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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>
58Histogram 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>
71using <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>
79The 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>
82It 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>
94Note 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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