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    65. 

  65.       
    66. 

  66.   <div class="section" id="tutorial-histogram">
    67. 

  67. <h1>Tutorial: Histogram</h1>
    68. 

  68. <div class="contents local topic" id="contents">
    69. 

  69. <ul class="simple">
    70. 

  70. <li><a class="reference internal" href="#original-implementation" id="id1">Original implementation</a></li>
    71. 

  71. <li><a class="reference internal" href="#gil-implementation" id="id2">GIL implementation</a></li>
    72. 

  72. </ul>
    73. 

  73. </div>
    74. 

  74. <p>This is a short tutorial presenting an example of a very simple sample of code
    75. 

  75. from an existing code base that calculates histogram of an image.
    76. 

  76. Next, the program is rewritten using GIL featres.</p>
    77. 

  77. <div class="section" id="original-implementation">
    78. 

  78. <h2><a class="toc-backref" href="#id1">Original implementation</a></h2>
    79. 

  79. <p>Actual code from a commercial software product that computes the luminosity
    80. 

  80. histogram (variable names have been changed and unrelated parts removed):</p>
    81. 

  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. 

  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. 

  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. 

  84. <span class="p">{</span>
    85. 

  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">&lt;</span> <span class="n">rows</span><span class="p">;</span> <span class="n">r</span><span class="o">++</span><span class="p">)</span>
    86. 

  86.     <span class="p">{</span>
    87. 

  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">&lt;</span> <span class="n">cols</span><span class="p">;</span> <span class="n">c</span><span class="o">++</span><span class="p">)</span>
    88. 

  88.         <span class="p">{</span>
    89. 

  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. 

  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. 

  91.         <span class="p">}</span>
    92. 

  92.         <span class="n">r</span> <span class="o">+=</span> <span class="n">sRowBytes</span><span class="p">;</span>
    93. 

  93.         <span class="n">g</span> <span class="o">+=</span> <span class="n">sRowBytes</span><span class="p">;</span>
    94. 

  94.         <span class="n">b</span> <span class="o">+=</span> <span class="n">sRowBytes</span><span class="p">;</span>
    95. 

  95.     <span class="p">}</span>
    96. 

  96. <span class="p">}</span>
    97. 

  97. </pre></div>
    98. 

  98. </div>
    99. 

  99. <p>Let&#8217;s consider the following issues of the implementation above:</p>
    100. 

  100. <ul class="simple">
    101. 

  101. <li>Works only for RGB (duplicate versions exist for other color spaces)</li>
    102. 

  102. <li>Works only for 8-bit images (duplicate versions exist)</li>
    103. 

  103. <li>Works only for planar images</li>
    104. 

  104. </ul>
    105. 

  105. </div>
    106. 

  106. <div class="section" id="gil-implementation">
    107. 

  107. <h2><a class="toc-backref" href="#id2">GIL implementation</a></h2>
    108. 

  108. <div class="highlight-cpp"><div class="highlight"><pre><span class="k">template</span> <span class="o">&lt;</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">&gt;</span>
    109. 

  109. <span class="kt">void</span> <span class="n">grayimage_histogram</span><span class="p">(</span><span class="n">GrayView</span><span class="o">&amp;</span> <span class="n">img</span><span class="p">,</span> <span class="n">R</span><span class="o">&amp;</span> <span class="n">hist</span><span class="p">)</span>
    110. 

  110. <span class="p">{</span>
    111. 

  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. 

  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. 

  113. <span class="p">}</span>
    114. 

  114. 

  115. <span class="k">template</span> <span class="o">&lt;</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">&gt;</span>
    116. 

  116. <span class="kt">void</span> <span class="n">luminosity8bit_hist</span><span class="p">(</span><span class="n">View</span><span class="o">&amp;</span> <span class="n">img</span><span class="p">,</span> <span class="n">R</span><span class="o">&amp;</span> <span class="n">hist</span><span class="p">)</span>
    117. 

  117. <span class="p">{</span>
    118. 

  118.     <span class="n">grayimage_histogram</span><span class="p">(</span><span class="n">color_converted_view</span><span class="o">&lt;</span><span class="n">gray8_pixel_t</span><span class="o">&gt;</span><span class="p">(</span><span class="n">img</span><span class="p">),</span><span class="n">hist</span><span class="p">);</span>
    119. 

  119. <span class="p">}</span>
    120. 

  120. </pre></div>
    121. 

  121. </div>
    122. 

  122. <p>Using the Boost.Lambda library (or C++11 lambda) features it can written
    123. 

  123. even simpler:</p>
    124. 

  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. 

  125. 

  126. <span class="k">template</span> <span class="o">&lt;</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">&gt;</span>
    127. 

  127. <span class="kt">void</span> <span class="n">grayimage_histogram</span><span class="p">(</span><span class="n">GrayView</span><span class="o">&amp;</span> <span class="n">img</span><span class="p">,</span> <span class="n">R</span><span class="o">&amp;</span> <span class="n">hist</span><span class="p">)</span>
    128. 

  128. <span class="p">{</span>
    129. 

  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. 

  130. <span class="p">}</span>
    131. 

  131. </pre></div>
    132. 

  132. </div>
    133. 

  133. <p>Let&#8217;s consider the following advantages of the GIL version:</p>
    134. 

  134. <ul class="simple">
    135. 

  135. <li>Works with any supported channel depth, color space, channel ordering
    136. 

  136. (RGB vs BGR), and row alignment policy.</li>
    137. 

  137. <li>Works for both planar and interleaved images.</li>
    138. 

  138. <li>Works with new color spaces, channel depths and image types that can be
    139. 

  139. provided in future extensions of GIL</li>
    140. 

  140. <li>The second version is as efficient as the hand-coded version</li>
    141. 

  141. </ul>
    142. 

  142. <p>Shortly, it is also very flexible.</p>
    143. 

  143. <p>For example, to compute the histogram of the second channel of the top left
    144. 

  144. quadrant of the image, taking every other row and column, call:</p>
    145. 

  145. <div class="highlight-cpp"><div class="highlight"><pre><span class="n">grayimage_histogram</span><span class="p">(</span>
    146. 

  146.     <span class="n">nth_channel_view</span><span class="p">(</span>
    147. 

  147.         <span class="n">subsampled_view</span><span class="p">(</span>
    148. 

  148.             <span class="n">subimage_view</span><span class="p">(</span><span class="n">img</span><span class="p">,</span>
    149. 

  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. 

  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. 

  151.             <span class="p">),</span>
    152. 

  152.         <span class="mi">1</span>   <span class="c1">// index of the second channel (for example, green for RGB)</span>
    153. 

  153.     <span class="p">),</span>
    154. 

  154.     <span class="n">hist</span>
    155. 

  155. <span class="p">);</span>
    156. 

  156. </pre></div>
    157. 

  157. </div>
    158. 

  158. <p>Since GIL operates on the source pixels of <code class="docutils literal"><span class="pre">img</span></code> object directly, no extra
    159. 

  159. memory is allocated and no images are copied.</p>
    160. 

  160. </div>
    161. 

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    162. 

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    172. 

  172.     <div class="footer" role="contentinfo">
    173. 

  173.       Last updated on 2019-12-10 00:12:10.
    174. 

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