Implementing a Semi-Supervised GAN (SGAN) From Scratch in Keras

Semi-supervised learning is the challenging problem of training a classifier in a dataset that contains a small number of labeled examples and a much larger number of unlabeled examples.

The Generative Adversarial Network, or GAN, is an architecture that makes effective use of large, unlabeled datasets to train an image generator model via an image discriminator model. The discriminator model can be used as a starting point for developing a classifier model in some cases.

The semi-supervised GAN, or SGAN, model is an extension of the GAN architecture that involves the simultaneous training of a supervised discriminator, unsupervised discriminator, and a generator model. The result is both a supervised classification model that generalizes well to unseen examples and a generator model that outputs plausible examples of images from the domain.

In this tutorial, you will discover how to develop a Semi-Supervised Generative Adversarial Network from scratch.

After completing this tutorial, you will know:

• The semi-supervised GAN is an extension of the GAN architecture for training a classifier model while making use of labeled and unlabeled data.
• There are at least three approaches to implementing the supervised and unsupervised discriminator models in Keras used in the semi-supervised GAN.
• How to train a semi-supervised GAN from scratch on MNIST and load and use the trained classifier for making predictions.

Discover how to develop DCGANs, conditional GANs, Pix2Pix, CycleGANs, and more with Keras in my new GANs book, with 29 step-by-step tutorials and full source code.

Let’s get started.

Tutorial Overview

This tutorial is divided into four parts; they are:

1. What Is the Semi-Supervised GAN?
2. How to Implement the Semi-Supervised Discriminator Model
3. How to Develop a Semi-Supervised GAN for MNIST
4. How to Load and Use the Final SGAN Classifier Model

What Is the Semi-Supervised GAN?

Semi-supervised learning refers to a problem where a predictive model is required and there are few labeled examples and many unlabeled examples.

The most common example is a classification predictive modeling problem in which there may be a very large dataset of examples, but only a small fraction have target labels. The model must learn from the small set of labeled examples and somehow harness the larger dataset of unlabeled examples in order to generalize to classifying new examples in the future.

The Semi-Supervised GAN, or sometimes SGAN for short, is an extension of the Generative Adversarial Network architecture for addressing semi-supervised learning problems.

One of the primary goals of this work is to improve the effectiveness of generative adversarial networks for semi-supervised learning (improving the performance of a supervised task, in this case, classification, by learning on additional unlabeled examples).

— Improved Techniques for Training GANs, 2016.

The discriminator in a traditional GAN is trained to predict whether a given image is real (from the dataset) or fake (generated), allowing it to learn features from unlabeled images. The discriminator can then be used via transfer learning as a starting point when developing a classifier for the same dataset, allowing the supervised prediction task to benefit from the unsupervised training of the GAN.

In the Semi-Supervised GAN, the discriminator model is updated to predict K+1 classes, where K is the number of classes in the prediction problem and the additional class label is added for a new “fake” class. It involves directly training the discriminator model for both the unsupervised GAN task and the supervised classification task simultaneously.

We train a generative model G and a discriminator D on a dataset with inputs belonging to one of N classes. At training time, D is made to predict which of N+1 classes the input belongs to, where an extra class is added to correspond to the outputs of G.

— Semi-Supervised Learning with Generative Adversarial Networks, 2016.

As such, the discriminator is trained in two modes: a supervised and unsupervised mode.

• Unsupervised Training: In the unsupervised mode, the discriminator is trained in the same way as the traditional GAN, to predict whether the example is either real or fake.
• Supervised Training: In the supervised mode, the discriminator is trained to predict the class label of real examples.

Training in unsupervised mode allows the model to learn useful feature extraction capabilities from a large unlabeled dataset, whereas training in supervised mode allows the model to use the extracted features and apply class labels.

The result is a classifier model that can achieve state-of-the-art results on standard problems such as MNIST when trained on very few labeled examples, such as tens, hundreds, or one thousand. Additionally, the training process can also result in better quality images output by the generator model.

For example, Augustus Odena in his 2016 paper titled “Semi-Supervised Learning with Generative Adversarial Networks” shows how a GAN-trained classifier is able to perform as well as or better than a standalone CNN model on the MNIST handwritten digit recognition task when trained with 25, 50, 100, and 1,000 labeled examples.

Tim Salimans, et al. from OpenAI in their 2016 paper titled “Improved Techniques for Training GANs” achieved at the time state-of-the-art results on a number of image classification tasks using a semi-supervised GAN, including MNIST.

How to Implement the Semi-Supervised Discriminator Model

There are a number of ways that we can implement the discriminator model for the semi-supervised GAN.

In this section, we will review three candidate approaches.

Traditional Discriminator Model

Consider a discriminator model for the standard GAN model.

It must take an image as input and predict whether it is real or fake. More specifically, it predicts the likelihood of the input image being real. The output layer uses a sigmoid activation function to predict a probability value in [0,1] and the model is typically optimized using a binary cross entropy loss function.

For example, we can define a simple discriminator model that takes grayscale images as input with the size of 28×28 pixels and predicts a probability of the image being real. We can use best practices and downsample the image using convolutional layers with a 2×2 stride and a leaky ReLU activation function.

The define_discriminator() function below implements this and defines our standard discriminator model.

Running the example creates a plot of the discriminator model, clearly showing the 28x28x1 shape of the input image and the prediction of a single probability value.

Separate Discriminator Models With Shared Weights

Starting with the standard GAN discriminator model, we can update it to create two models that share feature extraction weights.

Specifically, we can define one classifier model that predicts whether an input image is real or fake, and a second classifier model that predicts the class of a given model.

• Binary Classifier Model. Predicts whether the image is real or fake, sigmoid activation function in the output layer, and optimized using the binary cross entropy loss function.
• Multi-Class Classifier Model. Predicts the class of the image, softmax activation function in the output layer, and optimized using the categorical cross entropy loss function.

Both models have different output layers but share all feature extraction layers. This means that updates to one of the classifier models will impact both models.

The example below creates the traditional discriminator model with binary output first, then re-uses the feature extraction layers and creates a new multi-class prediction model, in this case with 10 classes.

Running the example creates and plots both models.

The plot for the first model is the same as before.

The plot of the second model shows the same expected input shape and same feature extraction layers, with a new 10 class classification output layer.

Single Discriminator Model With Multiple Outputs

Another approach to implementing the semi-supervised discriminator model is to have a single model with multiple output layers.

Specifically, this is a single model with one output layer for the unsupervised task and one output layer for the supervised task.

This is like having separate models for the supervised and unsupervised tasks in that they both share the same feature extraction layers, except that in this case, each input image always has two output predictions, specifically a real/fake prediction and a supervised class prediction.

A problem with this approach is that when the model is updated unlabeled and generated images, there is no supervised class label. In that case, these images must have an output label of “unknown” or “fake” from the supervised output. This means that an additional class label is required for the supervised output layer.

The example below implements the multi-output single model approach for the discriminator model in the semi-supervised GAN architecture.

We can see that the model is defined with two output layers and that the output layer for the supervised task is defined with n_classes + 1. in this case 11, making room for the additional “unknown” class label.

We can also see that the model is compiled to two loss functions, one for each output layer of the model.

Running the example creates and plots the single multi-output model.

The plot clearly shows the shared layers and the separate unsupervised and supervised output layers.

Stacked Discriminator Models With Shared Weights

A final approach is very similar to the prior two approaches and involves creating separate logical unsupervised and supervised models but attempts to reuse the output layers of one model to feed as input into another model.

The approach is based on the definition of the semi-supervised model in the 2016 paper by Tim Salimans, et al. from OpenAI titled “Improved Techniques for Training GANs.”

In the paper, they describe an efficient implementation, where first the supervised model is created with K output classes and a softmax activation function. The unsupervised model is then defined that takes the output of the supervised model prior to the softmax activation, then calculates a normalized sum of the exponential outputs.

To make this clearer, we can implement this activation function in NumPy and run some sample activations through it to see what happens.

The complete example is listed below.

Remember, the output of the unsupervised model prior to the softmax activation function will be the activations of the nodes directly. They will be small positive or negative values, but not normalized, as this would be performed by the softmax activation.

The custom activation function will output a value between 0.0 and 1.0.

A value close to 0.0 is output for a small or negative activation and a value close to 1.0 for a positive or large activation. We can see this when we run the example.

This means that the model is encouraged to output a strong class prediction for real examples, and a small class prediction or low activation for fake examples. It’s a clever trick and allows the re-use of the same output nodes from the supervised model in both models.

The activation function can be implemented almost directly via the Keras backend and called from a Lambda layer, e.g. a layer that will apply a custom function to the input to the layer.

The complete example is listed below. First, the supervised model is defined with a softmax activation and categorical cross entropy loss function. The unsupervised model is stacked on top of the output layer of the supervised model before the softmax activation, and the activations of the nodes pass through our custom activation function via the Lambda layer.

No need for a sigmoid activation function as we have already normalized the activation. As before, the unsupervised model is fit using binary cross entropy loss.

Running the example creates and plots the two models, which look much the same as the two models in the first example.

Stacked version of the unsupervised discriminator model:

Stacked version of the supervised discriminator model:

Now that we have seen how to implement the discriminator model in the semi-supervised GAN, we can develop a complete example for image generation and semi-supervised classification.

How to Develop a Semi-Supervised GAN for MNIST

In this section, we will develop a semi-supervised GAN model for the MNIST handwritten digit dataset.

The dataset has 10 classes for the digits 0-9, therefore the classifier model will have 10 output nodes. The model will be fit on the training dataset that contains 60,000 examples. Only 100 of the images in the training dataset will be used with labels, 10 from each of the 10 classes.

We will start off by defining the models.

We will use the stacked discriminator model, exactly as defined in the previous section.

Next, we can define the generator model. In this case, the generator model will take as input a point in the latent space and will use transpose convolutional layers to output a 28×28 grayscale image. The define_generator() function below implements this and returns the defined generator model.

The generator model will be fit via the unsupervised discriminator model.

We will use the composite model architecture, common to training the generator model when implemented in Keras. Specifically, weight sharing is used where the output of the generator model is passed directly to the unsupervised discriminator model, and the weights of the discriminator are marked as not trainable.

The define_gan() function below implements this, taking the already-defined generator and discriminator models as input and returning the composite model used to train the weights of the generator model.

We can load the training dataset and scale the pixels to the range [-1, 1] to match the output values of the generator model.

We can also define a function to select a subset of the training dataset in which we keep the labels and train the supervised version of the discriminator model.

The select_supervised_samples() function below implements this and is careful to ensure that the selection of examples is random and that the classes are balanced. The number of labeled examples is parameterized and set at 100, meaning that each of the 10 classes will have 10 randomly selected examples.

Next, we can define a function for retrieving a batch of real training examples.

A sample of images and labels is selected, with replacement. This same function can be used to retrieve examples from the labeled and unlabeled dataset, later when we train the models. In the case of the “unlabeled dataset“, we will ignore the labels.

Next, we can define functions to help in generating images using the generator model.

First, the generate_latent_points() function will create a batch worth of random points in the latent space that can be used as input for generating images. The generate_fake_samples() function will call this function to generate a batch worth of images that can be fed to the unsupervised discriminator model or the composite GAN model during training.

Next, we can define a function to be called when we want to evaluate the performance of the model.

This function will generate and plot 100 images using the current state of the generator model. This plot of images can be used to subjectively evaluate the performance of the generator model.

The supervised discriminator model is then evaluated on the entire training dataset, and the classification accuracy is reported. Finally, the generator model and the supervised discriminator model are saved to file, to be used later.

The summarize_performance() function below implements this and can be called periodically, such as the end of every training epoch. The results can be reviewed at the end of the run to select a classifier and even generator models.

Next, we can define a function to train the models. The defined models and loaded training dataset are provided as arguments, and the number of training epochs and batch size are parameterized with default values, in this case 20 epochs and a batch size of 100.

The chosen model configuration was found to overfit the training dataset quickly, hence the relatively smaller number of training epochs. Increasing the epochs to 100 or more results in much higher-quality generated images, but a lower-quality classifier model. Balancing these two concerns might make a fun extension.

First, the labeled subset of the training dataset is selected, and the number of training steps is calculated.

The training process is almost identical to the training of a vanilla GAN model, with the addition of updating the supervised model with labeled examples.

A single cycle through updating the models involves first updating the supervised discriminator model with labeled examples, then updating the unsupervised discriminator model with unlabeled real and generated examples. Finally, the generator model is updated via the composite model.

The shared weights of the discriminator model get updated with 1.5 batches worth of samples, whereas the weights of the generator model are updated with one batch worth of samples each iteration. Changing this so that each model is updated by the same amount might improve the model training process.

Finally, we can define the models and call the function to train and save the models.

Tying all of this together, the complete example of training a semi-supervised GAN on the MNIST handwritten digit image classification task is listed below.

The example can be run on a workstation with a CPU or GPU hardware, although a GPU is recommended for faster execution.

Given the stochastic nature of the training algorithm, your specific results will vary. Consider running the example a few times.

At the start of the run, the size of the training dataset is summarized, as is the supervised subset, confirming our configuration.

The performance of each model is summarized at the end of each update, including the loss and accuracy of the supervised discriminator model (c), the loss of the unsupervised discriminator model on real and generated examples (d), and the loss of the generator model updated via the composite model (g).

The loss for the supervised model will shrink to a small value close to zero and accuracy will hit 100%, which will be maintained for the entire run. The loss of the unsupervised discriminator and generator should remain at modest values throughout the run if they are kept in equilibrium.

The supervised classification model is evaluated on the entire training dataset at the end of every training epoch, in this case after every 600 training updates. At this time, the performance of the model is summarized, showing that it rapidly achieves good skill.

This is surprising given that the model is only trained on 10 labeled examples of each class.

The models are also saved at the end of each training epoch and plots of generated images are also created.

The quality of the generated images is good given the relatively small number of training epochs.

How to Load and Use the Final SGAN Classifier Model

Now that we have trained the generator and discriminator models, we can make use of them.

In the case of the semi-supervised GAN, we are less interested in the generator model and more interested in the supervised model.

Reviewing the results for the specific run, we can select a specific saved model that is known to have good performance on the test dataset. In this case, the model saved after 12 training epochs, or 7,200 updates, that had a classification accuracy of about 95.432% on the training dataset.

We can load the model directly via the load_model() Keras function.

Once loaded, we can evaluate it on the entire training dataset again to confirm the finding, then evaluate it on the holdout test dataset.

Recall, the feature extraction layers expect the input images to have the pixel values scaled to the range [-1,1], therefore, this must be performed before any images are provided to the model.

The complete example of loading the saved semi-supervised classifier model and evaluating it in the complete MNIST dataset is listed below.

Running the example loads the model and evaluates it on the MNIST dataset.

We can see that, in this case, the model achieves the expected performance of 95.432% on the training dataset, confirming we have loaded the correct model.

We can also see that the accuracy on the holdout test dataset is as good, or slightly better, at about 95.920%. This shows that the learned classifier has good generalization.

We have successfully demonstrated the training and evaluation of a semi-supervised classifier model fit via the GAN architecture.

Extensions

This section lists some ideas for extending the tutorial that you may wish to explore.

• Standalone Classifier. Fit a standalone classifier model on the labeled dataset directly and compare performance to the SGAN model.
• Number of Labeled Examples. Repeat the example of more or fewer labeled examples and compare the performance of the model
• Model Tuning. Tune the performance of the discriminator and generator model to further lift the performance of the supervised model closer toward state-of-the-art results.

If you explore any of these extensions, I’d love to know.
Post your findings in the comments below.

Summary

In this tutorial, you discovered how to develop a Semi-Supervised Generative Adversarial Network from scratch.

Specifically, you learned:

• The semi-supervised GAN is an extension of the GAN architecture for training a classifier model while making use of labeled and unlabeled data.
• There are at least three approaches to implementing the supervised and unsupervised discriminator models in Keras used in the semi-supervised GAN.
• How to train a semi-supervised GAN from scratch on MNIST and load and use the trained classifier for making predictions.