This guide trains a neural network model to classify images of clothing, like sneakers and shirts.
This guide uses tf.keras, a high-level API to build and train models in TensorFlow.
# TensorFlow and tf.keras import tensorflow as tf # Helper libraries import numpy as np import matplotlib.pyplot as plt print(tf.__version__)
Import the Fashion MNIST dataset
This guide uses the Fashion MNIST dataset which contains 70,000 grayscale images in 10 categories. The images show individual articles of clothing at low resolution (28 by 28 pixels), as seen here:
Fashion MNIST is intended as a drop-in replacement for the classic MNIST dataset—often used as the "Hello, World" of machine learning programs for computer vision. The MNIST dataset contains images of handwritten digits (0, 1, 2, etc.) in a format identical to that of the articles of clothing you'll use here.
This guide uses Fashion MNIST for variety, and because it's a slightly more challenging problem than regular MNIST. Both datasets are relatively small and are used to verify that an algorithm works as expected. They're good starting points to test and debug code.
Here, 60,000 images are used to train the network and 10,000 images to evaluate how accurately the network learned to classify images. You can access the Fashion MNIST directly from TensorFlow. Import and load the Fashion MNIST data directly from TensorFlow:
fashion_mnist = tf.keras.datasets.fashion_mnist (train_images, train_labels), (test_images, test_labels) = fashion_mnist.load_data()
The images are 28x28 NumPy arrays, with pixel values ranging from 0 to 255. The labels are an array of integers, ranging from 0 to 9. These correspond to the class of clothing the image represents:
Each image is mapped to a single label. Since the class names are not included with the dataset, store them here to use later when plotting the images:
class_names = ['T-shirt/top', 'Trouser', 'Pullover', 'Dress', 'Coat', 'Sandal', 'Shirt', 'Sneaker', 'Bag', 'Ankle boot']
Preprocess the data
Scale these values to a range of 0 to 1 before feeding them to the neural network model. To do so, divide the values by 255. It's important that the training set and the testing set be preprocessed in the same way:
train_images = train_images / 255.0 test_images = test_images / 255.0
Build the model
Building the neural network requires configuring the layers of the model, then compiling the model.
Set up the layers
The basic building block of a neural network is the layer. Layers extract representations from the data fed into them. Hopefully, these representations are meaningful for the problem at hand.
Most of deep learning consists of chaining together simple layers. Most layers, such as tf.keras.layers.Dense, have parameters that are learned during training.
model = tf.keras.Sequential([ tf.keras.layers.Flatten(input_shape=(28, 28)), tf.keras.layers.Dense(128, activation='relu'), tf.keras.layers.Dense(10) ])
The first layer in this network, tf.keras.layers.Flatten, transforms the format of the images from a two-dimensional array (of 28 by 28 pixels) to a one-dimensional array (of 28 * 28 = 784 pixels). Think of this layer as unstacking rows of pixels in the image and lining them up. This layer has no parameters to learn; it only reformats the data.
After the pixels are flattened, the network consists of a sequence of two tf.keras.layers.Dense layers. These are densely connected, or fully connected, neural layers. The first Dense layer has 128 nodes (or neurons). The second (and last) layer returns a logits array with length of 10. Each node contains a score that indicates the current image belongs to one of the 10 classes.
Compile the model
Before the model is ready for training, it needs a few more settings. These are added during the model's compile step:
- Loss function —This measures how accurate the model is during training. You want to minimize this function to "steer" the model in the right direction.
- Optimizer —This is how the model is updated based on the data it sees and its loss function.
- Metrics —Used to monitor the training and testing steps. The following example uses accuracy, the fraction of the images that are correctly classified.
model.compile(optimizer='adam', loss=tf.keras.losses.SparseCategoricalCrossentropy(from_logits=True), metrics=['accuracy'])
Train the model
Training the neural network model requires the following steps:
- Feed the training data to the model. In this example, the training data is in the
- The model learns to associate images and labels.
- You ask the model to make predictions about a test set—in this example, the
- Verify that the predictions match the labels from the
Feed the model
To start training, call the model.fit method—so called because it "fits" the model to the training data:
model.fit(train_images, train_labels, epochs=10)
As the model trains, the loss and accuracy metrics are displayed.
test_loss, test_acc = model.evaluate(test_images, test_labels, verbose=2) print('Test accuracy:', test_acc)
It turns out that the accuracy on the test dataset is a little less than the accuracy on the training dataset. This gap between training accuracy and test accuracy represents overfitting. Overfitting happens when a machine learning model performs worse on new, previously unseen inputs than it does on the training data. An overfitted model "memorizes" the noise and details in the training dataset to a point where it negatively impacts the performance of the model on the new data
With the model trained, you can use it to make predictions about some images. The model's linear outputs, logits. Attach a softmax layer to convert the logits to probabilities, which are easier to interpret.
probability_model = tf.keras.Sequential([model, tf.keras.layers.Softmax()]) predictions = probability_model.predict(test_images)
Use the trained model
Finally, use the trained model to make a prediction about a single image.
# Grab an image from the test dataset. img = test_images print(img.shape)
tf.keras models are optimized to make predictions on a batch, or collection, of examples at once. Accordingly, even though you're using a single image, you need to add it to a list:
# Add the image to a batch where it's the only member. img = (np.expand_dims(img,0)) print(img.shape)
Now predict the correct label for this image:
predictions_single = probability_model.predict(img) print(predictions_single)