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  22     <TITLE>Generic Image Library: Histogram Example</TITLE>
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  39 <h1><a class="anchor" name="BeforeAfterExample">Histogram Example </a></h1>Actual commercial code that computes the luminosity histogram (variable names have been changed and unrelated parts removed):<p>
  40 <div class="fragment"><pre class="fragment"><span class="keywordtype">void</span> luminosity_hist(<span class="keyword">const</span> uint8 *r, <span class="keyword">const</span> uint8 *g, <span class="keyword">const</span> uint8 *b, <span class="keywordtype">int</span> rows, <span class="keywordtype">int</span> cols, <span class="keywordtype">int</span> sRowBytes, Histogram *hist)
  41 {
  42     <span class="keywordflow">for</span> (<span class="keywordtype">int</span> r=0; r&lt;rows; r++)
  43     {
  44         <span class="keywordflow">for</span> (<span class="keywordtype">int</span> c=0; c&lt;cols; c++)
  45         {
  46             <span class="keywordtype">int</span> v=RGBToGray(r[c],g[c],b[c]);
  47             (*hist)[v]++;
  48         }
  49         r+=sRowBytes;
  50         g+=sRowBytes;
  51         b+=sRowBytes;
  52     }
  53 }
  54 </pre></div><p>
  55 <ul>
  56 <li>Works only for RGB (duplicate versions exist for other color spaces)</li><li>Works only for 8-bit images (duplicate versions exist)</li><li>Works only for planar images</li></ul>
  57 <p>
  58 Histogram using GIL:<p>
  59 <div class="fragment"><pre class="fragment"><span class="keyword">template</span> &lt;<span class="keyword">typename</span> GrayView, <span class="keyword">typename</span> R&gt;
  60 <span class="keywordtype">void</span> grayimage_histogram(GrayView&amp; img, R&amp; hist) {
  61     <span class="keywordflow">for</span> (<span class="keyword">typename</span> GrayView::iterator it=img.begin(); it!=img.end(); ++it)
  62         ++hist[*it];
  63 }
  64
  65 <span class="keyword">template</span> &lt;<span class="keyword">typename</span> View, <span class="keyword">typename</span> R&gt;
  66 <span class="keywordtype">void</span> luminosity8bit_hist(View&amp; img, R&amp; hist)
  67 {
  68     grayimage_histogram(color_converted_view&lt;gray8_pixel_t&gt;(img),hist);
  69 }
  70 </pre></div><p>
  71 using <code>boost::lambda</code> the GIL version can be written even simpler: <div class="fragment"><pre class="fragment"><span class="keyword">using</span> boost::lambda;
  72
  73 <span class="keyword">template</span> &lt;<span class="keyword">typename</span> GrayView, <span class="keyword">typename</span> R&gt;
  74 <span class="keywordtype">void</span> grayimage_histogram(GrayView&amp; img, R&amp; hist)
  75 {
  76     for_each_pixel(img, ++var(hist)[_1]);
  77 }
  78 </pre></div><p>
  79 The GIL version:<ul>
  80 <li>Works with any supported channel depth, color space, channel ordering (RGB vs BGR), and row alignment policy.</li><li>Works for both planar and interleaved images.</li><li>Works with new color spaces, channel depths and image types that can be provided in future extensions of GIL</li><li>The second version is as efficient as the hand-coded version</li></ul>
  81 <p>
  82 It is also very flexible. For example, to compute the histogram of the second channel of the top left quadrant of the image, taking every other row and column, call:<p>
  83 <div class="fragment"><pre class="fragment">grayimage_histogram(
  84     nth_channel_view(
  85             subsampled_view(
  86                 subimage_view(img, 0,0, img.width()/2,img.height()/2),  <span class="comment">// upper left quadrant</span>
  87                 2, 2   <span class="comment">// skip every other row and column</span>
  88             ),
  89         1              <span class="comment">// index of the second channel (for example, green for RGB)</span>
  90     ),
  91     hist
  92 );
  93 </pre></div><p>
  94 Note that no extra memory is allocated and no images are copied - GIL operates on the source pixels of <code>img</code> directly. </div>
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