# dlib C++ Library - dnn_introduction3_ex.cpp

    // The contents of this file are in the public domain. See LICENSE_FOR_EXAMPLE_PROGRAMS.txt
    /*
        This is an example illustrating the use of the deep learning tools from the
        dlib C++ Library.  I'm assuming you have already read the dnn_introduction_ex.cpp and
        the dnn_introduction2_ex.cpp examples.  So in this example program I'm going to go
        over a transfer learning example, which includes:
            - Defining a layer visitor to modify the some network parameters for fine-tuning
            - Using pretrained layers of a network for another task
    */

    #include <dlib/dnn.h>
    #include <iostream>

    // This header file includes a generic definition of the most common ResNet architectures
    #include "resnet.h"

    using namespace std;
    using namespace dlib;

    // In this simple example we will show how to load a pretrained network and use it for a
    // different task.  In particular, we will load a ResNet50 trained on ImageNet, adjust
    // some of its parameters and use it as a pretrained backbone for some metric learning
    // task.

    // Let's start by defining a network that will use the ResNet50 backbone from resnet.h
    namespace model
    {
        template<template<typename> class BN>
        using net_type = loss_metric<
            fc_no_bias<128,
            avg_pool_everything<
            typename resnet::def<BN>::template backbone_50<
            input_rgb_image
            >>>>;

        using train = net_type<bn_con>;
        using infer = net_type<affine>;
    }

    // Next, we define a layer visitor that will modify the weight decay of a network.  The
    // main interest of this class is to show how one can define custom visitors that modify
    // some network parameters.
    class visitor_weight_decay_multiplier
    {
    public:

        visitor_weight_decay_multiplier(double new_weight_decay_multiplier_) :
            new_weight_decay_multiplier(new_weight_decay_multiplier_) {}

        template <typename layer>
        void operator()(layer& l) const
        {
            set_weight_decay_multiplier(l, new_weight_decay_multiplier);
        }

    private:

        double new_weight_decay_multiplier;
    };

    int main() try
    {
        // Let's instantiate our network in train mode.
        model::train net;

        // We create a new scope so that resources from the loaded network are freed
        // automatically when leaving the scope.
        {
            // Now, let's define the classic ResNet50 network and load the pretrained model on
            // ImageNet.
            resnet::train_50 resnet50;
            std::vector<string> labels;
            deserialize("resnet50_1000_imagenet_classifier.dnn") >> resnet50 >> labels;

            // For transfer learning, we are only interested in the ResNet50's backbone, which
            // lays below the loss and the fc layers, so we can extract it as:
            auto backbone = std::move(resnet50.subnet().subnet());

            // We can now assign ResNet50's backbone to our network skipping the different
            // layers, in our case, the loss layer and the fc layer:
            net.subnet().subnet() = backbone;

            // An alternative way to use the pretrained network on a different
            // network is to extract the relevant part of the network (we remove
            // loss and fc layers), stack the new layers on top of it and assign
            // the network.
            using net_type = loss_metric<fc_no_bias<128, decltype(backbone)>>;
            net_type net2;
            net2.subnet().subnet() = backbone;
        }

        // We can use the visit_layers function to modify the weight decay of the entire
        // network:
        visit_computational_layers(net, visitor_weight_decay_multiplier(0.001));

        // We can also use predefined visitors to affect the learning rate of the whole
        // network.
        set_all_learning_rate_multipliers(net, 0.5);

        // Modifying the learning rates of a network is a common practice for fine tuning, for
        // this reason it is already provided. However, it is implemented internally using a
        // visitor that is very similar to the one defined in this example.

        // Usually, we want to freeze the network, except for the top layers:
        visit_computational_layers(net.subnet().subnet(), visitor_weight_decay_multiplier(0));
        set_all_learning_rate_multipliers(net.subnet().subnet(), 0);

        // Alternatively, we can use the visit_layers_range to modify only a specific set of
        // layers:
        visit_computational_layers_range<0, 2>(net, visitor_weight_decay_multiplier(1));

        // Sometimes we might want to set the learning rate differently throughout the network.
        // Here we show how to use adjust the learning rate at the different ResNet50's
        // convolutional blocks:
        set_learning_rate_multipliers_range<  0,   2>(net, 1);
        set_learning_rate_multipliers_range<  2,  38>(net, 0.1);
        set_learning_rate_multipliers_range< 38, 107>(net, 0.01);
        set_learning_rate_multipliers_range<107, 154>(net, 0.001);
        set_learning_rate_multipliers_range<154, 193>(net, 0.0001);

        // Finally, we can check the results by printing the network.  But before, if we
        // forward an image through the network, we will see tensors shape at every layer.
        matrix<rgb_pixel> image(224, 224);
        assign_all_pixels(image, rgb_pixel(0, 0, 0));
        std::vector<matrix<rgb_pixel>> minibatch(1, image);
        resizable_tensor input;
        net.to_tensor(minibatch.begin(), minibatch.end(), input);
        net.forward(input);
        cout << net << endl;
        cout << "input size=(" <<
           "num:" << input.num_samples() << ", " <<
           "k:" << input.k() << ", " <<
           "nr:" << input.nr() << ", "
           "nc:" << input.nc() << ")" << endl;

        // We can also print the number of parameters of the network:
        cout << "number of network parameters: " << count_parameters(net) << endl;

        // From this point on, we can fine-tune the new network using this pretrained backbone
        // on another task, such as the one showed in dnn_metric_learning_on_images_ex.cpp.

        return EXIT_SUCCESS;
    }
    catch (const serialization_error& e)
    {
        cout << e.what() << endl;
        cout << "You need to download a copy of the file resnet50_1000_imagenet_classifier.dnn" << endl;
        cout << "available at http://dlib.net/files/resnet50_1000_imagenet_classifier.dnn.bz2" << endl;
        cout << endl;
        return EXIT_FAILURE;
    }
    catch (const exception& e)
    {
        cout << e.what() << endl;
        return EXIT_FAILURE;
    }