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</tr> 54 </table> 55 </div> 56 <hr/> 57 <div class="content"> 58 <div class="navbar" style="text-align:right;"> 59 60 61 <a class="prev" title="Tutorial: Video Lecture" href="video.html"><img src="../_static/prev.png" alt="prev"/></a> 62 <a class="next" title="Tutorial: Image Gradient" href="gradient.html"><img src="../_static/next.png" alt="next"/></a> 63 64 </div> 65 66 <div class="section" id="tutorial-histogram"> 67<h1>Tutorial: Histogram</h1> 68<div class="contents local topic" id="contents"> 69<ul class="simple"> 70<li><a class="reference internal" href="#original-implementation" id="id1">Original implementation</a></li> 71<li><a class="reference internal" href="#gil-implementation" id="id2">GIL implementation</a></li> 72</ul> 73</div> 74<p>This is a short tutorial presenting an example of a very simple sample of code 75from an existing code base that calculates histogram of an image. 76Next, the program is rewritten using GIL featres.</p> 77<div class="section" id="original-implementation"> 78<h2><a class="toc-backref" href="#id1">Original implementation</a></h2> 79<p>Actual code from a commercial software product that computes the luminosity 80histogram (variable names have been changed and unrelated parts removed):</p> 81<div class="highlight-cpp"><div class="highlight"><pre><span class="kt">void</span> <span class="nf">luminosity_hist</span><span class="p">(</span> 82 <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> 83 <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> 84<span class="p">{</span> 85 <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> 86 <span class="p">{</span> 87 <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> 88 <span class="p">{</span> 89 <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> 90 <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> 91 <span class="p">}</span> 92 <span class="n">r</span> <span class="o">+=</span> <span class="n">sRowBytes</span><span class="p">;</span> 93 <span class="n">g</span> <span class="o">+=</span> <span class="n">sRowBytes</span><span class="p">;</span> 94 <span class="n">b</span> <span class="o">+=</span> <span class="n">sRowBytes</span><span class="p">;</span> 95 <span class="p">}</span> 96<span class="p">}</span> 97</pre></div> 98</div> 99<p>Let’s consider the following issues of the implementation above:</p> 100<ul class="simple"> 101<li>Works only for RGB (duplicate versions exist for other color spaces)</li> 102<li>Works only for 8-bit images (duplicate versions exist)</li> 103<li>Works only for planar images</li> 104</ul> 105</div> 106<div class="section" id="gil-implementation"> 107<h2><a class="toc-backref" href="#id2">GIL implementation</a></h2> 108<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> 109<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> 110<span class="p">{</span> 111 <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> 112 <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> 113<span class="p">}</span> 114 115<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> 116<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> 117<span class="p">{</span> 118 <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> 119<span class="p">}</span> 120</pre></div> 121</div> 122<p>Using the Boost.Lambda library (or C++11 lambda) features it can written 123even simpler:</p> 124<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> 125 126<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> 127<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> 128<span class="p">{</span> 129 <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> 130<span class="p">}</span> 131</pre></div> 132</div> 133<p>Let’s consider the following advantages of the GIL version:</p> 134<ul class="simple"> 135<li>Works with any supported channel depth, color space, channel ordering 136(RGB vs BGR), and row alignment policy.</li> 137<li>Works for both planar and interleaved images.</li> 138<li>Works with new color spaces, channel depths and image types that can be 139provided in future extensions of GIL</li> 140<li>The second version is as efficient as the hand-coded version</li> 141</ul> 142<p>Shortly, it is also very flexible.</p> 143<p>For example, to compute the histogram of the second channel of the top left 144quadrant of the image, taking every other row and column, call:</p> 145<div class="highlight-cpp"><div class="highlight"><pre><span class="n">grayimage_histogram</span><span class="p">(</span> 146 <span class="n">nth_channel_view</span><span class="p">(</span> 147 <span class="n">subsampled_view</span><span class="p">(</span> 148 <span class="n">subimage_view</span><span class="p">(</span><span class="n">img</span><span class="p">,</span> 149 <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> 150 <span class="mi">2</span><span class="p">,</span> <span class="mi">2</span> <span class="c1">// skip every other row and column</span> 151 <span class="p">),</span> 152 <span class="mi">1</span> <span class="c1">// index of the second channel (for example, green for RGB)</span> 153 <span class="p">),</span> 154 <span class="n">hist</span> 155<span class="p">);</span> 156</pre></div> 157</div> 158<p>Since GIL operates on the source pixels of <code class="docutils literal"><span class="pre">img</span></code> object directly, no extra 159memory is allocated and no images are copied.</p> 160</div> 161</div> 162 163 164 <div class="navbar" style="text-align:right;"> 165 166 167 <a class="prev" title="Tutorial: Video Lecture" href="video.html"><img src="../_static/prev.png" alt="prev"/></a> 168 <a class="next" title="Tutorial: Image Gradient" href="gradient.html"><img src="../_static/next.png" alt="next"/></a> 169 170 </div> 171 </div> 172 <div class="footer" role="contentinfo"> 173 Last updated on 2020-08-11 15:08:48. 174 Created using <a href="http://sphinx-doc.org/">Sphinx</a> 1.5.6. 175 </div> 176 </body> 177</html>
