YouTube Transcript:
What is a convolutional neural network (CNN)?

(light music)
Narrator: What is a convolutional neural network?
Let's start with the basics.
A convolutional neural network is a type of neural network
that is most often applied to image processing problems.
You've probably seen them in action
anywhere a computer is identifying objects in an image,
but you can also use convolutional neural networks
in natural language processing projects too.
The fact that they are useful for these fast-growing areas
is one of the main reasons they're so important
in deep learning and artificial intelligence today.
Once you understand how a convolutional neural network works
and what makes it unique from other neural networks,
you can see why they're so effective
for processing and classifying images.
But let's first take a regular neural network.
A regular neural network has an input layer,
hidden layers, and an output layer.
The input layer accepts inputs in different forms
while the hidden layers perform calculations
on these inputs.
The output layer then delivers the outcome
of the calculations and extractions.
Each of these layers contain neurons
that are connected to neurons in the previous layer
and each neuron has its own weight.
This means you aren't making any assumptions
about the data being fed into the network.
Great usually but not if you're working with images
or a language.
Convolutional neural networks work differently
as they treat data as spatial.
Instead of neurons being connected to every neuron
in the previous layer, they are instead only connected
to neurons close to it and all have the same weight.
The simplification in the connections
means the network upholds
the spatial aspect of the data set.
It means your network doesn't think an eye
is all over the image.
The word convolutional refers to the filtering process
that happens in this type of network.
Think of it this way, an image is complex.
A convolutional neural network simplifies it,
so it can be better processed and understood.
Let's look now at what's inside
a convolutional neural network.
Like a normal neural network,
a convolutional neural network
is made up of multiple layers.
There are a couple of layers that make it unique,
the convolutional layer and the pooling layer.
However, like other neural networks,
it will also have a ReLU or a rectified linear unit layer
and a fully connected layer.
The ReLU layer acts as an activation function,
ensuring non-linearity as the data moves through each layer
in the network.
Without it, the data being fed into each layer
would lose the dimensionality that we want to maintain.
The fully connected layer meanwhile
allows you to perform classification on your data set.
The convolutional layer is the most important,
so let's start there.
It works by placing a filter over an array of image pixels.
This then creates what's called a convolved feature map.
It's a bit like looking at an image through a window,
which allows you to see specific features
you might not otherwise be able to see.
Next, we have the pooling layer.
This downsamples or reduces the sample size
of a particular feature map.
This also makes processing much faster
as it reduces the number of parameters
the network needs to process.
The output of this is a pooled feature map.
There are two ways of doing this:
max pooling, which takes the maximum input
of a particular convolved feature,
or average pooling, which simply takes the average.
These steps amount to feature extraction
whereby the network builds up a picture of the image data
according to its own mathematical rules.
If you want to perform classification,
you'll need to move into the fully connected layer.
To do this, you'll need to flatten things out.
Remember, a neural network with a more complex set
of connections can only process linear data.
There are a number of ways you can train
a convolutional neural network.
If you're working with unlabeled data,
you can use unsupervised learning methods.
One of the best popular ways of doing this
is using autoencoders.
This allows you to squeeze data
in a space with low dimensions,
performing calculations in the first part
of the convolutional neural network.
Once this is done, you'll then need to reconstruct
with additional layers that upsample the data you have.
Another option is to use
generative adversarial networks or GANs.
With a GAN, you train two networks.
The first gives you artificial data samples
that should resemble data in the training set
while the second is a discriminative network.
It should distinguish
between the artificial and the true model.
A lot of people seem to be asking what the difference is
between a convolutional neural network
and a recurrent network.
It's actually quite simple.
Whereas a convolutional neural network
is a feedforward network that filters spatial data,
a recurrent neural network, as the name implies,
feeds data back into itself.
From this perspective, recurrent neural networks
are better suited to sequential data.
Think of it like this, a convolutional network
is able to perceive patterns across space,
a recurrent neural network can see them over time.
If you want to get started
with convolutional neural networks,
Python and TensorFlow are great tools to begin with.
It's worth exploring MNIST data set too.
This is a database of handwritten digits
that you can use to get started
with building your first convolutional neural network.
If you want to learn more
about convolutional neural networks, deep learning, and AI,
visit Packt today.