home *** CD-ROM | disk | FTP | other *** search
/ Mac Easy 2010 May / Mac Life Ubuntu.iso / casper / filesystem.squashfs / usr / share / gimp / 2.0 / help / en / plug-in-unsharp-mask.html < prev    next >
Encoding:
Extensible Markup Language  |  2008-05-03  |  14.6 KB  |  378 lines
  1. <?xml version="1.0" encoding="UTF-8" standalone="no"?>
  2. <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
  3. <html xmlns="http://www.w3.org/1999/xhtml">
  4.   <head>
  5.     <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
  6.     <title>5.7.¬† Unsharp Mask</title>
  7.     <link rel="stylesheet" href="gimp-help-plain.css" type="text/css" />
  8.     <link rel="stylesheet" href="gimp-help-screen.css" type="text/css" />
  9.     <link rel="stylesheet" href="gimp-help-custom.css" type="text/css" />
  10.     <link rel="alternate stylesheet" href="gimp22.css" type="text/css" title="gimp22" />
  11.     <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
  12.     <link rel="start" href="index.html" title="GNU Image Manipulation Program" />
  13.     <link rel="up" href="filters-enhance.html" title="5.¬† Enhance Filters" />
  14.     <link rel="prev" href="plug-in-destripe.html" title="5.6.¬† Destripe" />
  15.     <link rel="next" href="plug-in-sharpen.html" title="5.8.¬† Sharpen" />
  16.   </head>
  17.   <body>
  18.     <div class="navheader">
  19.       <table width="100%" summary="Navigation header">
  20.         <tr>
  21.           <th colspan="3" align="center">5.7.¬†
  22.     <span lang="en" xml:lang="en">Unsharp Mask</span>
  23.   </th>
  24.         </tr>
  25.         <tr>
  26.           <td width="20%" align="left"><a accesskey="p" href="plug-in-destripe.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-sharpen.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-unsharp-mask"></a>5.7.¬†
  40.     <span lang="en" xml:lang="en">Unsharp Mask</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: 2395 $</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="id2739036" class="indexterm"></a>
  62.       <a id="id2739054" 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="id2739063"></a>5.7.1.¬†
  68.       <span lang="en" xml:lang="en">Overview</span>
  69.     </h4>
  70.             </div>
  71.           </div>
  72.         </div>
  73.         <div class="figure">
  74.           <a id="id2739074"></a>
  75.           <p class="title">
  76.             <b>Figure¬†16.50.¬†
  77.         <span lang="en" xml:lang="en">
  78.           Applying example for the Unsharp Mask filter
  79.         </span>
  80.       </b>
  81.           </p>
  82.           <div class="figure-contents">
  83.             <div class="mediaobject">
  84.               <img src="../images/filters/examples/taj_orig.jpg" alt="Applying example for the Unsharp Mask filter" />
  85.             </div>
  86.             <div class="mediaobject">
  87.               <img src="../images/filters/examples/enhance-taj-unsharp-mask.jpg" alt="Applying example for the Unsharp Mask filter" />
  88.             </div>
  89.           </div>
  90.         </div>
  91.         <br class="figure-break" />
  92.         <p>
  93.       You can find this filter through
  94.       <span class="guimenu">Filters</span> ‚Üí <span class="guisubmenu">Enhance</span> ‚Üí <span class="guimenuitem">Unsharp Mask</span>.
  95.     </p>
  96.         <p>
  97.       Out-of-focus photographs and most digitized images often need a
  98.       sharpness correction. This is due to the digitizing process that must
  99.       chop a color continuum up in points with slightly different colors:
  100.       elements thinner than sampling frequency will be averaged into an
  101.       uniform color. So sharp borders are rendered a little blurred. The same
  102.       phenomenon appears when printing color dots on paper.
  103.     </p>
  104.         <p>
  105.       The Unsharp Mask filter (what an odd name!) sharpens edges of the
  106.       elements without increasing noise or blemish. It is the king of the
  107.       sharpen filters.
  108.     </p>
  109.         <p>
  110.       Some scanners apply a sharpen filter while scanning. It's worth
  111.       disabling it so that you keep control on your image.
  112.     </p>
  113.         <div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;">
  114.           <table border="0" summary="Tip">
  115.             <tr>
  116.               <td rowspan="2" align="center" valign="top" width="25">
  117.                 <img alt="[Tip]" src="../images/tip.png" />
  118.               </td>
  119.               <th align="left">Tip</th>
  120.             </tr>
  121.             <tr>
  122.               <td align="left" valign="top">
  123.                 <p>
  124.         Some imaging devices like digital cameras or scanners offer to sharpen
  125.         the created images for you. We strongly recommend you disable the
  126.         sharpening in this devices and use the GIMP filters instead. This way
  127.         you regain the full control over the sharpening of your images.
  128.       </p>
  129.               </td>
  130.             </tr>
  131.           </table>
  132.         </div>
  133.         <div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;">
  134.           <table border="0" summary="Tip">
  135.             <tr>
  136.               <td rowspan="2" align="center" valign="top" width="25">
  137.                 <img alt="[Tip]" src="../images/tip.png" />
  138.               </td>
  139.               <th align="left">Tip</th>
  140.             </tr>
  141.             <tr>
  142.               <td align="left" valign="top">
  143.                 <p>
  144.         To prevent color distortion while sharpening, Decompose your image to
  145.         HSV and work only on Value. Then Compose the image to HSV. Go to
  146.         Image/Mode and click on Decompose. Make sure the
  147.         <span class="guilabel">Decompose to Layers</span>
  148.         box is checked. Choose HSV and click OK. You will get a new grey-level
  149.         image with three layers, one for Hue, one for Saturation, and one for
  150.         Value. (Close the original image so you won't get confused). Select
  151.         the Value layer and apply your sharpening to it. When you are done,
  152.         with that same layer selected, reverse the process. Go to Image/Mode
  153.         and click on Compose. Again choose HSV and click OK. You will get back
  154.         your original image except that it will have been sharpened in the
  155.         Value component.
  156.       </p>
  157.               </td>
  158.             </tr>
  159.           </table>
  160.         </div>
  161.       </div>
  162.       <div class="sect3" lang="en" xml:lang="en">
  163.         <div class="titlepage">
  164.           <div>
  165.             <div>
  166.               <h4 class="title"><a id="id2739219"></a>5.7.2.¬†
  167.       <span lang="en" xml:lang="en">Options</span>
  168.     </h4>
  169.             </div>
  170.           </div>
  171.         </div>
  172.         <div class="figure">
  173.           <a id="id2739230"></a>
  174.           <p class="title">
  175.             <b>Figure¬†16.51.¬†
  176.         <span lang="en" xml:lang="en">
  177.           ‚Äú<span class="quote">Unsharp Mask</span>‚Äù filter options
  178.         </span>
  179.       </b>
  180.           </p>
  181.           <div class="figure-contents">
  182.             <div class="mediaobject">
  183.               <img src="../images/filters/unsharp_mask-options.png" alt="Unsharp Mask filter options" />
  184.             </div>
  185.           </div>
  186.         </div>
  187.         <br class="figure-break" />
  188.         <div class="variablelist">
  189.           <dl>
  190.             <dt>
  191.               <span class="term">Preview</span>
  192.             </dt>
  193.             <dd>
  194.               <p>
  195.             Parameter setting results are interactively displayed in preview.
  196.             Scroll bars allow you to move around the image.
  197.           </p>
  198.             </dd>
  199.             <dt>
  200.               <span class="term">Radius</span>
  201.             </dt>
  202.             <dd>
  203.               <p>
  204.             <span class="guilabel">Radius</span>
  205.             : slider and input boxes (0.1-120) allow you to set how many
  206.             pixels on either side of an edge will be affected by sharpening.
  207.             High resolution images allow higher radius. It is better to always
  208.             sharpen an image at its final resolution.
  209.           </p>
  210.             </dd>
  211.             <dt>
  212.               <span class="term">Amount</span>
  213.             </dt>
  214.             <dd>
  215.               <p>
  216.             <span class="guilabel">Amount</span>: slider and input boxes
  217.             (0.00-5.00) allow you to set strength of sharpening.
  218.           </p>
  219.             </dd>
  220.             <dt>
  221.               <span class="term">Threshold</span>
  222.             </dt>
  223.             <dd>
  224.               <p>
  225.             <span class="guilabel">Threshold</span>
  226.             : slider and input boxes (0-255) allow you to set the minimum
  227.             difference in pixel values that indicates an edge where sharpen
  228.             must be applied. So you can protect areas of smooth tonal
  229.             transition from sharpening, and avoid creation of blemishes in
  230.             face, sky or water surface.
  231.           </p>
  232.             </dd>
  233.           </dl>
  234.         </div>
  235.       </div>
  236.       <div class="sect3" lang="en" xml:lang="en">
  237.         <div class="titlepage">
  238.           <div>
  239.             <div>
  240.               <h4 class="title"><a id="id2739368"></a>5.7.3.¬†
  241.       <span lang="en" xml:lang="en">How does an unsharp mask work?</span>
  242.     </h4>
  243.             </div>
  244.           </div>
  245.         </div>
  246.         <p>
  247.       Using an unsharp mask to sharpen an image can seem rather weird. Here is
  248.       the explanation:
  249.     </p>
  250.         <p>
  251.       Think of an image with a contrast in some place. The intensity curve of
  252.       the pixels on a line going through this contrast will show an abrupt
  253.       increase of intensity: like a stair if contrast is perfectly sharp, like
  254.       an S if there is some blur.
  255.     </p>
  256.         <div class="mediaobject">
  257.           <img src="../images/filters/examples/unsharp-mask-1.png" />
  258.         </div>
  259.         <p>
  260.       Now, we have an image with some blur we want to sharpen (black curve).
  261.       We apply some more blur: the intensity variation will be more gradual
  262.       (green curve).
  263.     </p>
  264.         <p>
  265.       Let us substract the blurredness intensity from the intensity of the
  266.       image. We get the red curve, which is more abrupt : contrast and
  267.       sharpness are increased. QED.
  268.     </p>
  269.         <div class="mediaobject">
  270.           <img src="../images/filters/examples/unsharp-mask-2.png" />
  271.         </div>
  272.         <p>
  273.       Unsharp mask has first been used in silver photography. The photograph
  274.       first creates a copy of the original negative by contact, on a film,
  275.       placing a thin glass plate between both; that will produce a blurred
  276.       copy because of light diffusion. Then he places both films, exactly
  277.       corresponding, in a photo enlarger, to reproduce them on paper. The dark
  278.       areas of the positive blurred film, opposed to the clear areas of the
  279.       original negative will prevent light to go through and so will be
  280.       subtracted from the light going through the original film.
  281.     </p>
  282.         <p>
  283.       In digital photography, with GIMP, you will go through the following
  284.       steps:
  285.     </p>
  286.         <div class="procedure">
  287.           <ol type="1">
  288.             <li>
  289.               <p>
  290.           Open your image and duplicate it
  291.           <span class="guimenu">Image</span> ‚Üí <span class="guimenuitem">Duplicate</span>
  292.         </p>
  293.             </li>
  294.             <li>
  295.               <p>
  296.           In the copy, duplicate the layer
  297.           <span class="guimenu">Layer</span> ‚Üí <span class="guimenuitem">Duplicate layer</span>, then drop the Filters menu down and apply
  298.           <span class="guimenu">Blur</span> ‚Üí <span class="guimenuitem">Gaussian Blur</span>
  299.           to the duplicated layer with the default IIR option and radius 5.
  300.         </p>
  301.             </li>
  302.             <li>
  303.               <p>
  304.           In the layer dialog of the duplicated image, change Mode to
  305.           ‚Äú<span class="quote">Subtract</span>‚Äù, and in the right-clic menu, select
  306.           ‚Äú<span class="quote">Merge down</span>‚Äù.
  307.         </p>
  308.             </li>
  309.             <li>
  310.               <p>
  311.           Click and drag the only layer you got into the original image, where
  312.           it appears as a new layer.
  313.         </p>
  314.             </li>
  315.             <li>
  316.               <p>
  317.           Change the Mode in this layer dialog to ‚Äú<span class="quote">Addition</span>‚Äù.
  318.         </p>
  319.             </li>
  320.           </ol>
  321.         </div>
  322.         <p>
  323.       Voil√†. The<a class="link" href="plug-in-unsharp-mask.html" title="5.7.¬† Unsharp Mask">Unsharp Mask</a>
  324.       plug-in does the same for you.
  325.     </p>
  326.         <p>
  327.       At the beginning of the curve, you can see a dip. If blurring is
  328.       important, this dip is very deep; the result of the subtraction can be
  329.       negative, and a complementary color stripe will appear along the
  330.       contrast, or a black halo around a star on the light background of a
  331.       nebula (black eye effect).
  332.     </p>
  333.         <div class="figure">
  334.           <a id="id2739615"></a>
  335.           <p class="title">
  336.             <b>Figure¬†16.52.¬†
  337.         <span lang="en" xml:lang="en">Black eye effect</span>
  338.       </b>
  339.           </p>
  340.           <div class="figure-contents">
  341.             <div class="mediaobject">
  342.               <img src="../images/filters/examples/unsharp-mask-3.png" alt="Black eye effect" />
  343.             </div>
  344.           </div>
  345.         </div>
  346.         <br class="figure-break" />
  347.       </div>
  348.     </div>
  349.     <div class="navfooter">
  350.       <hr />
  351.       <table width="100%" summary="Navigation footer">
  352.         <tr>
  353.           <td width="40%" align="left"><a accesskey="p" href="plug-in-destripe.html"><img src="../images/prev.png" alt="Prev" /></a>¬†</td>
  354.           <td width="20%" align="center">
  355.             <a accesskey="u" href="filters-enhance.html">
  356.               <img src="../images/up.png" alt="Up" />
  357.             </a>
  358.           </td>
  359.           <td width="40%" align="right">¬†<a accesskey="n" href="plug-in-sharpen.html"><img src="../images/next.png" alt="Next" /></a></td>
  360.         </tr>
  361.         <tr>
  362.           <td width="40%" align="left" valign="top"><a accesskey="p" href="plug-in-destripe.html">5.6.¬†
  363.     <span lang="en" xml:lang="en">Destripe</span>
  364.   </a>¬†</td>
  365.           <td width="20%" align="center">
  366.             <a accesskey="h" href="index.html">
  367.               <img src="../images/home.png" alt="Home" />
  368.             </a>
  369.           </td>
  370.           <td width="40%" align="right" valign="top">¬†<a accesskey="n" href="plug-in-sharpen.html">5.8.¬†
  371.     <span lang="en" xml:lang="en">Sharpen</span>
  372.   </a></td>
  373.         </tr>
  374.       </table>
  375.     </div>
  376.   </body>
  377. </html>
  378.