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<h1>ImageMagick Compare Command-line Tool</h1>
<p class="navigation-index">[<a href="compare.html#usage">Example Usage</a> • <a href="compare.html#options">Option Summary</a>]</p>
<div class="doc-section">
<p>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>
</div>
<h2><a id="usage"></a>Example Usage</h2>
<div class="doc-section">
<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>
<p class="crt"><span class="crtprompt"> $ </span><span class="crtin">convert rose.jpg -sharpen 0x1 reconstruct.jpg</span><span class="crtout"></span><span class="crtprompt"> $ </span><span class="crtin">compare rose.jpg reconstruct.jpg difference.png</span><span class="crtout"></span><span class="crtprompt"> $ </span><span class="crtin">compare -compose src rose.jpg reconstruct.jpg difference.png</span></p>
<div class="image">
<p>
<a href="../images/rose.jpg"><img src="../images/rose.jpg" width="70" height="46" alt="rose"></img></a>
<a href="../images/reconstruct.jpg"><img src="../images/reconstruct.jpg" width="70" height="46" alt="rose"></img></a>
<img style="margin:13px 0;" src="../images/right.gif" width="20" height="20" alt="==>"></img>
<a href="../images/difference.png"><img src="../images/difference.png" width="70" height="46" alt="rose"></img></a>
</p>
</div>
<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>
<p class="crt"><span class="crtprompt"> $ </span><span class="crtin">compare -verbose -metric mae rose.jpg reconstruct.jpg difference.png</span><span class="crtout">Image: rose.jpg <br></br>
Channel distortion: MAE <br></br>
red: 2282.91 (0.034835) <br></br>
green: 1853.99 (0.0282901) <br></br>
blue: 2008.67 (0.0306503) <br></br>
all: 1536.39 (0.0234439) <br></br></span></p><p>Or, if you just want the red channel distortion, use this command:</p>
<p class="crt"><span class="crtprompt"> $ </span><span class="crtin">compare -channel red -metric PSNR rose.jpg reconstruct.jpg difference.png</span><span class="crtout">19.63</span></p>
<p>Or, if you just want the overall image distortion, use this command:</p>
<p class="crt"><span class="crtprompt"> $ </span><span class="crtin">compare -metric PSNR rose.jpg reconstruct.jpg difference.png</span><span class="crtout">28.31</span></p>
<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>
<p class="crt"><span class="crtprompt"> $ </span><span class="crtin">compare -metric RMSE -subimage-search logo.png wizard.jpg similarity.gif</span><span class="crtout">85.05 (0.00129778) @ 353,157</span></p>
<p>You can find additional examples of using <code>compare</code> in <a href="http://www-106.ibm.com/developerworks/library/l-graf/?ca=dnt-428">Graphics from the Command Line</a>. Further discussion is available in <a href="http://www-106.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>
</div>
<h2><a id="options"></a>Option Summary</h2>
<div class="doc-section">
<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="doc">
<tbody>
<tr>
<th align="left">Option</th>
<th align="left">Description</th>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#alpha">-alpha</a></td>
<td valign="top">on, activate, off, deactivate, set, opaque, copy",
transparent, extract, background, or shape the alpha channel</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#authenticate">-authenticate <em class="option">value</em></a></td>
<td valign="top">decrypt image with this password</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#channel">-channel <em class="option">type</em></a></td>
<td valign="top">apply option to select image channels</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#colorspace">-colorspace <em class="option">type</em></a></td>
<td valign="top">set image colorspace</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#compose">-compose <em class="option">operator</em></a></td>
<td valign="top">set image composite operator</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#decipher">-decipher <em class="option">filename</em></a></td>
<td valign="top">convert cipher pixels to plain</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#debug">-debug <em class="option">events</em></a></td>
<td valign="top">display copious debugging information</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#define">-define <em class="option">format:option</em></a></td>
<td valign="top">define one or more image format options</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#density">-density <em class="option">geometry</em></a></td>
<td valign="top">horizontal and vertical density of the image</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#depth">-depth <em class="option">value</em></a></td>
<td valign="top">image depth</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#dissimilarity-threshold">-dissimilarity-threshold <em class="option">value</em></a></td>
<td valign="top">maximum distortion for (sub)image match (default 0.2)</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#encipher">-encipher <em class="option">filename</em></a></td>
<td valign="top">convert plain pixels to cipher pixels</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#extract">-extract <em class="option">geometry</em></a></td>
<td valign="top">extract area from image</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#fuzz">-fuzz <em class="option">distance</em></a></td>
<td valign="top">colors within this distance are considered equal</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#help">-help</a></td>
<td valign="top">print program options</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#highlight-color">-highlight-color <em class="option">color</em></a></td>
<td valign="top">emphasize pixel differences with this color</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#identify">-identify</a></td>
<td valign="top">identify the format and characteristics of the image</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#interlace">-interlace <em class="option">type</em></a></td>
<td valign="top">type of image interlacing scheme</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#limit">-limit <em class="option">type value</em></a></td>
<td valign="top">pixel cache resource limit</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#log">-log <em class="option">format</em></a></td>
<td valign="top">format of debugging information</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#lowlight-color">-lowlight-color <em class="option">color</em></a></td>
<td valign="top">de-emphasize pixel differences with this color</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#metric">-metric <em class="option">type</em></a></td>
<td valign="top">measure differences between images with this metric</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#profile">-profile <em class="option">filename</em></a></td>
<td valign="top">add, delete, or apply an image profile</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#quality">-quality <em class="option">value</em></a></td>
<td valign="top">JPEG/MIFF/PNG compression level</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#quantize">-quantize <em class="option">colorspace</em></a></td>
<td valign="top">reduce image colors in this colorspace</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#quiet">-quiet</a></td>
<td valign="top">suppress all warning messages</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#regard-warnings">-regard-warnings</a></td>
<td valign="top">pay attention to warning messages.</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#respect-parentheses">-respect-parentheses</a></td>
<td valign="top">settings remain in effect until parenthesis boundary.</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#sampling-factor">-sampling-factor <em class="option">geometry</em></a></td>
<td valign="top">horizontal and vertical sampling factor</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#seed">-seed <em class="option">value</em></a></td>
<td valign="top">seed a new sequence of pseudo-random numbers</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#set">-set <em class="option">attribute value</em></a></td>
<td valign="top">set an image attribute</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#similarity-threshold">-similarity-threshold <em class="option">value</em></a></td>
<td valign="top">minimum distortion for (sub)image match (default 0.0)</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#size">-size <em class="option">geometry</em></a></td>
<td valign="top">width and height of image</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#subimage-search">-subimage-search</a></td>
<td valign="top">search for subimage</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#synchronize">-synchronize</a></td>
<td valign="top">synchronize image to storage device</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#taint">-taint</a></td>
<td valign="top">mark the image as modified</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#transparent-color">-transparent-color <em class="option">color</em></a></td>
<td valign="top">transparent color</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#verbose">-verbose</a></td>
<td valign="top">print detailed information about the image</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#version">-version</a></td>
<td valign="top">print version information</td>
</tr>
<tr>
<td valign="top"><a href="command-line-options.html#virtual-pixel">-virtual-pixel <em class="option">method</em></a></td>
<td valign="top">access method for pixels outside the boundaries of the image</td>
</tr>
</tbody>
</table>
</div>
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