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<div class="section" id="tutorial-histogram">
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<h1>Tutorial: Histogram</h1>
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<div class="contents local topic" id="contents">
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<ul class="simple">
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<li><a class="reference internal" href="#original-implementation" id="id1">Original implementation</a></li>
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<li><a class="reference internal" href="#gil-implementation" id="id2">GIL implementation</a></li>
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</ul>
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</div>
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<p>This is a short tutorial presenting an example of a very simple sample of code
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from an existing code base that calculates histogram of an image.
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Next, the program is rewritten using GIL featres.</p>
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<div class="section" id="original-implementation">
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<h2><a class="toc-backref" href="#id1">Original implementation</a></h2>
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<p>Actual code from a commercial software product that computes the luminosity
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histogram (variable names have been changed and unrelated parts removed):</p>
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<div class="highlight-cpp"><div class="highlight"><pre><span class="kt">void</span> <span class="nf">luminosity_hist</span><span class="p">(</span>
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<span class="n">std</span><span class="o">::</span><span class="kt">uint8_t</span> <span class="k">const</span><span class="o">*</span> <span class="n">r</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="kt">uint8_t</span> <span class="k">const</span><span class="o">*</span> <span class="n">g</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="kt">uint8_t</span> <span class="k">const</span><span class="o">*</span> <span class="n">b</span><span class="p">,</span>
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<span class="kt">int</span> <span class="n">rows</span><span class="p">,</span> <span class="kt">int</span> <span class="n">cols</span><span class="p">,</span> <span class="kt">int</span> <span class="n">sRowBytes</span><span class="p">,</span> <span class="n">Histogram</span><span class="o">*</span> <span class="n">hist</span><span class="p">)</span>
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<span class="p">{</span>
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<span class="k">for</span> <span class="p">(</span><span class="kt">int</span> <span class="n">r</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">r</span> <span class="o"><</span> <span class="n">rows</span><span class="p">;</span> <span class="n">r</span><span class="o">++</span><span class="p">)</span>
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<span class="p">{</span>
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<span class="k">for</span> <span class="p">(</span><span class="kt">int</span> <span class="n">c</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">c</span> <span class="o"><</span> <span class="n">cols</span><span class="p">;</span> <span class="n">c</span><span class="o">++</span><span class="p">)</span>
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<span class="p">{</span>
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<span class="kt">int</span> <span class="n">v</span> <span class="o">=</span> <span class="n">RGBToGray</span><span class="p">(</span><span class="n">r</span><span class="p">[</span><span class="n">c</span><span class="p">],</span> <span class="n">g</span><span class="p">[</span><span class="n">c</span><span class="p">],</span> <span class="n">b</span><span class="p">[</span><span class="n">c</span><span class="p">]);</span> <span class="c1">// call internal function or macro</span>
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<span class="p">(</span><span class="o">*</span><span class="n">hist</span><span class="p">)[</span><span class="n">v</span><span class="p">]</span><span class="o">++</span><span class="p">;</span>
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<span class="p">}</span>
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<span class="n">r</span> <span class="o">+=</span> <span class="n">sRowBytes</span><span class="p">;</span>
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<span class="n">g</span> <span class="o">+=</span> <span class="n">sRowBytes</span><span class="p">;</span>
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<span class="n">b</span> <span class="o">+=</span> <span class="n">sRowBytes</span><span class="p">;</span>
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<span class="p">}</span>
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<span class="p">}</span>
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</pre></div>
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</div>
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<p>Let’s consider the following issues of the implementation above:</p>
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<ul class="simple">
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<li>Works only for RGB (duplicate versions exist for other color spaces)</li>
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<li>Works only for 8-bit images (duplicate versions exist)</li>
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<li>Works only for planar images</li>
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</ul>
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</div>
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<div class="section" id="gil-implementation">
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<h2><a class="toc-backref" href="#id2">GIL implementation</a></h2>
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<div class="highlight-cpp"><div class="highlight"><pre><span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">GrayView</span><span class="p">,</span> <span class="k">typename</span> <span class="n">R</span><span class="o">></span>
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<span class="kt">void</span> <span class="n">grayimage_histogram</span><span class="p">(</span><span class="n">GrayView</span><span class="o">&</span> <span class="n">img</span><span class="p">,</span> <span class="n">R</span><span class="o">&</span> <span class="n">hist</span><span class="p">)</span>
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<span class="p">{</span>
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<span class="k">for</span> <span class="p">(</span><span class="k">typename</span> <span class="n">GrayView</span><span class="o">::</span><span class="n">iterator</span> <span class="n">it</span><span class="o">=</span><span class="n">img</span><span class="p">.</span><span class="n">begin</span><span class="p">();</span> <span class="n">it</span><span class="o">!=</span><span class="n">img</span><span class="p">.</span><span class="n">end</span><span class="p">();</span> <span class="o">++</span><span class="n">it</span><span class="p">)</span>
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<span class="o">++</span><span class="n">hist</span><span class="p">[</span><span class="o">*</span><span class="n">it</span><span class="p">];</span>
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<span class="p">}</span>
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<span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">View</span><span class="p">,</span> <span class="k">typename</span> <span class="n">R</span><span class="o">></span>
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<span class="kt">void</span> <span class="n">luminosity8bit_hist</span><span class="p">(</span><span class="n">View</span><span class="o">&</span> <span class="n">img</span><span class="p">,</span> <span class="n">R</span><span class="o">&</span> <span class="n">hist</span><span class="p">)</span>
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<span class="p">{</span>
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<span class="n">grayimage_histogram</span><span class="p">(</span><span class="n">color_converted_view</span><span class="o"><</span><span class="n">gray8_pixel_t</span><span class="o">></span><span class="p">(</span><span class="n">img</span><span class="p">),</span><span class="n">hist</span><span class="p">);</span>
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<span class="p">}</span>
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</pre></div>
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</div>
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<p>Using the Boost.Lambda library (or C++11 lambda) features it can written
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even simpler:</p>
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<div class="highlight-cpp"><div class="highlight"><pre><span class="k">using</span> <span class="n">boost</span><span class="o">::</span><span class="n">lambda</span><span class="p">;</span>
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<span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">GrayView</span><span class="p">,</span> <span class="k">typename</span> <span class="n">R</span><span class="o">></span>
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<span class="kt">void</span> <span class="n">grayimage_histogram</span><span class="p">(</span><span class="n">GrayView</span><span class="o">&</span> <span class="n">img</span><span class="p">,</span> <span class="n">R</span><span class="o">&</span> <span class="n">hist</span><span class="p">)</span>
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<span class="p">{</span>
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<span class="n">for_each_pixel</span><span class="p">(</span><span class="n">img</span><span class="p">,</span> <span class="o">++</span><span class="n">var</span><span class="p">(</span><span class="n">hist</span><span class="p">)[</span><span class="n">_1</span><span class="p">]);</span>
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<span class="p">}</span>
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</pre></div>
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</div>
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<p>Let’s consider the following advantages of the GIL version:</p>
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<ul class="simple">
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<li>Works with any supported channel depth, color space, channel ordering
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(RGB vs BGR), and row alignment policy.</li>
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<li>Works for both planar and interleaved images.</li>
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<li>Works with new color spaces, channel depths and image types that can be
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provided in future extensions of GIL</li>
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<li>The second version is as efficient as the hand-coded version</li>
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</ul>
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<p>Shortly, it is also very flexible.</p>
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<p>For example, to compute the histogram of the second channel of the top left
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quadrant of the image, taking every other row and column, call:</p>
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<div class="highlight-cpp"><div class="highlight"><pre><span class="n">grayimage_histogram</span><span class="p">(</span>
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<span class="n">nth_channel_view</span><span class="p">(</span>
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<span class="n">subsampled_view</span><span class="p">(</span>
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<span class="n">subimage_view</span><span class="p">(</span><span class="n">img</span><span class="p">,</span>
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<span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span> <span class="n">img</span><span class="p">.</span><span class="n">width</span><span class="p">()</span> <span class="o">/</span> <span class="mi">2</span><span class="p">,</span> <span class="n">img</span><span class="p">.</span><span class="n">height</span><span class="p">()</span> <span class="o">/</span> <span class="mi">2</span><span class="p">),</span> <span class="c1">// upper left quadrant</span>
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<span class="mi">2</span><span class="p">,</span> <span class="mi">2</span> <span class="c1">// skip every other row and column</span>
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<span class="p">),</span>
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<span class="mi">1</span> <span class="c1">// index of the second channel (for example, green for RGB)</span>
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<span class="p">),</span>
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<span class="n">hist</span>
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<span class="p">);</span>
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</pre></div>
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</div>
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<p>Since GIL operates on the source pixels of <code class="docutils literal"><span class="pre">img</span></code> object directly, no extra
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memory is allocated and no images are copied.</p>
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