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  6.     <title>5.4.¬† NL Filter</title>
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  15.     <link rel="next" href="plug-in-red-eye-removal.html" title="5.5.¬† Red Eye Removal" />
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  18.     <div class="navheader">
  19.       <table width="100%" summary="Navigation header">
  20.         <tr>
  21.           <th colspan="3" align="center">5.4.¬†
  22.     <span lang="en" xml:lang="en">NL Filter</span>
  23.   </th>
  24.         </tr>
  25.         <tr>
  26.           <td width="20%" align="left"><a accesskey="p" href="plug-in-despeckle.html"><img src="../images/prev.png" alt="Prev" /></a>¬†</td>
  27.           <th width="60%" align="center">5.¬†
  28.     <span lang="en" xml:lang="en">Enhance Filters</span>
  29.   </th>
  30.           <td width="20%" align="right">¬†<a accesskey="n" href="plug-in-red-eye-removal.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-nlfilt"></a>5.4.¬†
  40.     <span lang="en" xml:lang="en">NL Filter</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: 2442 $</td>
  53.                   <td align="left">2006-11-26</td>
  54.                   <td align="left">j.h</td>
  55.                 </tr>
  56.               </table>
  57.             </div>
  58.           </div>
  59.         </div>
  60.       </div>
  61.       <a id="id2737802" class="indexterm"></a>
  62.       <a id="id2737820" 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="id2737829"></a>5.4.1.¬†
  68.       <span lang="en" xml:lang="en">Overview</span>
  69.     </h4>
  70.             </div>
  71.           </div>
  72.         </div>
  73.         <p>
  74.       You can find this filter through
  75.       <span class="guimenu">Filters</span> ‚Üí <span class="guisubmenu">Enhance</span> ‚Üí <span class="guimenuitem"><span class="accel">N</span>L Filter</span>.
  76.       NL means "Non Linear". Derived
  77.       from the Unix <span class="command"><strong>pnmnlfilt</strong></span> program, it joins
  78.       smoothing, despeckle and sharpen enhancement functions. It works
  79.       on the whole image, not on the selection.
  80.     </p>
  81.         <p>
  82.       This is something of a swiss army knife filter. It has 3 distinct
  83.       operating modes. In all of the modes each pixel in the image is examined
  84.       and processed according to it and its surrounding pixels values. Rather
  85.       than using 9 pixels in a 3x3 block, it uses an hexagonal block whose
  86.       size can be set with the Radius option.
  87.     </p>
  88.       </div>
  89.       <div class="sect3" lang="en" xml:lang="en">
  90.         <div class="titlepage">
  91.           <div>
  92.             <div>
  93.               <h4 class="title"><a id="id2737892"></a>5.4.2.¬†
  94.       <span lang="en" xml:lang="en">Options</span>
  95.     </h4>
  96.             </div>
  97.           </div>
  98.         </div>
  99.         <div class="figure">
  100.           <a id="id2737903"></a>
  101.           <p class="title">
  102.             <b>Figure¬†16.46.¬†
  103.         <span lang="en" xml:lang="en">
  104.           ‚Äú<span class="quote">NL Filter</span>‚Äù options
  105.         </span>
  106.       </b>
  107.           </p>
  108.           <div class="figure-contents">
  109.             <div class="mediaobject">
  110.               <img src="../images/filters/filters-enhance-nlfilter.png" alt="NL Filter options" />
  111.             </div>
  112.           </div>
  113.         </div>
  114.         <br class="figure-break" />
  115.         <div class="variablelist">
  116.           <dl>
  117.             <dt>
  118.               <span class="term">Preview</span>
  119.             </dt>
  120.             <dd>
  121.               <p>
  122.             When <span class="guilabel">Do preview</span> is checked, parameter
  123.             setting results are interactively displayed in preview.
  124.           </p>
  125.             </dd>
  126.             <dt>
  127.               <span class="term">Parameter settings</span>
  128.             </dt>
  129.             <dd>
  130.               <p>
  131.             </p>
  132.               <div class="itemizedlist">
  133.                 <ul type="disc">
  134.                   <li>
  135.                     <p>
  136.                   <span class="guilabel">Alpha</span>: Meaning of this value
  137.                   depends on the selected option.
  138.                 </p>
  139.                   </li>
  140.                   <li>
  141.                     <p>
  142.                   <span class="guilabel">Radius</span>:
  143.                   Controls the strength of the filter (0.33-1.00).
  144.                 </p>
  145.                   </li>
  146.                 </ul>
  147.               </div>
  148.               <p>
  149.           </p>
  150.             </dd>
  151.           </dl>
  152.         </div>
  153.       </div>
  154.       <div class="sect3" lang="en" xml:lang="en">
  155.         <div class="titlepage">
  156.           <div>
  157.             <div>
  158.               <h4 class="title"><a id="id2738015"></a>5.4.3.¬†
  159.       <span lang="en" xml:lang="en">Operating Modes</span>
  160.     </h4>
  161.             </div>
  162.           </div>
  163.         </div>
  164.         <p>
  165.       This filter can perform several distinct functions, depending on
  166.       the value of the parameter <span class="guilabel">alpha</span>.
  167.     </p>
  168.         <div class="variablelist">
  169.           <dl>
  170.             <dt>
  171.               <span class="term">
  172.           Alpha trimmed mean filter. (0.0 <=
  173.           <em class="parameter"><code>alpha</code></em> <= 0.5)
  174.         </span>
  175.             </dt>
  176.             <dd>
  177.               <p>
  178.             The value of the center pixel will be replaced by the mean of
  179.             the 7 hexagon values, but the 7 values are sorted by size and
  180.             the top and bottom <em class="parameter"><code>alpha</code></em> portion of the 7
  181.             are excluded from the mean. This implies that an
  182.             <em class="parameter"><code>alpha</code></em> value of 0.0 gives the same sort of
  183.             output as a normal convolution (ie. averaging or smoothing
  184.             filter), where <em class="parameter"><code>radius</code></em> will determine the
  185.             "strength" of the filter. A good value to start from for subtle
  186.             filtering is <em class="parameter"><code>alpha</code></em> = 0.0,
  187.             <em class="parameter"><code>radius</code></em> = 0.55.  For a more blatant
  188.             effect, try <em class="parameter"><code>alpha</code></em> 0.0 and
  189.             <em class="parameter"><code>radius</code></em> 1.0.
  190.           </p>
  191.               <p>
  192.             An <em class="parameter"><code>alpha</code></em> value of 0.5 will cause the
  193.             median value of the 7 hexagons to be used to replace the center
  194.             pixel value. This sort of filter is good for eliminating "pop"
  195.             or single pixel noise from an image without spreading the noise
  196.             out or smudging features on the image. Judicious use of the
  197.             <em class="parameter"><code>radius</code></em> parameter will fine tune the
  198.             filtering. Intermediate values of <em class="parameter"><code>alpha</code></em>
  199.             give effects somewhere between smoothing and "pop" noise
  200.             reduction. For subtle filtering try starting with values of
  201.             <em class="parameter"><code>alpha</code></em> = 0.4,
  202.             <em class="parameter"><code>radius</code></em> = 0.6.  For a more blatant effect
  203.             try <em class="parameter"><code>alpha</code></em> = 0.5,
  204.             <em class="parameter"><code>radius</code></em> = 1.0 .
  205.           </p>
  206.             </dd>
  207.             <dt>
  208.               <span class="term">
  209.           Optimal estimation smoothing. (1.0 <=
  210.           <em class="parameter"><code>alpha</code></em> <= 2.0)
  211.         </span>
  212.             </dt>
  213.             <dd>
  214.               <p>
  215.             This type of filter applies a smoothing filter adaptively over
  216.             the image. For each pixel the variance of the surrounding
  217.             hexagon values is calculated, and the amount of smoothing is
  218.             made inversely proportional to it. The idea is that if the
  219.             variance is small then it is due to noise in the image, while if
  220.             the variance is large, it is because of "wanted" image features.
  221.             As usual the  <em class="parameter"><code>radius</code></em> parameter  controls
  222.             the effective radius, but it probably advisable  to  leave  the
  223.             radius between 0.8 and 1.0 for the variance calculation to be
  224.             meaningful. The <em class="parameter"><code>alpha</code></em> parameter sets the
  225.             noise threshold, over which less smoothing will be done. This
  226.             means that small values of <em class="parameter"><code>alpha</code></em> will
  227.             give the most subtle filtering effect, while large values will
  228.             tend to smooth all parts of the image. You could start with
  229.             values like
  230.             <code class="code"><em class="parameter"><code>alpha</code></em>  =  1.2</code>,
  231.             <code class="code"><em class="parameter"><code>radius</code></em> = 1.0</code>,
  232.             and try increasing or decreasing the
  233.             <em class="parameter"><code>alpha</code></em> parameter  to  get the desired
  234.             effect. This type of filter is best for filtering out dithering
  235.             noise in both bitmap and color images.
  236.           </p>
  237.             </dd>
  238.             <dt>
  239.               <span class="term">
  240.           Edge enhancement. (-0.1 >= <em class="parameter"><code>alpha</code></em> >= -0.9)
  241.         </span>
  242.             </dt>
  243.             <dd>
  244.               <p>
  245.           This is the opposite type of filter to the smoothing filter. It
  246.           enhances edges. The <em class="parameter"><code>alpha</code></em> parameter
  247.           controls the amount of edge enhancement, from subtle (-0.1) to
  248.           blatant (-0.9). The <em class="parameter"><code>radius</code></em> parameter
  249.           controls the effective radius as usual, but useful values are
  250.           between 0.5 and 0.9. Try starting with values of
  251.           <code class="code">
  252.             <em class="parameter"><code>alpha</code></em> = 0.3
  253.           </code>,
  254.           <code class="code">
  255.             <em class="parameter"><code>radius</code></em> = 0.8
  256.           </code>.
  257.         </p>
  258.             </dd>
  259.             <dt>
  260.               <span class="term">Combination use</span>
  261.             </dt>
  262.             <dd>
  263.               <p>
  264.             The various operating modes can be used one after the other to
  265.             get the desired result. For instance to turn a monochrome
  266.             dithered image into grayscale image you could try one or two
  267.             passes of the smoothing filter, followed by a pass of the
  268.             optimal estimation filter, then some subtle edge enhancement.
  269.             Note that using edge enhancement is only likely to be useful
  270.             after  one  of  the  non-linear filters (alpha trimmed mean or
  271.             optimal estimation filter), as edge enhancement is the direct
  272.             opposite of smoothing.
  273.           </p>
  274.               <p>
  275.             For reducing color quantization noise in images (ie. turning
  276.             .gif files back into 24 bit files) you could try a pass of the
  277.             optimal estimation filter (<em class="parameter"><code>alpha</code></em> 1.2,
  278.             <em class="parameter"><code>radius</code></em> 1.0), a pass of the median filter
  279.             (<em class="parameter"><code>alpha</code></em> 0.5, <em class="parameter"><code>radius</code></em>
  280.             0.55), and possibly a pass of the edge enhancement filter.
  281.             Several passes of the optimal estimation filter with declining
  282.             <em class="parameter"><code>alpha</code></em> values are more effective than a
  283.             single pass with a large <em class="parameter"><code>alpha</code></em> value. As
  284.             usual, there is a trade-off between filtering effectiveness and
  285.             losing detail. Experimentation is encouraged.
  286.           </p>
  287.             </dd>
  288.           </dl>
  289.         </div>
  290.       </div>
  291.     </div>
  292.     <div class="navfooter">
  293.       <hr />
  294.       <table width="100%" summary="Navigation footer">
  295.         <tr>
  296.           <td width="40%" align="left"><a accesskey="p" href="plug-in-despeckle.html"><img src="../images/prev.png" alt="Prev" /></a>¬†</td>
  297.           <td width="20%" align="center">
  298.             <a accesskey="u" href="filters-enhance.html">
  299.               <img src="../images/up.png" alt="Up" />
  300.             </a>
  301.           </td>
  302.           <td width="40%" align="right">¬†<a accesskey="n" href="plug-in-red-eye-removal.html"><img src="../images/next.png" alt="Next" /></a></td>
  303.         </tr>
  304.         <tr>
  305.           <td width="40%" align="left" valign="top"><a accesskey="p" href="plug-in-despeckle.html">5.3.¬†
  306.     <span lang="en" xml:lang="en">Despeckle</span>
  307.   </a>¬†</td>
  308.           <td width="20%" align="center">
  309.             <a accesskey="h" href="index.html">
  310.               <img src="../images/home.png" alt="Home" />
  311.             </a>
  312.           </td>
  313.           <td width="40%" align="right" valign="top">¬†<a accesskey="n" href="plug-in-red-eye-removal.html">5.5.¬†
  314.     <span lang="en" xml:lang="en">Red Eye Removal</span>
  315.   </a></td>
  316.         </tr>
  317.       </table>
  318.     </div>
  319.   </body>
  320. </html>
  321.