dlib C++ Library - equalize_histogram.h

    // Copyright (C) 2006  Davis E. King (davisking@users.sourceforge.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #ifndef DLIB_EQUALIZE_HISTOGRAm_
    #define DLIB_EQUALIZE_HISTOGRAm_

    #include "../pixel.h"
    #include "equalize_histogram_abstract.h"
    #include <vector>
    #include "../enable_if.h"
    #include "../matrix.h"

    namespace dlib
    {

    // ---------------------------------------------------------------------------------------

        template <
            typename in_image_type,
            long R,
            long C,
            typename MM
            >
        void get_histogram (
            const in_image_type& in_img,
            matrix<unsigned long,R,C,MM>& hist
        )
        {
            // make sure hist is the right size
            if (R == 1)
                hist.set_size(1,pixel_traits<typename in_image_type::type>::max()+1);
            else
                hist.set_size(pixel_traits<typename in_image_type::type>::max()+1,1);

            set_all_elements(hist,0);

            // compute the histogram
            for (long r = 0; r < in_img.nr(); ++r)
            {
                for (long c = 0; c < in_img.nc(); ++c)
                {
                    unsigned long p = get_pixel_intensity(in_img[r][c]);
                    ++hist(p);
                }
            }
        }

    // ---------------------------------------------------------------------------------------

        template <
            typename in_image_type,
            typename out_image_type
            >
        void equalize_histogram (
            const in_image_type& in_img,
            out_image_type& out_img
        )
        {
            COMPILE_TIME_ASSERT( pixel_traits<typename in_image_type::type>::has_alpha == false );
            COMPILE_TIME_ASSERT( pixel_traits<typename out_image_type::type>::has_alpha == false );

            typedef typename in_image_type::type in_pixel_type;
            typedef typename out_image_type::type out_pixel_type;

            // if there isn't any input image then don't do anything
            if (in_img.size() == 0)
            {
                out_img.clear();
                return;
            }

            out_img.set_size(in_img.nr(),in_img.nc());

            unsigned long p;

            matrix<unsigned long,1,0,typename in_image_type::mem_manager_type> histogram;
            get_histogram(in_img, histogram);

            double scale = pixel_traits<out_pixel_type>::max();
            if (in_img.size() > histogram(0))
                scale /= in_img.size()-histogram(0);
            else
                scale = 0;

            // make the black pixels remain black in the output image
            histogram(0) = 0;

            // compute the transform function
            for (long i = 1; i < histogram.size(); ++i)
                histogram(i) += histogram(i-1);
            // scale so that it is in the range [0,pixel_traits<out_pixel_type>::max()]
            for (long i = 0; i < histogram.size(); ++i)
                histogram(i) = static_cast<unsigned long>(histogram(i)*scale);

            // now do the transform
            for (long row = 0; row < in_img.nr(); ++row)
            {
                for (long col = 0; col < in_img.nc(); ++col)
                {
                    p = histogram(get_pixel_intensity(in_img[row][col]));
                    assign_pixel(out_img[row][col], in_img[row][col]);
                    assign_pixel_intensity(out_img[row][col],p);
                }
            }

        }

    // ---------------------------------------------------------------------------------------

    }

    #endif // DLIB_EQUALIZE_HISTOGRAm_