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Title: Evaluating Lossy Compression Rates of Deep Generative Models

Paper content:
Deep generative models have achieved remarkable progress in recent years.
Despite this progress, quantitative evaluation and comparison of generative models remains as one of the important challenges.
One of the most popular metrics for evaluating generative models is the log-likelihood.
While the direct computation of log-likelihood can be intractable, it has been recently shown that the log-likelihood of some of the most interesting generative models such as variational autoencoders (VAE) or generative adversarial networks (GAN) can be efficiently estimated using annealed importance sampling (AIS).
In this work, we argue that the log-likelihood metric by itself cannot represent all the different performance characteristics of generative models, and propose to use rate distortion curves to evaluate and compare deep generative models.
We show that we can approximate the entire rate distortion curve using one single run of AIS for roughly the same computational cost as a single log-likelihood estimate.
We evaluate lossy compression rates of different deep generative models such as VAEs, GANs (and its variants) and adversarial autoencoders (AAE) on MNIST and CIFAR10, and arrive at a number of insights not obtainable from log-likelihoods alone.
Generative models of images represent one of the most exciting areas of rapid progress of AI (Brock et al., 2019; Karras et al., 2018b; a) .
However, evaluating the performance of generative models remains a significant challenge.
Many of the most successful models, most notably Generative Adversarial Networks (GANs) (Goodfellow et al., 2014) , are implicit generative models for which computation of log-likelihoods is intractable or even undefined.
Evaluation typically focuses on metrics such as the Inception score (Salimans et al., 2016) or the Fréchet Inception Distance (FID) score (Heusel et al., 2017) , which do not have nearly the same degree of theoretical underpinning as likelihood-based metrics.
Log-likelihoods are one of the most important measures of generative models.
Their utility is evidenced by the fact that likelihoods (or equivalent metrics such as perplexity or bits-per-dimension) are reported in nearly all cases where it's convenient to compute them.
Unfortunately, computation of log-likelihoods for implicit generative models remains a difficult problem.
Furthermore, log-likelihoods have important conceptual limitations.
For continuous inputs in the image domain, the metric is often dominated by the fine-grained distribution over pixels rather than the high-level structure.
For models with low-dimensional support, one needs to assign an observation model, such as (rather arbitrary) isotropic Gaussian noise (Wu et al., 2016) .
Lossless compression metrics for GANs often give absurdly large bits-per-dimension (e.g. 10 14 ) which fails to reflect the true performance of the model (Grover et al., 2018; Danihelka et al., 2017) .
See Theis et al. (2015) for more discussion of limitations of likelihood-based evaluation.
Typically, one is not interested in describing the pixels of an image directly, and it would be sufficient to generate images close to the true data distribution in some metric such as Euclidean distance.
For this reason, there has been much interest in Wasserstein distance as a criterion for generative models, since the measure exploits precisely this metric structure Gulrajani et al., 2017; Salimans et al., 2018) .
However, Wasserstein distance remains difficult to approximate, and hence it is not routinely used to evaluate generative models.
We aim to achieve the best of both worlds by measuring lossy compression rates of deep generative models.
In particular, we aim to estimate the rate distortion function, which measures the number of bits required to match a distribution to within a given distortion.
Like Wasserstein distance, it can exploit the metric structure of the observation space, but like log-likelihoods, it connects to the rich literature of probabilistic and information theoretic analysis of generative models.
By focusing on different parts of the rate distortion curve, one can achieve different tradeoffs between the description length and the fidelity of reconstruction -thereby fixing the problem whereby lossless compression focuses on the details at the expense of high-level structure.
It has the further advantage that the distortion metric need not have a probabilistic interpretation; hence, one is free to use more perceptually valid distortion metrics such as structural similarity (SSIM) (Wang et al., 2004) or distances between hidden representations of a convolutional network (Huang et al., 2018) .
Algorithmically, computing rate distortion functions raises similar challenges to estimating loglikelihoods.
We show that the rate distortion curve can be computed by finding the normalizing constants of a family of unnormalized probability distributions over the noise variables z.
Interestingly, when the distortion metric is squared error, these distributions correspond to the posterior distributions of z for Gaussian observation models with different variances; hence, the rate distortion analysis generalizes the evaluation of log-likelihoods with Gaussian observation models.
Annealed Importance Sampling (AIS) (Neal, 2001 ) is currently the most effective general-purpose method for estimating log-likelihoods of implicit generative models, and was used by Wu et al. (2016) to compare log-likelihoods of a variety of implicit generative models.
The algorithm is based on gradually interpolating between a tractable initial distribution and an intractable target distribution.
We show that when AIS is used to estimate log-likelihoods under a Gaussian observation model, the sequence of intermediate distributions corresponds to precisely the distributions needed to compute the rate distortion curve.
Since AIS maintains a stochastic lower bound on the normalizing constants of these distributions, it automatically produces an upper bound on the entire rate distortion curve.
Furthermore, the tightness of the bound can be validated on simulated data using bidirectional Monte Carlo (BDMC) (Grosse et al., 2015; Wu et al., 2016) .
Hence, we can approximate the entire rate distortion curve for roughly the same computational cost as a single log-likelihood estimate.
We use our rate distortion approximations to study a variety of variational autoencoders (VAEs) (Kingma & Welling, 2013) , GANs and adversarial autoencoders (AAE) (Makhzani et al., 2015) , and arrive at a number of insights not obtainable from log-likelihoods alone.
For instance, we observe that VAEs and GANs have different rate distortion tradeoffs: While VAEs with larger code size can generally achieve better lossless compression rates, their performances drop at lossy compression in the low-rate regime.
Conversely, expanding the capacity of GANs appears to bring substantial reductions in distortion at the high-rate regime without any corresponding deterioration in quality in the low-rate regime.
We find that increasing the capacity of GANs by increasing the code size (width) has a qualitatively different effect on the rate distortion tradeoffs than increasing the depth.
We also find that that different GAN variants with the same code size achieve nearly identical RD curves, and that the code size dominates the performance differences between GANs.
In this work, we studied rate distortion approximations for evaluating different generative models such as VAEs, GANs and AAEs.
We showed that rate distortion curves provide more insights about the model than the log-likelihood alone while requiring roughly the same computational cost.
For instance, we observed that while VAEs with larger code size can generally achieve better lossless compression rates, their performances drop at lossy compression in the low-rate regime.
Conversely, expanding the capacity of GANs appears to bring substantial reductions in distortion at the high-rate regime without any corresponding deterioration in quality in the low-rate regime.
This may help explain the success of large GAN architectures (Brock et al., 2019; Karras et al., 2018a; b) .
We also discovered that increasing the capacity of GANs by increasing the code size (width) has a very different effect than increasing the depth.
The former extends the rate distortion curves leftwards, while the latter pushes the curves down.
We also found that different GAN variants with the same code size has almost similar rate distortion curves, and that the code size dominates the algorithmic differences of GANs.
Overall, lossy compression yields a richer and more complete picture of the distribution modeling performance of generative models.
The ability to quantitatively measure performance tradeoffs should lead to algorithmic insights which can improve these models.

assistant: text: We study rate distortion approximations for evaluating deep generative models, and show that rate distortion curves provide more insights about the model than the log-likelihood alone while requiring roughly the same computational cost.