    // Copyright (C) 2010  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #ifndef DLIB_IMAGE_PYRaMID_H__
    #define DLIB_IMAGE_PYRaMID_H__

    #include "image_pyramid_abstract.h"
    #include "../pixel.h"
    #include "../array2d.h"
    #include "../geometry.h"
    #include "spatial_filtering.h"

    namespace dlib
    {

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

        class pyramid_disable : noncopyable
        {
        public:

            template <typename T>
            vector<double,2> point_down (
                const vector<T,2>&
            ) const
            {
                return vector<double,2>(0,0);
            }

            template <typename T>
            vector<double,2> point_up (
                const vector<T,2>&
            ) const
            {
                return vector<double,2>(0,0);
            }

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

            template <typename T>
            vector<double,2> point_down (
                const vector<T,2>& p,
                unsigned int levels
            ) const
            {
                if (levels == 0)
                    return p;
                else
                    return vector<double,2>(0,0);
            }

            template <typename T>
            vector<double,2> point_up (
                const vector<T,2>& p,
                unsigned int levels
            ) const
            {
                if (levels == 0)
                    return p;
                else
                    return vector<double,2>(0,0);
            }

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

            rectangle rect_up (
                const rectangle& rect
            ) const
            {
                return rectangle(point_up(rect.tl_corner()), point_up(rect.br_corner()));
            }

            rectangle rect_up (
                const rectangle& rect,
                unsigned int levels
            ) const
            {
                return rectangle(point_up(rect.tl_corner(),levels), point_up(rect.br_corner(),levels));
            }

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

            rectangle rect_down (
                const rectangle& rect
            ) const
            {
                return rectangle(point_down(rect.tl_corner()), point_down(rect.br_corner()));
            }

            rectangle rect_down (
                const rectangle& rect,
                unsigned int levels
            ) const
            {
                return rectangle(point_down(rect.tl_corner(),levels), point_down(rect.br_corner(),levels));
            }

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

        public:

            template <
                typename in_image_type,
                typename out_image_type
                >
            void operator() (
                // we do this #ifdef stuff to avoid compiler warnings about unused variables.
    #ifdef ENABLE_ASSERTS
                const in_image_type& original,
    #else
                const in_image_type& ,
    #endif
                out_image_type& down
            ) const
            {
                // make sure requires clause is not broken
                DLIB_ASSERT(is_same_object(original, down) == false,
                            "\t void pyramid_disable::operator()"
                            << "\n\t is_same_object(original, down): " << is_same_object(original, down)
                            << "\n\t this:                           " << this
                            );

                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 );

                down.clear();
            }
        };

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

        namespace impl
        {

            class pyramid_down_2_1 : noncopyable
            {
            public:

                template <typename T>
                vector<double,2> point_down (
                    const vector<T,2>& p
                ) const
                {
                    //do return (p - vector<T,2>(2,2))/2.0;
                    return p/2.0 - vector<double,2>(1,1);
                }

                template <typename T>
                vector<double,2> point_up (
                    const vector<T,2>& p
                ) const
                {
                    return p*2 + vector<T,2>(2,2);
                }

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

                template <typename T>
                vector<double,2> point_down (
                    const vector<T,2>& p,
                    unsigned int levels
                ) const
                {
                    vector<double,2> temp = p;
                    for (unsigned int i = 0; i < levels; ++i)
                        temp = point_down(temp);
                    return temp;
                }

                template <typename T>
                vector<double,2> point_up (
                    const vector<T,2>& p,
                    unsigned int levels
                ) const
                {
                    vector<double,2> temp = p;
                    for (unsigned int i = 0; i < levels; ++i)
                        temp = point_up(temp);
                    return temp;
                }

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

                rectangle rect_up (
                    const rectangle& rect
                ) const
                {
                    return rectangle(point_up(rect.tl_corner()), point_up(rect.br_corner()));
                }

                rectangle rect_up (
                    const rectangle& rect,
                    unsigned int levels
                ) const
                {
                    return rectangle(point_up(rect.tl_corner(),levels), point_up(rect.br_corner(),levels));
                }

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

                rectangle rect_down (
                    const rectangle& rect
                ) const
                {
                    return rectangle(point_down(rect.tl_corner()), point_down(rect.br_corner()));
                }

                rectangle rect_down (
                    const rectangle& rect,
                    unsigned int levels
                ) const
                {
                    return rectangle(point_down(rect.tl_corner(),levels), point_down(rect.br_corner(),levels));
                }

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

            private:
                template <typename T, typename U>
                struct both_images_rgb
                {
                    const static bool value = pixel_traits<typename T::type>::rgb &&
                                            pixel_traits<typename U::type>::rgb;
                };
            public:

                template <
                    typename in_image_type,
                    typename out_image_type
                    >
                typename disable_if<both_images_rgb<in_image_type,out_image_type> >::type operator() (
                    const in_image_type& original,
                    out_image_type& down
                ) const
                {
                    // make sure requires clause is not broken
                    DLIB_ASSERT( is_same_object(original, down) == false,
                                "\t void pyramid_down_2_1::operator()"
                                << "\n\t is_same_object(original, down): " << is_same_object(original, down)
                                << "\n\t this:                           " << this
                                );

                    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 );

                    if (original.nr() <= 8 || original.nc() <= 8)
                    {
                        down.clear();
                        return;
                    }

                    typedef typename pixel_traits<typename in_image_type::type>::basic_pixel_type bp_type;
                    typedef typename promote<bp_type>::type ptype;
                    array2d<ptype> temp_img;
                    temp_img.set_size(original.nr(), (original.nc()-3)/2);
                    down.set_size((original.nr()-3)/2, (original.nc()-3)/2);

                    // This function applies a 5x5 Gaussian filter to the image.  It
                    // does this by separating the filter into its horizontal and vertical
                    // components and then downsamples the image by dropping every other
                    // row and column.  Note that we can do these things all together in
                    // one step.

                    // apply row filter
                    for (long r = 0; r < temp_img.nr(); ++r)
                    {
                        long oc = 0;
                        for (long c = 0; c < temp_img.nc(); ++c)
                        {
                            ptype pix1;
                            ptype pix2;
                            ptype pix3;
                            ptype pix4;
                            ptype pix5;

                            assign_pixel(pix1, original[r][oc]);
                            assign_pixel(pix2, original[r][oc+1]);
                            assign_pixel(pix3, original[r][oc+2]);
                            assign_pixel(pix4, original[r][oc+3]);
                            assign_pixel(pix5, original[r][oc+4]);

                            pix2 *= 4;
                            pix3 *= 6;
                            pix4 *= 4;

                            assign_pixel(temp_img[r][c], pix1 + pix2 + pix3 + pix4 + pix5);
                            oc += 2;
                        }
                    }

                    // apply column filter
                    long dr = 0;
                    for (long r = 2; r < temp_img.nr()-2; r += 2)
                    {
                        for (long c = 0; c < temp_img.nc(); ++c)
                        {
                            ptype temp = temp_img[r-2][c] +
                                        temp_img[r-1][c]*4 +
                                        temp_img[r  ][c]*6 +
                                        temp_img[r-1][c]*4 +
                                        temp_img[r-2][c];

                            assign_pixel(down[dr][c],temp/256);
                        }
                        ++dr;
                    }

                }

            private:
                struct rgbptype
                {
                    uint16 red;
                    uint16 green;
                    uint16 blue;
                };
            public:
            // ------------------------------------------
            //       OVERLOAD FOR RGB TO RGB IMAGES
            // ------------------------------------------
                template <
                    typename in_image_type,
                    typename out_image_type
                    >
                typename enable_if<both_images_rgb<in_image_type,out_image_type> >::type operator() (
                    const in_image_type& original,
                    out_image_type& down
                ) const
                {
                    // make sure requires clause is not broken
                    DLIB_ASSERT( is_same_object(original, down) == false,
                                "\t void pyramid_down_2_1::operator()"
                                << "\n\t is_same_object(original, down): " << is_same_object(original, down)
                                << "\n\t this:                           " << this
                                );

                    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 );

                    if (original.nr() <= 8 || original.nc() <= 8)
                    {
                        down.clear();
                        return;
                    }

                    array2d<rgbptype> temp_img;
                    temp_img.set_size(original.nr(), (original.nc()-3)/2);
                    down.set_size((original.nr()-3)/2, (original.nc()-3)/2);

                    // This function applies a 5x5 Gaussian filter to the image.  It
                    // does this by separating the filter into its horizontal and vertical
                    // components and then downsamples the image by dropping every other
                    // row and column.  Note that we can do these things all together in
                    // one step.

                    // apply row filter
                    for (long r = 0; r < temp_img.nr(); ++r)
                    {
                        long oc = 0;
                        for (long c = 0; c < temp_img.nc(); ++c)
                        {
                            rgbptype pix1;
                            rgbptype pix2;
                            rgbptype pix3;
                            rgbptype pix4;
                            rgbptype pix5;

                            pix1.red = original[r][oc].red;
                            pix2.red = original[r][oc+1].red;
                            pix3.red = original[r][oc+2].red;
                            pix4.red = original[r][oc+3].red;
                            pix5.red = original[r][oc+4].red;
                            pix1.green = original[r][oc].green;
                            pix2.green = original[r][oc+1].green;
                            pix3.green = original[r][oc+2].green;
                            pix4.green = original[r][oc+3].green;
                            pix5.green = original[r][oc+4].green;
                            pix1.blue = original[r][oc].blue;
                            pix2.blue = original[r][oc+1].blue;
                            pix3.blue = original[r][oc+2].blue;
                            pix4.blue = original[r][oc+3].blue;
                            pix5.blue = original[r][oc+4].blue;

                            pix2.red *= 4;
                            pix3.red *= 6;
                            pix4.red *= 4;

                            pix2.green *= 4;
                            pix3.green *= 6;
                            pix4.green *= 4;

                            pix2.blue *= 4;
                            pix3.blue *= 6;
                            pix4.blue *= 4;

                            rgbptype temp;
                            temp.red = pix1.red + pix2.red + pix3.red + pix4.red + pix5.red;
                            temp.green = pix1.green + pix2.green + pix3.green + pix4.green + pix5.green;
                            temp.blue = pix1.blue + pix2.blue + pix3.blue + pix4.blue + pix5.blue;

                            temp_img[r][c] = temp;

                            oc += 2;
                        }
                    }

                    // apply column filter
                    long dr = 0;
                    for (long r = 2; r < temp_img.nr()-2; r += 2)
                    {
                        for (long c = 0; c < temp_img.nc(); ++c)
                        {
                            rgbptype temp;
                            temp.red = temp_img[r-2][c].red +
                                    temp_img[r-1][c].red*4 +
                                    temp_img[r  ][c].red*6 +
                                    temp_img[r-1][c].red*4 +
                                    temp_img[r-2][c].red;
                            temp.green = temp_img[r-2][c].green +
                                        temp_img[r-1][c].green*4 +
                                        temp_img[r  ][c].green*6 +
                                        temp_img[r-1][c].green*4 +
                                        temp_img[r-2][c].green;
                            temp.blue = temp_img[r-2][c].blue +
                                        temp_img[r-1][c].blue*4 +
                                        temp_img[r  ][c].blue*6 +
                                        temp_img[r-1][c].blue*4 +
                                        temp_img[r-2][c].blue;

                            down[dr][c].red = temp.red/256;
                            down[dr][c].green = temp.green/256;
                            down[dr][c].blue = temp.blue/256;
                        }
                        ++dr;
                    }

                }

            private:

            };

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

            class pyramid_down_3_2 : noncopyable
            {
            public:

                template <typename T>
                vector<double,2> point_down (
                    const vector<T,2>& p
                ) const
                {
                    const double ratio = 2.0/3.0;
                    //do return (p - vector<T,2>(1,1))*ratio;
                    return p*ratio - vector<double,2>(ratio,ratio);
                }

                template <typename T>
                vector<double,2> point_up (
                    const vector<T,2>& p
                ) const
                {
                    const double ratio = 3.0/2.0;
                    return p*ratio + vector<T,2>(1,1);
                }

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

                template <typename T>
                vector<double,2> point_down (
                    const vector<T,2>& p,
                    unsigned int levels
                ) const
                {
                    vector<double,2> temp = p;
                    for (unsigned int i = 0; i < levels; ++i)
                        temp = point_down(temp);
                    return temp;
                }

                template <typename T>
                vector<double,2> point_up (
                    const vector<T,2>& p,
                    unsigned int levels
                ) const
                {
                    vector<double,2> temp = p;
                    for (unsigned int i = 0; i < levels; ++i)
                        temp = point_up(temp);
                    return temp;
                }

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

                rectangle rect_up (
                    const rectangle& rect
                ) const
                {
                    return rectangle(point_up(rect.tl_corner()), point_up(rect.br_corner()));
                }

                rectangle rect_up (
                    const rectangle& rect,
                    unsigned int levels
                ) const
                {
                    return rectangle(point_up(rect.tl_corner(),levels), point_up(rect.br_corner(),levels));
                }

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

                rectangle rect_down (
                    const rectangle& rect
                ) const
                {
                    return rectangle(point_down(rect.tl_corner()), point_down(rect.br_corner()));
                }

                rectangle rect_down (
                    const rectangle& rect,
                    unsigned int levels
                ) const
                {
                    return rectangle(point_down(rect.tl_corner(),levels), point_down(rect.br_corner(),levels));
                }

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

            private:
                template <typename T, typename U>
                struct both_images_rgb
                {
                    const static bool value = pixel_traits<typename T::type>::rgb &&
                                            pixel_traits<typename U::type>::rgb;
                };
            public:

                template <
                    typename in_image_type,
                    typename out_image_type
                    >
                typename disable_if<both_images_rgb<in_image_type,out_image_type> >::type operator() (
                    const in_image_type& original,
                    out_image_type& down
                ) const
                {
                    // make sure requires clause is not broken
                    DLIB_ASSERT(is_same_object(original, down) == false,
                                "\t void pyramid_down_3_2::operator()"
                                << "\n\t is_same_object(original, down): " << is_same_object(original, down)
                                << "\n\t this:                           " << this
                                );

                    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 );

                    if (original.nr() <= 8 || original.nc() <= 8)
                    {
                        down.clear();
                        return;
                    }

                    const long size_in = 3;
                    const long size_out = 2;

                    typedef typename pixel_traits<typename in_image_type::type>::basic_pixel_type bp_type;
                    typedef typename promote<bp_type>::type ptype;
                    const long full_nr =  size_out*((original.nr()-2)/size_in);
                    const long part_nr = (size_out*(original.nr()-2))/size_in;
                    const long full_nc =  size_out*((original.nc()-2)/size_in);
                    const long part_nc = (size_out*(original.nc()-2))/size_in;
                    down.set_size(part_nr, part_nc);

                    long rr = 1;
                    long r;
                    for (r = 0; r < full_nr; r+=size_out)
                    {
                        long cc = 1;
                        long c;
                        for (c = 0; c < full_nc; c+=size_out)
                        {
                            ptype block[size_in][size_in];
                            separable_3x3_filter_block_grayscale(block, original, rr, cc, 2, 12, 2);

                            // bi-linearly interpolate block
                            assign_pixel(down[r][c]     , (block[0][0]*9 + block[1][0]*3 + block[0][1]*3 + block[1][1])/(16*256));
                            assign_pixel(down[r][c+1]   , (block[0][2]*9 + block[1][2]*3 + block[0][1]*3 + block[1][1])/(16*256));
                            assign_pixel(down[r+1][c]   , (block[2][0]*9 + block[1][0]*3 + block[2][1]*3 + block[1][1])/(16*256));
                            assign_pixel(down[r+1][c+1] , (block[2][2]*9 + block[1][2]*3 + block[2][1]*3 + block[1][1])/(16*256));

                            cc += size_in;
                        }
                        if (part_nc - full_nc == 1)
                        {
                            ptype block[size_in][2];
                            separable_3x3_filter_block_grayscale(block, original, rr, cc, 2, 12, 2);

                            // bi-linearly interpolate partial block
                            assign_pixel(down[r][c]     , (block[0][0]*9 + block[1][0]*3 + block[0][1]*3 + block[1][1])/(16*256));
                            assign_pixel(down[r+1][c]   , (block[2][0]*9 + block[1][0]*3 + block[2][1]*3 + block[1][1])/(16*256));
                        }
                        rr += size_in;
                    }
                    if (part_nr - full_nr == 1)
                    {
                        long cc = 1;
                        long c;
                        for (c = 0; c < full_nc; c+=size_out)
                        {
                            ptype block[2][size_in];
                            separable_3x3_filter_block_grayscale(block, original, rr, cc, 2, 12, 2);

                            // bi-linearly interpolate partial block
                            assign_pixel(down[r][c]     , (block[0][0]*9 + block[1][0]*3 + block[0][1]*3 + block[1][1])/(16*256));
                            assign_pixel(down[r][c+1]   , (block[0][2]*9 + block[1][2]*3 + block[0][1]*3 + block[1][1])/(16*256));

                            cc += size_in;
                        }
                        if (part_nc - full_nc == 1)
                        {
                            ptype block[2][2];
                            separable_3x3_filter_block_grayscale(block, original, rr, cc, 2, 12, 2);

                            // bi-linearly interpolate partial block
                            assign_pixel(down[r][c]     , (block[0][0]*9 + block[1][0]*3 + block[0][1]*3 + block[1][1])/(16*256));
                        }
                    }

                }

            private:
                struct rgbptype
                {
                    uint32 red;
                    uint32 green;
                    uint32 blue;
                };

            public:
            // ------------------------------------------
            //       OVERLOAD FOR RGB TO RGB IMAGES
            // ------------------------------------------
                template <
                    typename in_image_type,
                    typename out_image_type
                    >
                typename enable_if<both_images_rgb<in_image_type,out_image_type> >::type operator() (
                    const in_image_type& original,
                    out_image_type& down
                ) const
                {
                    // make sure requires clause is not broken
                    DLIB_ASSERT( is_same_object(original, down) == false,
                                "\t void pyramid_down_3_2::operator()"
                                << "\n\t is_same_object(original, down): " << is_same_object(original, down)
                                << "\n\t this:                           " << this
                                );

                    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 );

                    if (original.nr() <= 8 || original.nc() <= 8)
                    {
                        down.clear();
                        return;
                    }

                    const long size_in = 3;
                    const long size_out = 2;

                    const long full_nr =  size_out*((original.nr()-2)/size_in);
                    const long part_nr = (size_out*(original.nr()-2))/size_in;
                    const long full_nc =  size_out*((original.nc()-2)/size_in);
                    const long part_nc = (size_out*(original.nc()-2))/size_in;
                    down.set_size(part_nr, part_nc);

                    long rr = 1;
                    long r;
                    for (r = 0; r < full_nr; r+=size_out)
                    {
                        long cc = 1;
                        long c;
                        for (c = 0; c < full_nc; c+=size_out)
                        {
                            rgbptype block[size_in][size_in];
                            separable_3x3_filter_block_rgb(block, original, rr, cc, 2, 12, 2);

                            // bi-linearly interpolate block
                            down[r][c].red       = (block[0][0].red*9   + block[1][0].red*3   + block[0][1].red*3   + block[1][1].red)/(16*256);
                            down[r][c].green     = (block[0][0].green*9 + block[1][0].green*3 + block[0][1].green*3 + block[1][1].green)/(16*256);
                            down[r][c].blue      = (block[0][0].blue*9  + block[1][0].blue*3  + block[0][1].blue*3  + block[1][1].blue)/(16*256);

                            down[r][c+1].red     = (block[0][2].red*9   + block[1][2].red*3   + block[0][1].red*3   + block[1][1].red)/(16*256);
                            down[r][c+1].green   = (block[0][2].green*9 + block[1][2].green*3 + block[0][1].green*3 + block[1][1].green)/(16*256);
                            down[r][c+1].blue    = (block[0][2].blue*9  + block[1][2].blue*3  + block[0][1].blue*3  + block[1][1].blue)/(16*256);

                            down[r+1][c].red     = (block[2][0].red*9   + block[1][0].red*3   + block[2][1].red*3   + block[1][1].red)/(16*256);
                            down[r+1][c].green   = (block[2][0].green*9 + block[1][0].green*3 + block[2][1].green*3 + block[1][1].green)/(16*256);
                            down[r+1][c].blue    = (block[2][0].blue*9  + block[1][0].blue*3  + block[2][1].blue*3  + block[1][1].blue)/(16*256);

                            down[r+1][c+1].red   = (block[2][2].red*9   + block[1][2].red*3   + block[2][1].red*3   + block[1][1].red)/(16*256);
                            down[r+1][c+1].green = (block[2][2].green*9 + block[1][2].green*3 + block[2][1].green*3 + block[1][1].green)/(16*256);
                            down[r+1][c+1].blue  = (block[2][2].blue*9  + block[1][2].blue*3  + block[2][1].blue*3  + block[1][1].blue)/(16*256);

                            cc += size_in;
                        }
                        if (part_nc - full_nc == 1)
                        {
                            rgbptype block[size_in][2];
                            separable_3x3_filter_block_rgb(block, original, rr, cc, 2, 12, 2);

                            // bi-linearly interpolate partial block
                            down[r][c].red       = (block[0][0].red*9   + block[1][0].red*3   + block[0][1].red*3   + block[1][1].red)/(16*256);
                            down[r][c].green     = (block[0][0].green*9 + block[1][0].green*3 + block[0][1].green*3 + block[1][1].green)/(16*256);
                            down[r][c].blue      = (block[0][0].blue*9  + block[1][0].blue*3  + block[0][1].blue*3  + block[1][1].blue)/(16*256);

                            down[r+1][c].red     = (block[2][0].red*9   + block[1][0].red*3   + block[2][1].red*3   + block[1][1].red)/(16*256);
                            down[r+1][c].green   = (block[2][0].green*9 + block[1][0].green*3 + block[2][1].green*3 + block[1][1].green)/(16*256);
                            down[r+1][c].blue    = (block[2][0].blue*9  + block[1][0].blue*3  + block[2][1].blue*3  + block[1][1].blue)/(16*256);
                        }
                        rr += size_in;
                    }
                    if (part_nr - full_nr == 1)
                    {
                        long cc = 1;
                        long c;
                        for (c = 0; c < full_nc; c+=size_out)
                        {
                            rgbptype block[2][size_in];
                            separable_3x3_filter_block_rgb(block, original, rr, cc, 2, 12, 2);

                            // bi-linearly interpolate partial block
                            down[r][c].red       = (block[0][0].red*9   + block[1][0].red*3   + block[0][1].red*3   + block[1][1].red)/(16*256);
                            down[r][c].green     = (block[0][0].green*9 + block[1][0].green*3 + block[0][1].green*3 + block[1][1].green)/(16*256);
                            down[r][c].blue      = (block[0][0].blue*9  + block[1][0].blue*3  + block[0][1].blue*3  + block[1][1].blue)/(16*256);

                            down[r][c+1].red     = (block[0][2].red*9   + block[1][2].red*3   + block[0][1].red*3   + block[1][1].red)/(16*256);
                            down[r][c+1].green   = (block[0][2].green*9 + block[1][2].green*3 + block[0][1].green*3 + block[1][1].green)/(16*256);
                            down[r][c+1].blue    = (block[0][2].blue*9  + block[1][2].blue*3  + block[0][1].blue*3  + block[1][1].blue)/(16*256);

                            cc += size_in;
                        }
                        if (part_nc - full_nc == 1)
                        {
                            rgbptype block[2][2];
                            separable_3x3_filter_block_rgb(block, original, rr, cc, 2, 12, 2);

                            // bi-linearly interpolate partial block
                            down[r][c].red       = (block[0][0].red*9   + block[1][0].red*3   + block[0][1].red*3   + block[1][1].red)/(16*256);
                            down[r][c].green     = (block[0][0].green*9 + block[1][0].green*3 + block[0][1].green*3 + block[1][1].green)/(16*256);
                            down[r][c].blue      = (block[0][0].blue*9  + block[1][0].blue*3  + block[0][1].blue*3  + block[1][1].blue)/(16*256);
                        }
                    }
                }

            private:

            };

        }

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

        template <
            unsigned int N
            >
        class pyramid_down : noncopyable
        {
        public:

            COMPILE_TIME_ASSERT(N > 0);

            template <typename T>
            vector<double,2> point_down (
                const vector<T,2>& p
            ) const
            {
                const double ratio = (N-1.0)/N;
                return p*ratio;
            }

            template <typename T>
            vector<double,2> point_up (
                const vector<T,2>& p
            ) const
            {
                const double ratio = N/(N-1.0);
                return p*ratio;
            }

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

            template <typename T>
            vector<double,2> point_down (
                const vector<T,2>& p,
                unsigned int levels
            ) const
            {
                vector<double,2> temp = p;
                for (unsigned int i = 0; i < levels; ++i)
                    temp = point_down(temp);
                return temp;
            }

            template <typename T>
            vector<double,2> point_up (
                const vector<T,2>& p,
                unsigned int levels
            ) const
            {
                vector<double,2> temp = p;
                for (unsigned int i = 0; i < levels; ++i)
                    temp = point_up(temp);
                return temp;
            }

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

            rectangle rect_up (
                const rectangle& rect
            ) const
            {
                return rectangle(point_up(rect.tl_corner()), point_up(rect.br_corner()));
            }

            rectangle rect_up (
                const rectangle& rect,
                unsigned int levels
            ) const
            {
                return rectangle(point_up(rect.tl_corner(),levels), point_up(rect.br_corner(),levels));
            }

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

            rectangle rect_down (
                const rectangle& rect
            ) const
            {
                return rectangle(point_down(rect.tl_corner()), point_down(rect.br_corner()));
            }

            rectangle rect_down (
                const rectangle& rect,
                unsigned int levels
            ) const
            {
                return rectangle(point_down(rect.tl_corner(),levels), point_down(rect.br_corner(),levels));
            }

            template <
                typename in_image_type,
                typename out_image_type
                >
            void operator() (
                const in_image_type& original,
                out_image_type& down
            ) const
            {
                // make sure requires clause is not broken
                DLIB_ASSERT(is_same_object(original, down) == false,
                            "\t void pyramid_down::operator()"
                            << "\n\t is_same_object(original, down): " << is_same_object(original, down)
                            << "\n\t this:                           " << this
                            );

                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 );

                down.set_size(((N-1)*original.nr())/N,
                              ((N-1)*original.nc())/N);
                resize_image(original, down);
            }
        };

        template <>
        class pyramid_down<1> : public pyramid_disable {};

        template <>
        class pyramid_down<2> : public dlib::impl::pyramid_down_2_1 {};

        template <>
        class pyramid_down<3> : public dlib::impl::pyramid_down_3_2 {};

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

    }

    #endif // DLIB_IMAGE_PYRaMID_H__