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<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>
<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)
{
    <span class="keywordflow">for</span> (<span class="keywordtype">int</span> r=0; r&lt;rows; r++)
    {
        <span class="keywordflow">for</span> (<span class="keywordtype">int</span> c=0; c&lt;cols; c++)
        {
            <span class="keywordtype">int</span> v=RGBToGray(r[c],g[c],b[c]);
            (*hist)[v]++;
        }
        r+=sRowBytes;
        g+=sRowBytes;
        b+=sRowBytes;
    }
}
</pre></div><p>
<ul>
<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>
<p>
Histogram using GIL:<p>
<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;
<span class="keywordtype">void</span> grayimage_histogram(GrayView&amp; img, R&amp; hist) {
    <span class="keywordflow">for</span> (<span class="keyword">typename</span> GrayView::iterator it=img.begin(); it!=img.end(); ++it)
        ++hist[*it];
}

<span class="keyword">template</span> &lt;<span class="keyword">typename</span> View, <span class="keyword">typename</span> R&gt;
<span class="keywordtype">void</span> luminosity8bit_hist(View&amp; img, R&amp; hist) 
{
    grayimage_histogram(color_converted_view&lt;gray8_pixel_t&gt;(img),hist);
}
</pre></div><p>
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;

<span class="keyword">template</span> &lt;<span class="keyword">typename</span> GrayView, <span class="keyword">typename</span> R&gt;
<span class="keywordtype">void</span> grayimage_histogram(GrayView&amp; img, R&amp; hist)
{
    for_each_pixel(img, ++var(hist)[_1]);
}
</pre></div><p>
The GIL version:<ul>
<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>
<p>
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>
<div class="fragment"><pre class="fragment">grayimage_histogram(
    nth_channel_view(
            subsampled_view(
                subimage_view(img, 0,0, img.width()/2,img.height()/2),  <span class="comment">// upper left quadrant</span>
                2, 2   <span class="comment">// skip every other row and column</span>
            ),
        1              <span class="comment">// index of the second channel (for example, green for RGB)</span>
    ),
    hist
);
</pre></div><p>
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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<img src="doxygen.png" alt="doxygen" align="middle" border="0"></a> 1.5.6 </small></address>
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	21	<head>
	22	<TITLE>Generic Image Library: Histogram Example</TITLE>
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	28	<td width="100%" valign="top" style='padding-left: 10px; padding-right: 10px; padding-bottom: 10px'>
	29	<div class="qindex"><a class="qindex" href="index.html">Modules</a>
	30	\| <a class="qindex" href="classes.html">Alphabetical List</a>
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	38	<div class="contents">
	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<rows; r++)
	43	{
	44	<span class="keywordflow">for</span> (<span class="keywordtype">int</span> c=0; c<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> <<span class="keyword">typename</span> GrayView, <span class="keyword">typename</span> R>
	60	<span class="keywordtype">void</span> grayimage_histogram(GrayView& img, R& 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> <<span class="keyword">typename</span> View, <span class="keyword">typename</span> R>
66	<span class="keywordtype">void</span> luminosity8bit_hist(View& img, R& hist)
67	{
68	grayimage_histogram(color_converted_view<gray8_pixel_t>(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> <<span class="keyword">typename</span> GrayView, <span class="keyword">typename</span> R>
74	<span class="keywordtype">void</span> grayimage_histogram(GrayView& img, R& 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>
95	<hr size="1"><address style="text-align: right;"><small>Generated on Sat May 2 13:50:16 2009 for Generic Image Library by
96	<a href="http://www.doxygen.org/index.html">
97	<img src="doxygen.png" alt="doxygen" align="middle" border="0"></a> 1.5.6 </small></address>
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99	</html>