conditional_gan.ipynb

Kernel: Python 3

Conditional GAN

Author: Sayak Paul
Date created: 2021/07/13
Last modified: 2024/01/02
Description: Training a GAN conditioned on class labels to generate handwritten digits.

Generative Adversarial Networks (GANs) let us generate novel image data, video data, or audio data from a random input. Typically, the random input is sampled from a normal distribution, before going through a series of transformations that turn it into something plausible (image, video, audio, etc.).

However, a simple DCGAN doesn't let us control the appearance (e.g. class) of the samples we're generating. For instance, with a GAN that generates MNIST handwritten digits, a simple DCGAN wouldn't let us choose the class of digits we're generating. To be able to control what we generate, we need to condition the GAN output on a semantic input, such as the class of an image.

In this example, we'll build a Conditional GAN that can generate MNIST handwritten digits conditioned on a given class. Such a model can have various useful applications:

• let's say you are dealing with an imbalanced image dataset, and you'd like to gather more examples for the skewed class to balance the dataset. Data collection can be a costly process on its own. You could instead train a Conditional GAN and use it to generate novel images for the class that needs balancing.

• Since the generator learns to associate the generated samples with the class labels, its representations can also be used for other downstream tasks.

Following are the references used for developing this example:

• Conditional Generative Adversarial Nets

• Lecture on Conditional Generation from Coursera

If you need a refresher on GANs, you can refer to the "Generative adversarial networks" section of this resource.

This example requires TensorFlow 2.5 or higher, as well as TensorFlow Docs, which can be installed using the following command:

!pip install -q git+https://github.com/tensorflow/docs

Imports

import keras

from keras import layers
from keras import ops
from tensorflow_docs.vis import embed
import tensorflow as tf
import numpy as np
import imageio

Constants and hyperparameters

batch_size = 64
num_channels = 1
num_classes = 10
image_size = 28
latent_dim = 128

Loading the MNIST dataset and preprocessing it

# We'll use all the available examples from both the training and test
# sets.
(x_train, y_train), (x_test, y_test) = keras.datasets.mnist.load_data()
all_digits = np.concatenate([x_train, x_test])
all_labels = np.concatenate([y_train, y_test])

# Scale the pixel values to [0, 1] range, add a channel dimension to
# the images, and one-hot encode the labels.
all_digits = all_digits.astype("float32") / 255.0
all_digits = np.reshape(all_digits, (-1, 28, 28, 1))
all_labels = keras.utils.to_categorical(all_labels, 10)

# Create tf.data.Dataset.
dataset = tf.data.Dataset.from_tensor_slices((all_digits, all_labels))
dataset = dataset.shuffle(buffer_size=1024).batch(batch_size)

print(f"Shape of training images: {all_digits.shape}")
print(f"Shape of training labels: {all_labels.shape}")

Calculating the number of input channel for the generator and discriminator

In a regular (unconditional) GAN, we start by sampling noise (of some fixed dimension) from a normal distribution. In our case, we also need to account for the class labels. We will have to add the number of classes to the input channels of the generator (noise input) as well as the discriminator (generated image input).

generator_in_channels = latent_dim + num_classes
discriminator_in_channels = num_channels + num_classes
print(generator_in_channels, discriminator_in_channels)

Creating the discriminator and generator

The model definitions (discriminator, generator, and ConditionalGAN) have been adapted from this example.

# Create the discriminator.
discriminator = keras.Sequential(
    [
        keras.layers.InputLayer((28, 28, discriminator_in_channels)),
        layers.Conv2D(64, (3, 3), strides=(2, 2), padding="same"),
        layers.LeakyReLU(negative_slope=0.2),
        layers.Conv2D(128, (3, 3), strides=(2, 2), padding="same"),
        layers.LeakyReLU(negative_slope=0.2),
        layers.GlobalMaxPooling2D(),
        layers.Dense(1),
    ],
    name="discriminator",
)

# Create the generator.
generator = keras.Sequential(
    [
        keras.layers.InputLayer((generator_in_channels,)),
        # We want to generate 128 + num_classes coefficients to reshape into a
        # 7x7x(128 + num_classes) map.
        layers.Dense(7 * 7 * generator_in_channels),
        layers.LeakyReLU(negative_slope=0.2),
        layers.Reshape((7, 7, generator_in_channels)),
        layers.Conv2DTranspose(128, (4, 4), strides=(2, 2), padding="same"),
        layers.LeakyReLU(negative_slope=0.2),
        layers.Conv2DTranspose(128, (4, 4), strides=(2, 2), padding="same"),
        layers.LeakyReLU(negative_slope=0.2),
        layers.Conv2D(1, (7, 7), padding="same", activation="sigmoid"),
    ],
    name="generator",
)

Creating a ConditionalGAN model

class ConditionalGAN(keras.Model):
    def __init__(self, discriminator, generator, latent_dim):
        super().__init__()
        self.discriminator = discriminator
        self.generator = generator
        self.latent_dim = latent_dim
        self.seed_generator = keras.random.SeedGenerator(1337)
        self.gen_loss_tracker = keras.metrics.Mean(name="generator_loss")
        self.disc_loss_tracker = keras.metrics.Mean(name="discriminator_loss")

    @property
    def metrics(self):
        return [self.gen_loss_tracker, self.disc_loss_tracker]

    def compile(self, d_optimizer, g_optimizer, loss_fn):
        super().compile()
        self.d_optimizer = d_optimizer
        self.g_optimizer = g_optimizer
        self.loss_fn = loss_fn

    def train_step(self, data):
        # Unpack the data.
        real_images, one_hot_labels = data

        # Add dummy dimensions to the labels so that they can be concatenated with
        # the images. This is for the discriminator.
        image_one_hot_labels = one_hot_labels[:, :, None, None]
        image_one_hot_labels = ops.repeat(
            image_one_hot_labels, repeats=[image_size * image_size]
        )
        image_one_hot_labels = ops.reshape(
            image_one_hot_labels, (-1, image_size, image_size, num_classes)
        )

        # Sample random points in the latent space and concatenate the labels.
        # This is for the generator.
        batch_size = ops.shape(real_images)[0]
        random_latent_vectors = keras.random.normal(
            shape=(batch_size, self.latent_dim), seed=self.seed_generator
        )
        random_vector_labels = ops.concatenate(
            [random_latent_vectors, one_hot_labels], axis=1
        )

        # Decode the noise (guided by labels) to fake images.
        generated_images = self.generator(random_vector_labels)

        # Combine them with real images. Note that we are concatenating the labels
        # with these images here.
        fake_image_and_labels = ops.concatenate(
            [generated_images, image_one_hot_labels], -1
        )
        real_image_and_labels = ops.concatenate([real_images, image_one_hot_labels], -1)
        combined_images = ops.concatenate(
            [fake_image_and_labels, real_image_and_labels], axis=0
        )

        # Assemble labels discriminating real from fake images.
        labels = ops.concatenate(
            [ops.ones((batch_size, 1)), ops.zeros((batch_size, 1))], axis=0
        )

        # Train the discriminator.
        with tf.GradientTape() as tape:
            predictions = self.discriminator(combined_images)
            d_loss = self.loss_fn(labels, predictions)
        grads = tape.gradient(d_loss, self.discriminator.trainable_weights)
        self.d_optimizer.apply_gradients(
            zip(grads, self.discriminator.trainable_weights)
        )

        # Sample random points in the latent space.
        random_latent_vectors = keras.random.normal(
            shape=(batch_size, self.latent_dim), seed=self.seed_generator
        )
        random_vector_labels = ops.concatenate(
            [random_latent_vectors, one_hot_labels], axis=1
        )

        # Assemble labels that say "all real images".
        misleading_labels = ops.zeros((batch_size, 1))

        # Train the generator (note that we should *not* update the weights
        # of the discriminator)!
        with tf.GradientTape() as tape:
            fake_images = self.generator(random_vector_labels)
            fake_image_and_labels = ops.concatenate(
                [fake_images, image_one_hot_labels], -1
            )
            predictions = self.discriminator(fake_image_and_labels)
            g_loss = self.loss_fn(misleading_labels, predictions)
        grads = tape.gradient(g_loss, self.generator.trainable_weights)
        self.g_optimizer.apply_gradients(zip(grads, self.generator.trainable_weights))

        # Monitor loss.
        self.gen_loss_tracker.update_state(g_loss)
        self.disc_loss_tracker.update_state(d_loss)
        return {
            "g_loss": self.gen_loss_tracker.result(),
            "d_loss": self.disc_loss_tracker.result(),
        }

Training the Conditional GAN

cond_gan = ConditionalGAN(
    discriminator=discriminator, generator=generator, latent_dim=latent_dim
)
cond_gan.compile(
    d_optimizer=keras.optimizers.Adam(learning_rate=0.0003),
    g_optimizer=keras.optimizers.Adam(learning_rate=0.0003),
    loss_fn=keras.losses.BinaryCrossentropy(from_logits=True),
)

cond_gan.fit(dataset, epochs=20)

Interpolating between classes with the trained generator

# We first extract the trained generator from our Conditional GAN.
trained_gen = cond_gan.generator

# Choose the number of intermediate images that would be generated in
# between the interpolation + 2 (start and last images).
num_interpolation = 9  # @param {type:"integer"}

# Sample noise for the interpolation.
interpolation_noise = keras.random.normal(shape=(1, latent_dim))
interpolation_noise = ops.repeat(interpolation_noise, repeats=num_interpolation)
interpolation_noise = ops.reshape(interpolation_noise, (num_interpolation, latent_dim))


def interpolate_class(first_number, second_number):
    # Convert the start and end labels to one-hot encoded vectors.
    first_label = keras.utils.to_categorical([first_number], num_classes)
    second_label = keras.utils.to_categorical([second_number], num_classes)
    first_label = ops.cast(first_label, "float32")
    second_label = ops.cast(second_label, "float32")

    # Calculate the interpolation vector between the two labels.
    percent_second_label = ops.linspace(0, 1, num_interpolation)[:, None]
    percent_second_label = ops.cast(percent_second_label, "float32")
    interpolation_labels = (
        first_label * (1 - percent_second_label) + second_label * percent_second_label
    )

    # Combine the noise and the labels and run inference with the generator.
    noise_and_labels = ops.concatenate([interpolation_noise, interpolation_labels], 1)
    fake = trained_gen.predict(noise_and_labels)
    return fake


start_class = 2  # @param {type:"slider", min:0, max:9, step:1}
end_class = 6  # @param {type:"slider", min:0, max:9, step:1}

fake_images = interpolate_class(start_class, end_class)

Here, we first sample noise from a normal distribution and then we repeat that for num_interpolation times and reshape the result accordingly. We then distribute it uniformly for num_interpolation with the label identities being present in some proportion.

fake_images *= 255.0
converted_images = fake_images.astype(np.uint8)
converted_images = ops.image.resize(converted_images, (96, 96)).numpy().astype(np.uint8)
imageio.mimsave("animation.gif", converted_images[:, :, :, 0], fps=1)
embed.embed_file("animation.gif")

We can further improve the performance of this model with recipes like WGAN-GP. Conditional generation is also widely used in many modern image generation architectures like VQ-GANs, DALL-E, etc.

You can use the trained model hosted on Hugging Face Hub and try the demo on Hugging Face Spaces.