TOWARDS GAN BENCHMARKS WHICH REQUIRE GENERALIZATION
For many evaluation metrics commonly used as benchmarks for unconditional image generation, trivially memorizing the training set attains a better score than models which are considered state-of-the-art; we consider this problematic. We clarify a necessary condition for an evaluation metric not to behave this way: estimating the function must require a large sample from the model. In search of such a metric, we turn to neural network divergences (NNDs), which are defined in terms of a neural network trained to distinguish between distributions. The resulting benchmarks cannot be "won" by training set memorization, while still being perceptually correlated and computable only from samples. We survey past work on using NNDs for evaluation, implement an example black-box metric based on these ideas, and validate experimentally that it can measure a notion of generalization.
Summary:
The paper looks at the problem of benchmarking models that unconditionally generate images. In particular they focus on GAN models and discuss the Inception Score (IS) and Fréchet Inception Distance (FID) metrics. The authors argue that a good benchmark should not have a trivial solution (e.g. memorising the dataset) and find that a necessary condition for such a metric is a large number of samples. They also find that for IS and FID , a GAN is outperformed by a model that memorises the dataset, while a method based on neural network divergences (NND) does not show the same behaviour. NND works by training a discriminative model to discriminate between samples of the generative model and samples from a held out test set. The poorer the discriminative model performs, the better the generative model is.
The authors show a range of results using a CNN based divergence: on PixelCNN++, GANs, overfitted GANs, WGAN-GP and conclude that it’s a better metric than IS/FID at the expense of requiring much more computation to evaluate.  They also perform a test with limited compute and show that the results correlate well with a bigger dataset, but show some bias.
Review:
The paper is well written, with a clear description of the properties a good benchmark should have, an analysis of the current solutions and their shortcomings and an extensive experimental evaluation of the CNN divergence metric. The authors also compared with non GAN methods and experimented with small datasets, both are not necessarily within scope but a welcome addition. The authors also open source their code.
In the section “Outperforming Memorization”, the authors mention a way to tune capacity of the “critic” network and influence its ability to overfit on the sample. This means that if someone wants to compare the generalisation and diversity of samples between GANs, they would need to train the exact same critic CNN to be able to make a comparison. However the authors do not provide any principled way to determine the right size of the "critic" network. In general, given evaluating the metric requires training a network from scratch, it will be very difficult to make this consistent. This makes the proposed benchmark more impractical to use than its alternatives.
In the section “training against the metric”, the authors mention that a main criticism is the fact that a GAN directly optimises for the NND loss. In table 3 we indeed see that this is the case, however the authors argue that perhaps the GAN is simply the better model. I am worried by the fact that both PixelCNN++ and IAF-VAE perform worse than the training set on this benchmark. It seems like this particular benchmark would then work well specifically for GANs, but would (still) not allow us to compare with models trained using maximum likelihood.
In conclusion, I think the paper is well written and the authors clearly make progress towards a dependable benchmark for GANs. The paper does not introduce any new method, but instead has a thorough analysis and discussion of current methods which is worthwhile by itself.
Nits:
Page 7, second paragraph, fifth line, spurious “q”
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Revision
I would like to thank the authors for a thoughtful revision and response. I have updated my score to a 7 and think this paper is a worthy contribution to ICLR. The new drawback section is well written and informative.