  97.           </div>
  98.         </div>
  99.         <br class="figure-break" />
  100.         <p>
  101.       You can find this filter through
  102.       <span class="guimenu">Filters</span> ‚Üí <span class="guisubmenu">Edge detect</span> ‚Üí <span class="guimenuitem">Difference of Gaussians</span>
  103.     </p>
  104.         <p>
  105.       This filter does edge detection using the
  106.       so-called ‚Äú<span class="quote">Difference of Gaussians</span>‚Äù
  107.       algorithm, which works by performing two different Gaussian blurs on the
  108.       image, with a different blurring radius for each, and subtracting them
  109.       to yield the result. This algorithm is very widely used in artificial
  110.       vision (maybe in biological vision as well!), and is pretty fast because
  111.       there are very efficient methods for doing Gaussian blurs. The most
  112.       important parameters are the blurring radii for the two Gaussian blurs.
  113.       It is probably easiest to set them using the preview, but it may help to
  114.       know that increasing the smaller radius tends to give thicker-appearing
  115.       edges, and decreasing the larger radius tends to increase the
  116.       ‚Äú<span class="quote">threshold</span>‚Äù
  117.       for recognizing something as an edge. In most cases you will get nicer
  118.       results if Radius 2 is smaller than Radius 1, but nothing prevents you
  119.       from reversing them, and in situations where you have a light figure on
  120.       the dark background, reversing them may actually improve the result.
  121.     </p>
  122.       </div>
  123.       <div class="sect3" lang="en" xml:lang="en">
  124.         <div class="titlepage">
  125.           <div>
  126.             <div>
  127.               <h4 class="title"><a id="id2735440"></a>4.2.2.¬†
  128.       <span lang="en" xml:lang="en">Options</span>
  129.     </h4>
  130.             </div>
  131.           </div>
  132.         </div>
  133.         <div class="figure">
  134.           <a id="id2735451"></a>
  135.           <p class="title">
  136.             <b>Figure¬†16.32.¬†
  137.         <span lang="en" xml:lang="en">Gaussian Difference filter options</span>
  138.       </b>
  139.           </p>
  140.           <div class="figure-contents">
  141.             <div class="mediaobject">
  142.               <img src="../images/filters/diff-gauss-options.png" alt="Gaussian Difference filter options" />
  143.             </div>
  144.           </div>
  145.         </div>
  146.         <br class="figure-break" />
  147.         <div class="variablelist">
  148.           <dl>
  149.             <dt>
  150.               <span class="term">Smoothing parameters</span>
  151.             </dt>
  152.             <dd>
  153.               <p>
  154.             Radius 1 and Radius 2 are the blurring radii for the two Gaussian
  155.             blurs. The only constraints on them is that they cannot be equal,
  156.             or else the result will be a blank image. If you want to produce
  157.             something that looks like a sketch, in most cases setting Radius 2
  158.             smaller than Radius 1 will give better results.
  159.           </p>
  160.             </dd>
  161.             <dt>
  162.               <span class="term">Normalize</span>
  163.             </dt>
  164.             <dd>
  165.               <p>
  166.             Checking this box causes the brightness range in the result to be
  167.             stretched as much as possible, increasing contrast. Note that in
  168.             the preview, only the part of the image that is shown is taken
  169.             into account, so with <span class="guilabel">Normalize</span>
  170.             checked the preview is not completely accurate. (It is accurate
  171.             except in terms of global contrast, though.)
  172.           </p>
  173.             </dd>
  174.             <dt>
  175.               <span class="term">Invert</span>
  176.             </dt>
  177.             <dd>
  178.               <p>
  179.             Checking this box inverts the result, so that you see dark edges
  180.             on a white background, giving something that looks more like a
  181.             drawing.
  182.           </p>
  183.             </dd>
  184.           </dl>
  185.         </div>
  186.       </div>
  187.     </div>
  188.     <div class="navfooter">
  189.       <hr />
  190.       <table width="100%" summary="Navigation footer">
  191.         <tr>
  192.           <td width="40%" align="left"><a accesskey="p" href="filters-edge.html"><img src="../images/prev.png" alt="Prev" /></a>¬†</td>
  193.           <td width="20%" align="center">
  194.             <a accesskey="u" href="filters-edge.html">
  195.               <img src="../images/up.png" alt="Up" />
  196.             </a>
  197.           </td>
  198.           <td width="40%" align="right">¬†<a accesskey="n" href="plug-in-edge.html"><img src="../images/next.png" alt="Next" /></a></td>
  199.         </tr>
  200.         <tr>
  201.           <td width="40%" align="left" valign="top"><a accesskey="p" href="filters-edge.html">4.¬†
  202.     <span lang="en" xml:lang="en">Edge-Detect Filters</span>
  203.   </a>¬†</td>
  204.           <td width="20%" align="center">
  205.             <a accesskey="h" href="index.html">
  206.               <img src="../images/home.png" alt="Home" />
  207.             </a>
  208.           </td>
  209.           <td width="40%" align="right" valign="top">¬†<a accesskey="n" href="plug-in-edge.html">4.3.¬†
  210.     <span lang="en" xml:lang="en">Edge</span>
  211.   </a></td>
  212.         </tr>
  213.       </table>
  214.     </div>
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  217.