dlib C++ Library - thresholding.h

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

    #include "../pixel.h"
    #include "thresholding_abstract.h"
    #include "equalize_histogram.h"

    namespace dlib
    {

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

        const unsigned char on_pixel = 255;
        const unsigned char off_pixel = 0;

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

        template <
            typename in_image_type,
            typename out_image_type
            >
        void threshold_image (
            const in_image_type& in_img,
            out_image_type& out_img,
            unsigned long thresh
        )
        {
            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 );

            COMPILE_TIME_ASSERT(pixel_traits<typename out_image_type::type>::grayscale);

            // 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());

            for (long r = 0; r < in_img.nr(); ++r)
            {
                for (long c = 0; c < in_img.nc(); ++c)
                {
                    typename out_image_type::type p;
                    assign_pixel(p,in_img[r][c]);
                    if (p >= thresh)
                        p = on_pixel;
                    else
                        p = off_pixel;
                    out_img[r][c] = p;
                }
            }
        }

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

        template <
            typename in_image_type,
            typename out_image_type
            >
        void auto_threshold_image (
            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 );

            COMPILE_TIME_ASSERT(pixel_traits<typename out_image_type::type>::grayscale);

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

            unsigned long thresh;
            // find the threshold we should use
            matrix<unsigned long,1> hist;
            get_histogram(in_img,hist);

            // Start our two means (a and b) out at the ends of the histogram
            long a = 0;
            long b = hist.size()-1;
            bool moved_a = true;
            bool moved_b = true;
            while (moved_a || moved_b)
            {
                moved_a = false;
                moved_b = false;

                // catch the degenerate case where the histogram is empty
                if (a >= b)
                    break;

                if (hist(a) == 0)
                {
                    ++a;
                    moved_a = true;
                }

                if (hist(b) == 0)
                {
                    --b;
                    moved_b = true;
                }
            }

            // now do k-means clustering with k = 2 on the histogram.
            moved_a = true;
            moved_b = true;
            while (moved_a || moved_b)
            {
                moved_a = false;
                moved_b = false;

                long a_hits = 0;
                long b_hits = 0;
                long a_mass = 0;
                long b_mass = 0;

                for (long i = 0; i < hist.size(); ++i)
                {
                    // if i is closer to a
                    if (std::abs(i-a) < std::abs(i-b))
                    {
                        a_mass += hist(i)*i;
                        a_hits += hist(i);
                    }
                    else // if i is closer to b
                    {
                        b_mass += hist(i)*i;
                        b_hits += hist(i);
                    }
                }

                long new_a = (a_mass + a_hits/2)/a_hits;
                long new_b = (b_mass + b_hits/2)/b_hits;

                if (new_a != a)
                {
                    moved_a = true;
                    a = new_a;
                }

                if (new_b != b)
                {
                    moved_b = true;
                    b = new_b;
                }
            }

            // put the threshold between the two means we found
            thresh = (a + b)/2;

            // now actually apply the threshold
            threshold_image(in_img,out_img,thresh);
        }

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

        template <
            typename in_image_type,
            typename out_image_type
            >
        void hysteresis_threshold (
            const in_image_type& in_img,
            out_image_type& out_img,
            unsigned long lower_thresh,
            unsigned long upper_thresh
        )
        {
            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 );

            COMPILE_TIME_ASSERT(pixel_traits<typename out_image_type::type>::grayscale);

            DLIB_ASSERT( lower_thresh <= upper_thresh,
                "\tvoid hysteresis_threshold(in_img, out_img, lower_thresh, upper_thresh)"
                << "\n\tYou can't use an upper_thresh that is less than your lower_thresh"
                << "\n\tlower_thresh: " << lower_thresh
                << "\n\tupper_thresh: " << upper_thresh
                );

            // 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());
            assign_all_pixels(out_img,0);

            const long size = 50;
            long rstack[size];
            long cstack[size];

            // now do the thresholding
            for (long r = 0; r < in_img.nr(); ++r)
            {
                for (long c = 0; c < in_img.nc(); ++c)
                {
                    unsigned long p;
                    assign_pixel(p,in_img[r][c]);
                    if (p >= upper_thresh)
                    {
                        // now do line following for pixels >= lower_thresh.
                        // set the stack position to 0.
                        long pos = 1;
                        rstack[0] = r;
                        cstack[0] = c;

                        while (pos > 0)
                        {
                            --pos;
                            const long r = rstack[pos];
                            const long c = cstack[pos];

                            // This is the base case of our recursion.  We want to stop if we hit a
                            // pixel we have already visited.
                            if (out_img[r][c] == on_pixel)
                                continue;

                            out_img[r][c] = on_pixel;

                            // put the neighbors of this pixel on the stack if they are bright enough
                            if (r-1 >= 0)
                            {
                                if (pos < size && get_pixel_intensity(in_img[r-1][c]) >= lower_thresh)
                                {
                                    rstack[pos] = r-1;
                                    cstack[pos] = c;
                                    ++pos;
                                }
                                if (pos < size && c-1 >= 0 && get_pixel_intensity(in_img[r-1][c-1]) >= lower_thresh)
                                {
                                    rstack[pos] = r-1;
                                    cstack[pos] = c-1;
                                    ++pos;
                                }
                                if (pos < size && c+1 < in_img.nc() && get_pixel_intensity(in_img[r-1][c+1]) >= lower_thresh)
                                {
                                    rstack[pos] = r-1;
                                    cstack[pos] = c+1;
                                    ++pos;
                                }
                            }

                            if (pos < size && c-1 >= 0 && get_pixel_intensity(in_img[r][c-1]) >= lower_thresh)
                            {
                                rstack[pos] = r;
                                cstack[pos] = c-1;
                                ++pos;
                            }
                            if (pos < size && c+1 < in_img.nc() && get_pixel_intensity(in_img[r][c+1]) >= lower_thresh)
                            {
                                rstack[pos] = r;
                                cstack[pos] = c+1;
                                ++pos;
                            }

                            if (r+1 < in_img.nr())
                            {
                                if (pos < size && get_pixel_intensity(in_img[r+1][c]) >= lower_thresh)
                                {
                                    rstack[pos] = r+1;
                                    cstack[pos] = c;
                                    ++pos;
                                }
                                if (pos < size && c-1 >= 0 && get_pixel_intensity(in_img[r+1][c-1]) >= lower_thresh)
                                {
                                    rstack[pos] = r+1;
                                    cstack[pos] = c-1;
                                    ++pos;
                                }
                                if (pos < size && c+1 < in_img.nc() && get_pixel_intensity(in_img[r+1][c+1]) >= lower_thresh)
                                {
                                    rstack[pos] = r+1;
                                    cstack[pos] = c+1;
                                    ++pos;
                                }
                            }

                        } // end while (pos >= 0)

                    }
                    else
                    {
                        out_img[r][c] = off_pixel;
                    }

                }
            }
        }

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

    }

    #endif // DLIB_THRESHOLDINg_