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    <title>ImageMagick: Command-line Tools: Compare</title>
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<p class="text-center"><a href="compare.html#usage">Example Usage</a> • <a href="compare.html#options">Option Summary</a></p>

<p class="lead magick-description">Use the <code>compare</code> program to mathematically and visually annotate the difference between an image and its reconstruction.  See <a href="command-line-processing.html">Command Line Processing</a> for advice on how to structure your <code>compare</code> command or see below for example usages of the command.</p>

<h2 class="magick-header"><a id="usage"></a>Example Usage</h2>

<p>We list a few examples of the <code>compare</code> command here to illustrate its usefulness and ease of use. To get started, lets compare an image to one thats been sharpened:</p>

<pre>
convert rose.jpg -sharpen 0x1 reconstruct.jpg
compare rose.jpg reconstruct.jpg difference.png
compare -compose src rose.jpg reconstruct.jpg difference.png
</pre>

<ul>
  <a href="../images/rose.jpg"><img src="../images/rose.jpg" width="70" height="46" alt="rose" /></a>
  <a href="../images/reconstruct.jpg"><img src="../images/reconstruct.jpg" width="70" height="46" alt="rose" /></a>
  <img style="margin:13px 0;" src="../images/right.gif" width="20" height="20" alt="==>" />
  <a href="../images/difference.png"><img src="../images/difference.png" width="70" height="46" alt="rose" /></a>
</ul>

<p>The red areas of the difference image emphasizes (highlight) pixels that are affected by the image sharpening, whereas white de-emphasizes (lowlight) pixels that are untouched by the sharpening process.</p>

<p>In addition to the visual interpretation of the difference in an image and its reconstruction, we report a mathematical measure of the difference:</p>

<pre>
-> compare -verbose -metric mae rose.jpg reconstruct.jpg difference.png
Image: rose.jpg
 Channel distortion: MAE
  red: 2282.91 (0.034835)
  green: 1853.99 (0.0282901)
  blue: 2008.67 (0.0306503)
  all: 1536.39 (0.0234439)
</pre>
<p>Or, if you just want the red channel distortion, use this command:</p>

<pre>
-> compare -channel red -metric PSNR rose.jpg reconstruct.jpg difference.png
19.63
</pre>

<p>Or, if you just want the overall image distortion, use this command:</p>

<pre>
-> compare -metric PSNR rose.jpg reconstruct.jpg difference.png
28.31
</pre>

<p>If the reconstructed image is a subimage of the image, the compare program returns the best match offset.  In addition, it returns a similarity image such that an exact match location is completely white and if none of the pixels match, black, otherwise some gray level in-between:</p>

<pre>
-> compare -metric RMSE -subimage-search logo.png wizard.jpg similarity.gif
85.05 (0.00129778) @ 353,157
</pre>

<p>You can find additional examples of using <code>compare</code> in <a href="http://www.ibm.com/developerworks/library/l-graf/?ca=dnt-428">Graphics from the Command Line</a>.  Further discussion is available in <a href="http://www.ibm.com/developerworks/library/l-graf2/?ca=dgr-lnxw15GraphicsLine">More Graphics from the Command Line</a> and <a href="http://www.imagemagick.org/Usage/">Examples of ImageMagick Usage</a>.</p>

<p>The compare program returns 2 on error otherwise 0 if the images are similar or 1 if they are dissimilar.</p>

<h2 class="magick-header"><a id="options"></a>Option Summary</h2>

<p>The <code>compare</code> command recognizes these options.  Click on an option to get more details about how that option works.</p>

<table class="table table-condensed table-striped">
  <tbody>
  <tr>
    <th align="left">Option</th>
    <th align="left">Description</th>
  </tr>

  <tr>
    <td><a href="command-line-options.html#alpha">-alpha</a></td>
    <td>on, activate, off, deactivate, set, opaque, copy",
transparent, extract, background, or shape the alpha channel</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#authenticate">-authenticate <var>value</var></a></td>
    <td>decrypt image with this password</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#channel">-channel <var>type</var></a></td>
    <td>apply option to select image channels</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#colorspace">-colorspace <var>type</var></a></td>
    <td>set image colorspace</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#compose">-compose <var>operator</var></a></td>
    <td>set image composite operator</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#decipher">-decipher <var>filename</var></a></td>
    <td>convert cipher pixels to plain</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#debug">-debug <var>events</var></a></td>
    <td>display copious debugging information</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#define">-define <var>format:option</var></a></td>
    <td>define one or more image format options</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#density">-density <var>geometry</var></a></td>
    <td>horizontal and vertical density of the image</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#depth">-depth <var>value</var></a></td>
    <td>image depth</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#dissimilarity-threshold">-dissimilarity-threshold <var>value</var></a></td>
    <td>maximum distortion for (sub)image match (default 0.2)</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#encipher">-encipher <var>filename</var></a></td>
    <td>convert plain pixels to cipher pixels</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#extract">-extract <var>geometry</var></a></td>
    <td>extract area from image</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#fuzz">-fuzz <var>distance</var></a></td>
    <td>colors within this distance are considered equal</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#help">-help</a></td>
    <td>print program options</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#highlight-color">-highlight-color <var>color</var></a></td>
    <td>emphasize pixel differences with this color</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#identify">-identify</a></td>
    <td>identify the format and characteristics of the image</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#interlace">-interlace <var>type</var></a></td>
    <td>type of image interlacing scheme</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#limit">-limit <var>type value</var></a></td>
    <td>pixel cache resource limit</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#log">-log <var>format</var></a></td>
    <td>format of debugging information</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#lowlight-color">-lowlight-color <var>color</var></a></td>
    <td>de-emphasize pixel differences with this color</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#metric">-metric <var>type</var></a></td>
    <td>measure differences between images with this metric</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#profile">-profile <var>filename</var></a></td>
    <td>add, delete, or apply an image profile</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#quality">-quality <var>value</var></a></td>
    <td>JPEG/MIFF/PNG compression level</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#quantize">-quantize <var>colorspace</var></a></td>
    <td>reduce image colors in this colorspace</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#quiet">-quiet</a></td>
    <td>suppress all warning messages</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#regard-warnings">-regard-warnings</a></td>
    <td>pay attention to warning messages.</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#respect-parentheses">-respect-parentheses</a></td>
    <td>settings remain in effect until parenthesis boundary.</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#sampling-factor">-sampling-factor <var>geometry</var></a></td>
    <td>horizontal and vertical sampling factor</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#seed">-seed <var>value</var></a></td>
    <td>seed a new sequence of pseudo-random numbers</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#set">-set <var>attribute value</var></a></td>
    <td>set an image attribute</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#similarity-threshold">-similarity-threshold <var>value</var></a></td>
    <td>minimum distortion for (sub)image match (default 0.0)</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#size">-size <var>geometry</var></a></td>
    <td>width and height of image</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#subimage-search">-subimage-search</a></td>
    <td>search for subimage</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#synchronize">-synchronize</a></td>
    <td>synchronize image to storage device</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#taint">-taint</a></td>
    <td>mark the image as modified</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#transparent-color">-transparent-color <var>color</var></a></td>
    <td>transparent color</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#verbose">-verbose</a></td>
    <td>print detailed information about the image</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#version">-version</a></td>
    <td>print version information</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#virtual-pixel">-virtual-pixel <var>method</var></a></td>
    <td>access method for pixels outside the boundaries of the image</td>
  </tr>

  </tbody>
</table>
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