boost_1_45_0/libs/gil/doc/doxygen/before_after.dox

/*!
\page BeforeAfterExample Histogram Example

Actual commercial code that computes the luminosity histogram (variable names have been changed and unrelated parts removed):

\code
void luminosity_hist(const uint8 *r, const uint8 *g, const uint8 *b, int rows, int cols, int sRowBytes, Histogram *hist)
{
    for (int r=0; r<rows; r++)
    {
        for (int c=0; c<cols; c++)
        {
            int v=RGBToGray(r[c],g[c],b[c]);
            (*hist)[v]++;
        }
        r+=sRowBytes;
        g+=sRowBytes;
        b+=sRowBytes;
    }
}
\endcode

- Works only for RGB (duplicate versions exist for other color spaces)
- Works only for 8-bit images (duplicate versions exist)
- Works only for planar images

<p> Histogram using GIL:


\code
template <typename GrayView, typename R>
void grayimage_histogram(GrayView& img, R& hist) {
    for (typename GrayView::iterator it=img.begin(); it!=img.end(); ++it)
        ++hist[*it];
}

template <typename View, typename R>
void luminosity8bit_hist(View& img, R& hist) 
{
    grayimage_histogram(color_converted_view<gray8_pixel_t>(img),hist);
}
\endcode

using \p boost::lambda the GIL version can be written even simpler:
\code
using boost::lambda;

template <typename GrayView, typename R>
void grayimage_histogram(GrayView& img, R& hist)
{
    for_each_pixel(img, ++var(hist)[_1]);
}
\endcode

The GIL version:
- Works with any supported channel depth, color space, channel ordering (RGB vs BGR), and row alignment policy.
- Works for both planar and interleaved images.
- Works with new color spaces, channel depths and image types that can be provided in future extensions of GIL
- The second version is as efficient as the hand-coded version

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:

\code
grayimage_histogram(
    nth_channel_view(
            subsampled_view(
                subimage_view(img, 0,0, img.width()/2,img.height()/2),  // upper left quadrant
                2, 2   // skip every other row and column
            ),
        1              // index of the second channel (for example, green for RGB)
    ),
    hist
);
\endcode

Note that no extra memory is allocated and no images are copied - GIL operates on the source pixels of \p img directly.
*/