# Pre-trained Networks and Transfer Learning

Training CNNs can take a lot of time, and a lot of data is required for that task. However, much of the time is spent to learn the best low-level filters that a network is using to extract patterns from images. A natural question arises - can we use a neural network trained on one dataset and adapt it to classifying different images without full training process?

This approach is called **transfer learning**, because we transfer some knowledge from one neural network model to another. In transfer learning, we typically start with a pre-trained model, which has been trained on some large image dataset, such as **ImageNet**. Those models can already do a good job extracting different features from generic images, and in many cases just building a classifier on top of those extracted features can yield a good result.

## Pre-Trained Models as Feature Extractors

Convolutional networks that we have talked about in previous section contained a number of layers, each of which is supposed to extract some features from the image, starting from low-level pixel combinations (such as horizontal/vertical line or stroke), up to higher level combinations of features, corresponding to things like an eye of a flame. If we train CNN on sufficiently large dataset of generic and diverse images, the network should learn to extract those common features.

Both Keras and PyTorch contain functions to easily load pre-trained neural network weights for some common architectures, most of which were trained on ImageNet images. The most often used ones are described in [CNN Architectures](../07-ConvNets/CNN_Architectures.md) page. In particular, you may want to consider using one of the following:

* **VGG-16/VGG-19** are relatively simple models, but they give good accuracy. Often using VGG as a first attempt is a good choice to see how transfer learning is working.
* **ResNet** is a family of models proposed by Microsoft Research in 2015. They have more layers, and thus take more resources.
* **MobileNet** is a family of models with reduced size, suitable for mobile devices. Use them if you are short in resources, and can sacrifice a little bit of accuracy.

Here are sample features extracted from a picture of a cat by VGG-16 network:

![Features extracted by VGG-16](images/features.png)

## Cats vs. Dogs Dataset

In this example, we will use a dataset of [Cats and Dogs](https://www.microsoft.com/en-us/download/details.aspx?id=54765&WT.mc_id=academic-33554-dmitryso), which is very close to a real-life image classification scenario. 

## Continue in Notebook

Let's see transfer learning in action in corresponding notebooks:

* [Transfer Learning - PyTorch](TransferLearningPyTorch.ipynb)
* [Transfer Learning - TensorFlow](TransferLearningTF.ipynb)

