To this point, you built
an image classifier that worked using
a deep neural network and you saw how to
improve its performance by adding convolutions.
One limitation though was that it used
a dataset of very uniform images.
Images of clothing that was
staged and framed in 28 by 28.
But what happens when you use larger images and
where the feature might be in different locations?
For example, how about these images of horses and humans?
They have different sizes and different aspect ratios.
The subject can be in different locations.
In some cases, there may even be multiple subjects.
In addition to that,
the earlier examples with
a fashion data used a built-in dataset.
All of the data was handily split into training
and test sets for you and labels were available.
In many scenarios, that's not going to be
the case and you'll have to do it for yourself.
So in this lesson, we'll take a look at some of
the APIs that are available to make that easier for you.
In particular, the image generator in TensorFlow.
One feature of the image generator is
that you can point it at a directory and
then the sub-directories of that will
automatically generate labels for you.
So for example, consider this directory structure.
You have an images directory and in that,
you have sub-directories for training and validation.
When you put sub-directories in these for horses
and humans and store the requisite images in there,
the image generator can create a feeder
for those images and auto label them for you.
So for example, if I point
an image generator at the training directory,
the labels will be horses and humans and all of
the images in each directory
will be loaded and labeled accordingly.
Similarly, if I point one at the validation directory,
the same thing will happen.
So let's take a look at this in code.
The image generator class
is available in Keras.preprocessing.image.
You can then instantiate an image generator like this.
I'm going to pass rescale to it to normalize the data.
You can then call the flow from
directory method on it to get it to
load images from that directory and its sub-directories.
It's a common mistake that people
point the generator at the sub-directory.
It will fail in that circumstance.
You should always point it at the directory that
contains sub-directories that contain your images.
The names of the sub-directories will be
the labels for your images
that are contained within them.
So make sure that the directory you're
pointing to is the correct one.
You put it in the second parameter like this.
Now, images might come in all shapes and
sizes and unfortunately for training a neural network,
the input data all has to be the same size,
so the images will need to be
resized to make them consistent.
The nice thing about this code is that
the images are resized for you as they're loaded.
So you don't need to preprocess
thousands of images on your file system.
But you could have done that if you wanted to.
The advantage of doing it at
runtime like this is that you can then
experiment with different sizes
without impacting your source data.
While the horses and humans dataset
is already in 300 by 300,
when you use other datasets they may
not always be uniformly sized.
So this is really useful for you.
The images will be loaded for training and validation in
batches where it's more
efficient than doing it one by one.
Now, there's a whole science to calculating
batch size that's beyond the scope of this course,
but you can experiment with different sizes to see
the impact on the performance by changing this parameter.
Finally, there's the class mode.
Now, this is a binary classifier i.e.
it picks between two different things;
horses and humans, so we specify that here.
Other options in particular for
more than two things
will be explored later in the course.
The validation generator should be exactly the
same except of course it points at a different directory,
the one containing the sub-directories
containing the test images.
When you go through the workbook shortly,
you'll see how to download the images as a zip,
and then sort them into
training and test sub-directories,
and then put horses and humans sub-directories in each.
That's just pure Python.
It's not TensorFlow or any other deep learning stuff.
But it's all explained for you in the notebook.
Laurence MoroneyAI Advocate
