CSC321 Lecture 19: Generative Adversarial Networks
Roger Grosse
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Overview
In generative modeling, we’d like to train a network that models a
distribution, such as a distribution over images.
One way to judge the quality of the model is to sample from it.
This field has seen rapid progress:
2018
2009 2015
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Overview
Four modern approaches to generative modeling:
Generative adversarial networks (today)
Reversible architectures (next lecture)
Autoregressive models (Lecture 7, and next lecture)
Variational autoencoders (CSC412)
All four approaches have different pros and cons.
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Implicit Generative Models
Implicit generative models implicitly define a probability distribution
Start by sampling the code vector z from a fixed, simple distribution
(e.g. spherical Gaussian)
The generator network computes a differentiable function G mapping
z to an x in data space
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Implicit Generative Models
A 1-dimensional example:
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Implicit Generative Models
https://blog.openai.com/generative-models/
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Implicit Generative Models
This sort of architecture sounded preposterous to many of us, but
amazingly, it works.
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Generative Adversarial Networks
The advantage of implicit generative models: if you have some
criterion for evaluating the quality of samples, then you can compute
its gradient with respect to the network parameters, and update the
network’s parameters to make the sample a little better
The idea behind Generative Adversarial Networks (GANs): train two
different networks
The generator network tries to produce realistic-looking samples
The discriminator network tries to figure out whether an image came
from the training set or the generator network
The generator network tries to fool the discriminator network
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Generative Adversarial Networks
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Generative Adversarial Networks
Let D denote the discriminator’s predicted probability of being data
Discriminator’s cost function: cross-entropy loss for task of classifying
real vs. fake images
JD = Ex∼D [− log D(x)] + Ez [− log(1 − D(G (z)))]
One possible cost function for the generator: the opposite of the
discriminator’s
JG = −JD
= const + Ez [log(1 − D(G (z)))]
This is called the minimax formulation, since the generator and
discriminator are playing a zero-sum game against each other:
max min JD
G D
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Generative Adversarial Networks
Updating the discriminator:
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Generative Adversarial Networks
Updating the generator:
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Generative Adversarial Networks
Alternating training of the generator and discriminator:
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A Better Cost Function
We introduced the minimax cost function for the generator:
JG = Ez [log(1 − D(G (z)))]
One problem with this is saturation.
Recall from our lecture on classification: when the prediction is really
wrong,
“Logistic + squared error” gets a weak gradient signal
“Logistic + cross-entropy” gets a strong gradient signal
Here, if the generated sample is really bad, the discriminator’s
prediction is close to 0, and the generator’s cost is flat.
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A Better Cost Function
Original minimax cost:
JG = Ez [log(1 − D(G (z)))]
Modified generator cost:
JG = Ez [− log D(G (z))]
This fixes the saturation problem.
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Generative Adversarial Networks
Since GANs were introduced in 2014, there have been hundreds of
papers introducing various architectures and training methods.
Most modern architectures are based on the Deep Convolutional GAN
(DC-GAN), where the generator and discriminator are both conv nets.
GAN Zoo: https://github.com/hindupuravinash/the-gan-zoo
Good source of horrible puns (VEEGAN, Checkhov GAN, etc.)
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GAN Samples
Celebrities:
Karras et al., 2017. Progressive growing of GANs for improved quality, stability, and variation
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GAN Samples
Bedrooms:
Karras et al., 2017. Progressive growing of GANs for improved quality, stability, and variation
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GAN Samples
Objects:
Karras et al., 2017. Progressive growing of GANs for improved quality, stability, and variation
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GAN Samples
GANs revolutionized generative modeling by producing crisp,
high-resolution images.
The catch: we don’t know how well they’re modeling the distribution.
Can’t measure the log-likelihood they assign to held-out data.
Could they be memorizing training examples? (E.g., maybe they
sometimes produce photos of real celebrities?)
We have no way to tell if they are dropping important modes from the
distribution.
See Wu et al., “On the quantitative analysis of decoder-based
generative models” for partial answers to these questions.
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CycleGAN
Style transfer problem: change the style of an image while preserving the
content.
Data: Two unrelated collections of images, one for each style
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CycleGAN
If we had paired data (same content in both styles), this would be a
supervised learning problem. But this is hard to find.
The CycleGAN architecture learns to do it from unpaired data.
Train two different generator nets to go from style 1 to style 2, and
vice versa.
Make sure the generated samples of style 2 are indistinguishable from
real images by a discriminator net.
Make sure the generators are cycle-consistent: mapping from style 1 to
style 2 and back again should give you almost the original image.
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CycleGAN
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CycleGAN
Style transfer between aerial photos and maps:
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CycleGAN
Style transfer between road scenes and semantic segmentations (labels of
every pixel in an image by object category):
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