/Src/Dependencies/Boost/libs/gil/doc/html/g_i_l_0009.html

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  22.     <TITLE>Generic Image Library: Histogram Example</TITLE>
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  29. <div class="qindex"><a class="qindex" href="index.html">Modules</a> 
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  38. <div class="contents">
    39. 

  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. 

  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. 

  41. {
    42. 

  42.     <span class="keywordflow">for</span> (<span class="keywordtype">int</span> r=0; r&lt;rows; r++)
    43. 

  43.     {
    44. 

  44.         <span class="keywordflow">for</span> (<span class="keywordtype">int</span> c=0; c&lt;cols; c++)
    45. 

  45.         {
    46. 

  46.             <span class="keywordtype">int</span> v=RGBToGray(r[c],g[c],b[c]);
    47. 

  47.             (*hist)[v]++;
    48. 

  48.         }
    49. 

  49.         r+=sRowBytes;
    50. 

  50.         g+=sRowBytes;
    51. 

  51.         b+=sRowBytes;
    52. 

  52.     }
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  53. }
    54. 

  54. </pre></div><p>
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  55. <ul>
    56. 

  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. 

  57. <p>
    58. 

  58. Histogram using GIL:<p>
    59. 

  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;
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  60. <span class="keywordtype">void</span> grayimage_histogram(GrayView&amp; img, R&amp; hist) {
    61. 

  61.     <span class="keywordflow">for</span> (<span class="keyword">typename</span> GrayView::iterator it=img.begin(); it!=img.end(); ++it)
    62. 

  62.         ++hist[*it];
    63. 

  63. }
    64. 

  64. 

  65. <span class="keyword">template</span> &lt;<span class="keyword">typename</span> View, <span class="keyword">typename</span> R&gt;
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  66. <span class="keywordtype">void</span> luminosity8bit_hist(View&amp; img, R&amp; hist) 
    67. 

  67. {
    68. 

  68.     grayimage_histogram(color_converted_view&lt;gray8_pixel_t&gt;(img),hist);
    69. 

  69. }
    70. 

  70. </pre></div><p>
    71. 

  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;
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  72. 

  73. <span class="keyword">template</span> &lt;<span class="keyword">typename</span> GrayView, <span class="keyword">typename</span> R&gt;
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  74. <span class="keywordtype">void</span> grayimage_histogram(GrayView&amp; img, R&amp; hist)
    75. 

  75. {
    76. 

  76.     for_each_pixel(img, ++var(hist)[_1]);
    77. 

  77. }
    78. 

  78. </pre></div><p>
    79. 

  79. The GIL version:<ul>
    80. 

  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. 

  81. <p>
    82. 

  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. 

  83. <div class="fragment"><pre class="fragment">grayimage_histogram(
    84. 

  84.     nth_channel_view(
    85. 

  85.             subsampled_view(
    86. 

  86.                 subimage_view(img, 0,0, img.width()/2,img.height()/2),  <span class="comment">// upper left quadrant</span>
    87. 

  87.                 2, 2   <span class="comment">// skip every other row and column</span>
    88. 

  88.             ),
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  89.         1              <span class="comment">// index of the second channel (for example, green for RGB)</span>
    90. 

  90.     ),
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  91.     hist
    92. 

  92. );
    93. 

  93. </pre></div><p>
    94. 

  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>
    95. 

  95. <hr size="1"><address style="text-align: right;"><small>Generated on Sat May 2 13:50:16 2009 for Generic Image Library by&nbsp;
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  96. <a href="http://www.doxygen.org/index.html">
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  97. <img src="doxygen.png" alt="doxygen" align="middle" border="0"></a> 1.5.6 </small></address>
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