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| <TITLE>Generic Image Library: Histogram Example</TITLE> |
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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<rows; r++) |
| { |
| <span class="keywordflow">for</span> (<span class="keywordtype">int</span> c=0; c<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> <<span class="keyword">typename</span> GrayView, <span class="keyword">typename</span> R> |
| <span class="keywordtype">void</span> grayimage_histogram(GrayView& img, R& 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> <<span class="keyword">typename</span> View, <span class="keyword">typename</span> R> |
| <span class="keywordtype">void</span> luminosity8bit_hist(View& img, R& hist) |
| { |
| grayimage_histogram(color_converted_view<gray8_pixel_t>(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> <<span class="keyword">typename</span> GrayView, <span class="keyword">typename</span> R> |
| <span class="keywordtype">void</span> grayimage_histogram(GrayView& img, R& 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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