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Train Generative Adversarial Network (GAN)

This example shows how to train a generative adversarial network (GAN) to generate images.

A generative adversarial network (GAN) is a type of deep learning network that can generate data with similar characteristics as the input real data.

A GAN consists of two networks that train together:

  1. Generator — Given a vector of random values (latent inputs) as input, this network generates data with the same structure as the training data.

  2. Discriminator — Given batches of data containing observations from both the training data, and generated data from the generator, this network attempts to classify the observations as "real" or "generated".

To train a GAN, train both networks simultaneously to maximize the performance of both:

  • Train the generator to generate data that "fools" the discriminator.

  • Train the discriminator to distinguish between real and generated data.

To optimize the performance of the generator, maximize the loss of the discriminator when given generated data. That is, the objective of the generator is to generate data that the discriminator classifies as "real".

To optimize the performance of the discriminator, minimize the loss of the discriminator when given batches of both real and generated data. That is, the objective of the discriminator is to not be "fooled" by the generator.

Ideally, these strategies result in a generator that generates convincingly realistic data and a discriminator that has learned strong feature representations that are characteristic of the training data.

Load Training Data

Download and extract the Flowers data set [1].

url = 'http://download.tensorflow.org/example_images/flower_photos.tgz';
downloadFolder = tempdir;
filename = fullfile(downloadFolder,'flower_dataset.tgz');

imageFolder = fullfile(downloadFolder,'flower_photos');
if ~exist(imageFolder,'dir')
    disp('Downloading Flowers data set (218 MB)...')
    websave(filename,url);
    untar(filename,downloadFolder)
end

Create an image datastore containing the photos of the flowers.

datasetFolder = fullfile(imageFolder);

imds = imageDatastore(datasetFolder, ...
    'IncludeSubfolders',true);

Augment the data to include random horizontal flipping and resize the images to have size 64-by-64.

augmenter = imageDataAugmenter('RandXReflection',true);
augimds = augmentedImageDatastore([64 64],imds,'DataAugmentation',augmenter);    

Define Generator Network

Define the following network architecture, which generates images from 1-by-1-by-100 arrays of random values:

This network:

  • Converts the 1-by-1-by-100 arrays of noise to 7-by-7-by-128 arrays using a project and reshape layer.

  • Upscales the resulting arrays to 64-by-64-by-3 arrays using a series of transposed convolution layers with batch normalization and ReLU layers.

Define this network architecture as a layer graph and specify the following network properties.

  • For the transposed convolution layers, specify 5-by-5 filters with a decreasing number of filters for each layer, a stride of 2 and cropping of the output on each edge.

  • For the final transposed convolution layer, specify three 5-by-5 filters corresponding to the three RGB channels of the generated images, and the output size of the previous layer.

  • At the end of the network, include a tanh layer.

To project and reshape the noise input, use the custom layer projectAndReshapeLayer, attached to this example as a supporting file. The projectAndReshapeLayer layer upscales the input using a fully connected operation and reshapes the output to the specified size.

filterSize = 5;
numFilters = 64;
numLatentInputs = 100;

projectionSize = [4 4 512];

layersGenerator = [
    imageInputLayer([1 1 numLatentInputs],'Normalization','none','Name','in')
    projectAndReshapeLayer(projectionSize,numLatentInputs,'proj');
    transposedConv2dLayer(filterSize,4*numFilters,'Name','tconv1')
    batchNormalizationLayer('Name','bnorm1')
    reluLayer('Name','relu1')
    transposedConv2dLayer(filterSize,2*numFilters,'Stride',2,'Cropping','same','Name','tconv2')
    batchNormalizationLayer('Name','bnorm2')
    reluLayer('Name','relu2')
    transposedConv2dLayer(filterSize,numFilters,'Stride',2,'Cropping','same','Name','tconv3')
    batchNormalizationLayer('Name','bnorm3')
    reluLayer('Name','relu3')
    transposedConv2dLayer(filterSize,3,'Stride',2,'Cropping','same','Name','tconv4')
    tanhLayer('Name','tanh')];

lgraphGenerator = layerGraph(layersGenerator);

To train the network with a custom training loop and enable automatic differentiation, convert the layer graph to a dlnetwork object.

dlnetGenerator = dlnetwork(lgraphGenerator);

Define Discriminator Network

Define the following network, which classifies real and generated 64-by-64 images.

Create a network that takes 64-by-64-by-3 images and returns a scalar prediction score using a series of convolution layers with batch normalization and leaky ReLU layers. Add noise to the input images using dropout.

  • For the dropout layer, specify a dropout probability of 0.5.

  • For the convolution layers, specify 5-by-5 filters with an increasing number of filters for each layer. Also specify a stride of 2 and a padding of the output.

  • For the leaky ReLU layers, specify a scale of 0.2.

  • For the final layer, specify a convolutional layer with one 4-by-4 filter.

To output the probabilities in the range [0,1], use the sigmoid function in the Model Gradients Function.

dropoutProb = 0.5;
numFilters = 64;
scale = 0.2;

inputSize = [64 64 3];
filterSize = 5;

layersDiscriminator = [
    imageInputLayer(inputSize,'Normalization','none','Name','in')
    dropoutLayer(0.5,'Name','dropout')
    convolution2dLayer(filterSize,numFilters,'Stride',2,'Padding','same','Name','conv1')
    leakyReluLayer(scale,'Name','lrelu1')
    convolution2dLayer(filterSize,2*numFilters,'Stride',2,'Padding','same','Name','conv2')
    batchNormalizationLayer('Name','bn2')
    leakyReluLayer(scale,'Name','lrelu2')
    convolution2dLayer(filterSize,4*numFilters,'Stride',2,'Padding','same','Name','conv3')
    batchNormalizationLayer('Name','bn3')
    leakyReluLayer(scale,'Name','lrelu3')
    convolution2dLayer(filterSize,8*numFilters,'Stride',2,'Padding','same','Name','conv4')
    batchNormalizationLayer('Name','bn4')
    leakyReluLayer(scale,'Name','lrelu4')
    convolution2dLayer(4,1,'Name','conv5')];

lgraphDiscriminator = layerGraph(layersDiscriminator);

To train the network with a custom training loop and enable automatic differentiation, convert the layer graph to a dlnetwork object.

dlnetDiscriminator = dlnetwork(lgraphDiscriminator);

Define Model Gradients, Loss Functions and Scores

Create the function modelGradients, listed in the Model Gradients Function section of the example, which takes as input the generator and discriminator networks, a mini-batch of input data, an array of random values and the flip factor, and returns the gradients of the loss with respect to the learnable parameters in the networks and the scores of the two networks.

Specify Training Options

Train with a mini-batch size of 128 for 500 epochs. For larger datasets, you might not need to train for as many epochs.

numEpochs = 500;
miniBatchSize = 128;

Specify the options for Adam optimization. For both networks, specify

  • A learning rate of 0.0002

  • A gradient decay factor of 0.5

  • A squared gradient decay factor of 0.999

learnRate = 0.0002;
gradientDecayFactor = 0.5;
squaredGradientDecayFactor = 0.999;

If the discriminator learns to discriminate between real and generated images too quickly, then the generator may fail to train. To better balance the learning of the discriminator and the generator, add noise to the real data by randomly flipping the labels.

Specify to flip 30% of the real labels. This means that 15% of the total number of labels are flipped during training. Note that this does not impair the generator as all the generated images are still labelled correctly.

flipFactor = 0.3;

Display the generated validation images every 100 iterations.

validationFrequency = 100;

Train Model

Use minibatchqueue to process and manage the mini-batches of images. For each mini-batch:

  • Use the custom mini-batch preprocessing function preprocessMiniBatch (defined at the end of this example) to rescale the images in the range [-1,1].

  • Discard any partial mini-batches with less than 128 observations.

  • Format the image data with the dimension labels 'SSCB' (spatial, spatial, channel, batch). By default, the minibatchqueue object converts the data to dlarray objects with underlying type single.

  • Train on a GPU if one is available. When the 'OutputEnvironment' option of minibatchqueue is "auto", minibatchqueue converts each output to a gpuArray if a GPU is available. Using a GPU requires Parallel Computing Toolbox™ and a CUDA® enabled NVIDIA® GPU with compute capability 3.0 or higher.

augimds.MiniBatchSize = miniBatchSize;

executionEnvironment = "auto";

mbq = minibatchqueue(augimds,...
    'MiniBatchSize',miniBatchSize,...
    'PartialMiniBatch','discard',...
    'MiniBatchFcn', @preprocessMiniBatch,...
    'MiniBatchFormat','SSCB',...
    'OutputEnvironment',executionEnvironment);

Train the model using a custom training loop. Loop over the training data and update the network parameters at each iteration. To monitor the training progress, display a batch of generated images using a held-out array of random values to input into the generator as well as a plot of the scores.

Initialize the parameters for Adam.

trailingAvgGenerator = [];
trailingAvgSqGenerator = [];
trailingAvgDiscriminator = [];
trailingAvgSqDiscriminator = [];

To monitor training progress, display a batch of generated images using a held-out batch of fixed arrays of random values fed into the generator and plot the network scores.

Create an array of held-out random values.

numValidationImages = 25;
ZValidation = randn(1,1,numLatentInputs,numValidationImages,'single');

Convert the data to dlarray objects and specify the dimension labels 'SSCB' (spatial, spatial, channel, batch).

dlZValidation = dlarray(ZValidation,'SSCB');

For GPU training, convert the data to gpuArray objects.

if (executionEnvironment == "auto" && canUseGPU) || executionEnvironment == "gpu"
    dlZValidation = gpuArray(dlZValidation);
end

Initialize the training progress plots. Create a figure and resize it to have twice the width.

f = figure;
f.Position(3) = 2*f.Position(3);

Create a subplot for the generated images and the network scores.

imageAxes = subplot(1,2,1);
scoreAxes = subplot(1,2,2);

Initialize the animated lines for the scores plot.

lineScoreGenerator = animatedline(scoreAxes,'Color',[0 0.447 0.741]);
lineScoreDiscriminator = animatedline(scoreAxes, 'Color', [0.85 0.325 0.098]);
legend('Generator','Discriminator');
ylim([0 1])
xlabel("Iteration")
ylabel("Score")
grid on

Train the GAN. For each epoch, shuffle the datastore and loop over mini-batches of data.

For each mini-batch:

  • Evaluate the model gradients using dlfeval and the modelGradients function.

  • Update the network parameters using the adamupdate function.

  • Plot the scores of the two networks.

  • After every validationFrequency iterations, display a batch of generated images for a fixed held-out generator input.

Training can take some time to run.

iteration = 0;
start = tic;

% Loop over epochs.
for epoch = 1:numEpochs
    
    % Reset and shuffle datastore.
    shuffle(mbq);
    
    % Loop over mini-batches.
    while hasdata(mbq)
        iteration = iteration + 1;
        
        % Read mini-batch of data.
        dlX = next(mbq);
        
        % Generate latent inputs for the generator network. Convert to
        % dlarray and specify the dimension labels 'SSCB' (spatial,
        % spatial, channel, batch). If training on a GPU, then convert
        % latent inputs to gpuArray.
        Z = randn(1,1,numLatentInputs,size(dlX,4),'single');
        dlZ = dlarray(Z,'SSCB');        
        
        if (executionEnvironment == "auto" && canUseGPU) || executionEnvironment == "gpu"
            dlZ = gpuArray(dlZ);
        end
        
        % Evaluate the model gradients and the generator state using
        % dlfeval and the modelGradients function listed at the end of the
        % example.
        [gradientsGenerator, gradientsDiscriminator, stateGenerator, scoreGenerator, scoreDiscriminator] = ...
            dlfeval(@modelGradients, dlnetGenerator, dlnetDiscriminator, dlX, dlZ, flipFactor);
        dlnetGenerator.State = stateGenerator;
        
        % Update the discriminator network parameters.
        [dlnetDiscriminator,trailingAvgDiscriminator,trailingAvgSqDiscriminator] = ...
            adamupdate(dlnetDiscriminator, gradientsDiscriminator, ...
            trailingAvgDiscriminator, trailingAvgSqDiscriminator, iteration, ...
            learnRate, gradientDecayFactor, squaredGradientDecayFactor);
        
        % Update the generator network parameters.
        [dlnetGenerator,trailingAvgGenerator,trailingAvgSqGenerator] = ...
            adamupdate(dlnetGenerator, gradientsGenerator, ...
            trailingAvgGenerator, trailingAvgSqGenerator, iteration, ...
            learnRate, gradientDecayFactor, squaredGradientDecayFactor);
        
        % Every validationFrequency iterations, display batch of generated images using the
        % held-out generator input
        if mod(iteration,validationFrequency) == 0 || iteration == 1
            % Generate images using the held-out generator input.
            dlXGeneratedValidation = predict(dlnetGenerator,dlZValidation);
            
            % Tile and rescale the images in the range [0 1].
            I = imtile(extractdata(dlXGeneratedValidation));
            I = rescale(I);
            
            % Display the images.
            subplot(1,2,1);
            image(imageAxes,I)
            xticklabels([]);
            yticklabels([]);
            title("Generated Images");
        end
        
        % Update the scores plot
        subplot(1,2,2)
        addpoints(lineScoreGenerator,iteration,...
            double(gather(extractdata(scoreGenerator))));
        
        addpoints(lineScoreDiscriminator,iteration,...
            double(gather(extractdata(scoreDiscriminator))));
        
        % Update the title with training progress information.
        D = duration(0,0,toc(start),'Format','hh:mm:ss');
        title(...
            "Epoch: " + epoch + ", " + ...
            "Iteration: " + iteration + ", " + ...
            "Elapsed: " + string(D))
        
        drawnow
    end
end

Here, the discriminator has learned a strong feature representation that identifies real images among generated images. In turn, the generator has learned a similarly strong feature representation that allows it to generate realistic looking data.

The training plot shows the scores of the generator and discriminator networks. To learn more about how to interpret the network scores, see Monitor GAN Training Progress and Identify Common Failure Modes.

Generate New Images

To generate new images, use the predict function on the generator with a dlarray object containing a batch of 1-by-1-by-100 arrays of random values. To display the images together, use the imtile function and rescale the images using the rescale function.

Create a dlarray object containing a batch of 25 1-by-1-by-100 arrays of random values to input into the generator network.

ZNew = randn(1,1,numLatentInputs,25,'single');
dlZNew = dlarray(ZNew,'SSCB');

To generate images using the GPU, also convert the data to gpuArray objects.

if (executionEnvironment == "auto" && canUseGPU) || executionEnvironment == "gpu"
    dlZNew = gpuArray(dlZNew);
end

Generate new images using the predict function with the generator and the input data.

dlXGeneratedNew = predict(dlnetGenerator,dlZNew);

Display the images.

I = imtile(extractdata(dlXGeneratedNew));
I = rescale(I);
figure
image(I)
axis off
title("Generated Images")

Model Gradients Function

The function modelGradients takes as input the generator and discriminator dlnetwork objects dlnetGenerator and dlnetDiscriminator, a mini-batch of input data dlX, an array of random values dlZ and the percentage of real labels to flip flipFactor, and returns the gradients of the loss with respect to the learnable parameters in the networks, the generator state, and the scores of the two networks. Because the discriminator output is not in the range [0,1], modelGradients applies the sigmoid function to convert it into probabilities.

function [gradientsGenerator, gradientsDiscriminator, stateGenerator, scoreGenerator, scoreDiscriminator] = ...
    modelGradients(dlnetGenerator, dlnetDiscriminator, dlX, dlZ, flipFactor)

% Calculate the predictions for real data with the discriminator network.
dlYPred = forward(dlnetDiscriminator, dlX);

% Calculate the predictions for generated data with the discriminator network.
[dlXGenerated,stateGenerator] = forward(dlnetGenerator,dlZ);
dlYPredGenerated = forward(dlnetDiscriminator, dlXGenerated);

% Convert the discriminator outputs to probabilities.
probGenerated = sigmoid(dlYPredGenerated);
probReal = sigmoid(dlYPred);

% Calculate the score of the discriminator.
scoreDiscriminator = ((mean(probReal)+mean(1-probGenerated))/2);

% Calculate the score of the generator.
scoreGenerator = mean(probGenerated);

% Randomly flip a fraction of the labels of the real images.
numObservations = size(probReal,4);
idx = randperm(numObservations,floor(flipFactor * numObservations));

% Flip the labels
probReal(:,:,:,idx) = 1-probReal(:,:,:,idx);

% Calculate the GAN loss.
[lossGenerator, lossDiscriminator] = ganLoss(probReal,probGenerated);

% For each network, calculate the gradients with respect to the loss.
gradientsGenerator = dlgradient(lossGenerator, dlnetGenerator.Learnables,'RetainData',true);
gradientsDiscriminator = dlgradient(lossDiscriminator, dlnetDiscriminator.Learnables);

end

GAN Loss Function and Scores

The objective of the generator is to generate data that the discriminator classifies as "real". To maximize the probability that images from the generator are classified as real by the discriminator, minimize the negative log likelihood function.

Given the output Y of the discriminator:

  • Yˆ=σ(Y) is the probability that the input image belongs to the class "real".

  • 1-Yˆ is the probability that the input image belongs to the class "generated".

Note that the sigmoid operation σ happens in the modelGradients function. The loss function for the generator is given by

lossGenerator=-mean(log(YˆGenerated)),

where YˆGenerated contains the discriminator output probabilities for the generated images.

The objective of the discriminator is to not be "fooled" by the generator. To maximize the probability that the discriminator successfully discriminates between the real and generated images, minimize the sum of the corresponding negative log likelihood functions.

The loss function for the discriminator is given by

lossDiscriminator=-mean(log(YˆReal))-mean(log(1-YˆGenerated)),

where YˆReal contains the discriminator output probabilities for the real images.

To measure on a scale from 0 to 1 how well the generator and discriminator achieve their respective goals you can use the concept of score.

The generator score is the average of the probabilities corresponding to the discriminator output for the generated images:

scoreGenerator=mean(YˆGenerated).

The discriminator score is the average of the probabilities corresponding to the discriminator output for both the real and generated images:

scoreDiscriminator=12mean(YˆReal)+12mean(1-YˆGenerated).

The score is inversely proportional to the loss but effectively contains the same information.

function [lossGenerator, lossDiscriminator] = ganLoss(probReal,probGenerated)

% Calculate the loss for the discriminator network.
lossDiscriminator =  -mean(log(probReal)) -mean(log(1-probGenerated));

% Calculate the loss for the generator network.
lossGenerator = -mean(log(probGenerated));

end

Mini-Batch Preprocessing Function

The preprocessMiniBatch function preprocesses the data using the following steps:

  1. Extract the image data from the incoming cell array and concatenate into a numeric array.

  2. Rescale the images to be in the range [-1,1].

function X = preprocessMiniBatch(data)
    % Concatenate mini-batch
    X = cat(4,data{:});
    
    % Rescale the images in the range [-1 1].
    X = rescale(X,-1,1,'InputMin',0,'InputMax',255);
end

References

  1. The TensorFlow Team. Flowers http://download.tensorflow.org/example_images/flower_photos.tgz

  2. Radford, Alec, Luke Metz, and Soumith Chintala. "Unsupervised representation learning with deep convolutional generative adversarial networks." arXiv preprint arXiv:1511.06434 (2015).

See Also

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