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<div class="section" id="histogram-matching">
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<span id="hm"></span><h1>Histogram Matching</h1>
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<div class="section" id="description">
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<h2>Description</h2>
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<p>Histogram Matching is a technique to match the histograms of two images.</p>
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<p>One use case of this would be when two images of the same location have been taken
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under the same local illumination but with different sensors, bringing out different
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features in either image.</p>
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<p>The famous histogram equalization is a special case of this algorithm when the reference image
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is expected to have a uniform histogram.</p>
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<div class="section" id="algorithm">
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<h2>Algorithm</h2>
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<ol class="arabic simple">
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<li>Calculate the histogram corresponding to input image and reference image.</li>
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<li>If it is a multi channeled image (e.g. RGB), convert both to an independent color space
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(like YCbCr, HSV etc.).</li>
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<li>Then calculate the cumulative histogram over the input image and reference image.</li>
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<li>Normalize both the histogram to bring bin values between 0-1. For multi-channeled images
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normalize each channel independently (by the number of pixels in image).</li>
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<li>If the cumulative histogram of input image is H(p<sub>x</sub>) and of reference image is R(p<sub>x’</sub>)
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p<sub>x</sub> & p<sub>x’</sub> in [0, 255], then apply the transformation
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p<sub>x’</sub> = R<sup>-1</sup>(H(p<sub>x</sub>))</li>
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</ol>
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<p><strong>Explanation</strong></p>
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<p>Since we will be transforming the image to match a reference image, we match
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the cumulative histogram of the image to the cumulative histogram of the reference histogram.</p>
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<p>Hence,</p>
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<blockquote>
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<div><p>=> R(p<sub>x’</sub>) = H(p<sub>x</sub>)</p>
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<p>=> p<sub>x’</sub> = R<sup>-1</sup>(H(p<sub>x</sub>))</p>
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</div></blockquote>
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</div>
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<div class="section" id="results">
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<h2>Results</h2>
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<p>The algorithm is applied on a few standard images. One of the transformations in shown below:</p>
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<p><strong>Original Image(left) & Reference Image(right)</strong></p>
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<div class="align-center figure">
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<a class="reference internal image-reference" href="https://github.com/boost-gil/test-images/blob/master/jpeg/suite/matching.jpg"><img alt="Could not load image." src="https://github.com/boost-gil/test-images/blob/master/jpeg/suite/matching.jpg" style="width: 600px; height: 300px;" /></a>
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</div>
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<p><strong>Histogram matched Image</strong></p>
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<div class="align-center figure">
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<a class="reference internal image-reference" href="https://github.com/boost-gil/test-images/blob/master/jpeg/suite/matching_out.jpg"><img alt="Could not load image." src="https://github.com/boost-gil/test-images/blob/master/jpeg/suite/matching_out.jpg" style="width: 300px; height: 300px;" /></a>
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</div>
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</div>
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<div class="section" id="demo">
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<h2>Demo</h2>
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<p>Usage Syntax:</p>
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<blockquote>
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<div><div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="n">gray8_image_t</span> <span class="n">inp_img</span><span class="p">,</span> <span class="n">ref_img</span><span class="p">;</span>
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<span class="n">read_image</span><span class="p">(</span><span class="s">"your_image.png"</span><span class="p">,</span> <span class="n">inp_img</span><span class="p">,</span> <span class="n">png_tag</span><span class="p">{});</span>
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<span class="n">read_image</span><span class="p">(</span><span class="s">"your_ref_image.png"</span><span class="p">,</span> <span class="n">ref_img</span><span class="p">,</span> <span class="n">png_tag</span><span class="p">{});</span>
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<span class="n">gray8_image_t</span> <span class="nf">dst_img</span><span class="p">(</span><span class="n">inp_img</span><span class="p">.</span><span class="n">dimensions</span><span class="p">());</span>
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<span class="n">histogram_matching</span><span class="p">(</span><span class="n">view</span><span class="p">(</span><span class="n">inp_img</span><span class="p">),</span> <span class="n">view</span><span class="p">(</span><span class="n">ref_image</span><span class="p">),</span> <span class="n">view</span><span class="p">(</span><span class="n">dst_img</span><span class="p">));</span>
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<span class="c1">// To specify mask over input image</span>
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<span class="n">vector</span><span class="o"><</span><span class="n">vector</span><span class="o"><</span><span class="kt">bool</span><span class="o">>></span> <span class="n">mask</span><span class="p">(</span><span class="n">inp_img</span><span class="p">.</span><span class="n">height</span><span class="p">(),</span> <span class="n">vector</span><span class="o"><</span><span class="kt">bool</span><span class="o">></span><span class="p">(</span><span class="n">inp_img</span><span class="p">.</span><span class="n">width</span><span class="p">(),</span> <span class="nb">true</span><span class="p">));</span>
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<span class="n">histogram_matching</span><span class="p">(</span><span class="n">view</span><span class="p">(</span><span class="n">inp_img</span><span class="p">),</span> <span class="n">view</span><span class="p">(</span><span class="n">ref_image</span><span class="p">),</span> <span class="n">view</span><span class="p">(</span><span class="n">dst_img</span><span class="p">),</span> <span class="nb">true</span><span class="p">,</span> <span class="n">mask</span><span class="p">);</span>
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</pre></div>
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</div>
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<div class="admonition tip">
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<p class="first admonition-title">Tip</p>
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<p class="last">Convert an RGB image to a channel independent color space
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before trying the histogram matching algorithm.</p>
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</div>
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</div></blockquote>
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Last updated on 2022-05-13 12:05:18.
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Created using <a href="http://sphinx-doc.org/">Sphinx</a> 1.8.5.
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