    // Copyright (C) 2008  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #ifndef DLIB_EDGE_DETECTOr_
    #define DLIB_EDGE_DETECTOr_

    #include "edge_detector_abstract.h"
    #include "../pixel.h"
    #include "../array2d.h"

    namespace dlib
    {

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

        template <
            typename T
            >
        inline char edge_orientation (
            const T& x_,
            const T& y_
        )
        {

            // if this is a perfectly horizontal gradient then return right away
            if (x_ == 0)
            {
                return '|';
            }
            else if (y_ == 0) // if this is a perfectly vertical gradient then return right away
            {
                return '-';
            }

            // Promote x so that when we multiply by 128 later we know overflow won't happen.
            typedef typename promote<T>::type type;
            type x = x_;
            type y = y_;

            if (x < 0)
            {
                x = -x;
                if (y < 0)
                {
                    y = -y;
                    x *= 128;
                    const type temp = x/y;
                    if (temp > 309)
                        return '-';
                    else if (temp > 53)
                        return '/';
                    else
                        return '|';
                }
                else
                {
                    x *= 128;
                    const type temp = x/y;
                    if (temp > 309)
                        return '-';
                    else if (temp > 53)
                        return '\\';
                    else
                        return '|';
                }
            }
            else
            {
                if (y < 0)
                {
                    y = -y;
                    x *= 128;

                    const type temp = x/y;
                    if (temp > 309)
                        return '-';
                    else if (temp > 53)
                        return '\\';
                    else
                        return '|';
                }
                else
                {
                    x *= 128;

                    const type temp = x/y;
                    if (temp > 309)
                        return '-';
                    else if (temp > 53)
                        return '/';
                    else
                        return '|';
                }
            }

        }

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

        template <
            typename in_image_type,
            typename out_image_type
            >
        void sobel_edge_detector (
            const in_image_type& in_img_,
            out_image_type& horz_,
            out_image_type& vert_
        )
        {
            typedef typename image_traits<out_image_type>::pixel_type pixel_type;
            COMPILE_TIME_ASSERT(pixel_traits<pixel_type>::is_unsigned == false);
            DLIB_ASSERT( !is_same_object(in_img_,horz_) && !is_same_object(in_img_,vert_) &&
                         !is_same_object(horz_,vert_),
                "\tvoid sobel_edge_detector(in_img_, horz_, vert_)"
                << "\n\t You can't give the same image as more than one argument"
                << "\n\t is_same_object(in_img_,horz_): " << is_same_object(in_img_,horz_)
                << "\n\t is_same_object(in_img_,vert_): " << is_same_object(in_img_,vert_)
                << "\n\t is_same_object(horz_,vert_):    " << is_same_object(horz_,vert_)
                );

            const int vert_filter[3][3] = {{-1,-2,-1},
            {0,0,0},
            {1,2,1}};
            const int horz_filter[3][3] = { {-1,0,1},
            {-2,0,2},
            {-1,0,1}};

            const long M = 3;
            const long N = 3;

            const_image_view<in_image_type> in_img(in_img_);
            image_view<out_image_type> horz(horz_);
            image_view<out_image_type> vert(vert_);

            horz.set_size(in_img.nr(),in_img.nc());
            vert.set_size(in_img.nr(),in_img.nc());

            assign_border_pixels(horz,1,1,0);
            assign_border_pixels(vert,1,1,0);

            // figure out the range that we should apply the filter to
            const long first_row = M/2;
            const long first_col = N/2;
            const long last_row = in_img.nr() - M/2;
            const long last_col = in_img.nc() - N/2;

            // apply the filter to the image
            for (long r = first_row; r < last_row; ++r)
            {
                for (long c = first_col; c < last_col; ++c)
                {
                    typedef typename pixel_traits<typename image_traits<in_image_type>::pixel_type>::basic_pixel_type bp_type;

                    typename promote<bp_type>::type p, horz_temp, vert_temp;
                    horz_temp = 0;
                    vert_temp = 0;
                    for (long m = 0; m < M; ++m)
                    {
                        for (long n = 0; n < N; ++n)
                        {
                            // pull out the current pixel and put it into p
                            p = get_pixel_intensity(in_img[r-M/2+m][c-N/2+n]);

                            horz_temp += p*horz_filter[m][n];
                            vert_temp += p*vert_filter[m][n];
                        }
                    }

                    assign_pixel(horz[r][c] , horz_temp);
                    assign_pixel(vert[r][c] , vert_temp);

                }
            }
        }

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

        namespace impl
        {
            template <typename T>
            typename promote<T>::type square (const T& a)
            {
                return static_cast<T>(a)*static_cast<T>(a);
            }
        }

        template <
            typename in_image_type,
            typename out_image_type
            >
        void suppress_non_maximum_edges (
            const in_image_type& horz_,
            const in_image_type& vert_,
            out_image_type& out_img_
        )
        {
            const_image_view<in_image_type> horz(horz_);
            const_image_view<in_image_type> vert(vert_);
            image_view<out_image_type> out_img(out_img_);

            COMPILE_TIME_ASSERT(is_signed_type<typename image_traits<in_image_type>::pixel_type>::value);
            DLIB_ASSERT( horz.nr() == vert.nr() && horz.nc() == vert.nc(),
                "\tvoid suppress_non_maximum_edges(horz, vert, out_img)"
                << "\n\tYou have to give horz and vert gradient images that are the same size"
                << "\n\thorz.nr():   " << horz.nr()
                << "\n\thorz.nc():   " << horz.nc()
                << "\n\tvert.nr():   " << vert.nr()
                << "\n\tvert.nc():   " << vert.nc()
                );
            DLIB_ASSERT( !is_same_object(out_img_,horz_) && !is_same_object(out_img_,vert_),
                "\tvoid suppress_non_maximum_edges(horz_, vert_, out_img_)"
                << "\n\t out_img can't be the same as one of the input images."
                << "\n\t is_same_object(out_img_,horz_):    " << is_same_object(out_img_,horz_)
                << "\n\t is_same_object(out_img_,vert_):    " << is_same_object(out_img_,vert_)
                );

            using std::min;
            using std::abs;

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

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

            zero_border_pixels(out_img,1,1);

            // now do non maximum suppression while we copy the
            const long M = 3;
            const long N = 3;

            // figure out the range that we should apply the filter to
            const long first_row = M/2;
            const long first_col = N/2;
            const long last_row = horz.nr() - M/2;
            const long last_col = horz.nc() - N/2;

            // apply the filter to the image
            for (long r = first_row; r < last_row; ++r)
            {
                for (long c = first_col; c < last_col; ++c)
                {
                    typedef typename promote<typename image_traits<in_image_type>::pixel_type>::type T;
                    const T y = horz[r][c];
                    const T x = vert[r][c];

                    using impl::square;

                    const T val = square(horz[r][c]) + square(vert[r][c]);

                    const char ori = edge_orientation(x,y);
                    const unsigned char zero = 0;
                    switch (ori)
                    {
                        case '-':
                            if (square(horz[r-1][c])+square(vert[r-1][c]) > val || square(horz[r+1][c]) + square(vert[r+1][c]) > val)
                                assign_pixel(out_img[r][c] , zero);
                            else
                                assign_pixel(out_img[r][c] , std::sqrt((double)val));
                            break;

                        case '|':
                            if (square(horz[r][c-1]) + square(vert[r][c-1]) > val || square(horz[r][c+1]) + square(vert[r][c+1]) > val)
                                assign_pixel(out_img[r][c] , zero);
                            else
                                assign_pixel(out_img[r][c] , std::sqrt((double)val));
                            break;

                        case '/':
                            if (square(horz[r-1][c-1]) + square(vert[r-1][c-1]) > val || square(horz[r+1][c+1]) + square(vert[r+1][c+1]) > val)
                                assign_pixel(out_img[r][c] , zero);
                            else
                                assign_pixel(out_img[r][c] , std::sqrt((double)val));
                            break;

                        case '\\':
                            if (square(horz[r+1][c-1]) + square(vert[r+1][c-1]) > val || square(horz[r-1][c+1]) + square(vert[r-1][c+1]) > val)
                                assign_pixel(out_img[r][c] , zero);
                            else
                                assign_pixel(out_img[r][c] , std::sqrt((double)val));
                            break;

                    }
                }
            }
        }

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

    }

    #endif // DLIB_EDGE_DETECTOr_