Champfer distance transform
===========================

.. code:: cpp

    template <typename Img>
    concept bool Image_with_Neighborhood_and_Extension =
        Image<Img> &&
        Neighborhood<Img> &&
        Extension<Img>;

    template <
        Image_with_Neighborhood_and_Extension Img,
        ConvexDomain<Img>,
        Applyable<Img::neiborhood_type, NeighborhoodMask>
    >
    Image chamfer_distance_transform(const Img& ima, const NeighborhoodMask& nbh_mask) {
        auto ima_res = copy(ima);
        extension::fill(ima, numeric_limits<Img::value_type>::max());

        // forward pass
        for(auto [&pix_res, pix] : zip(ima_res.pixels(), ima.pixels())) {
            if(pix->val()) {
                pix_res.val() = accumulate(
                    nbh_mask(pix_res.neighbors()),
                    numeric_limits<Img::value_type>::max(),
                    [&pix_res](auto vmin, auto&& neighbor) {
                        if (neighbor->index() < pix_res->index()) {
                            return min(vmin, neighbor->val() + neighbor->weight())
                        }
                    }
                );
            } else {
                pix_res->val() = 0;
            }
        }

        // backward pass
        for(auto [&pix_res, pix] : zip_backward(ima_res.pixels(), ima.pixels())) {
            pix_res.val() = accumulate(
                nbh_mask(pix_res.neighbors()),
                numeric_limits<Img::value_type>::max(),
                [&pix_res](auto vmin, auto&& neighbor) {
                    if (neighbor->index() > pix_res->index()) {
                        return min(vmin, neighbor->val() + neighbor->weight())
                    }
                }
            );
        }

        return ima_res;
    }