NeurIPS 2020

Hierarchical Quantized Autoencoders

Review 1

Summary and Contributions: Hierarchies of VQ-VAEs (e.g. VQ-VAE-2 and HAM) can be used for lossy compression at lower rates than simple VQ-VAEs. The authors propose modifications to current methods in this area, leading to higher quality reconstructions at very low rates without using slow autoregressive components. This includes a novel objective for training hierarchies of VQ-VAEs with stochastic posterior distributions.

Strengths: Although in my opinion not grounded and developed well enough, the argument in favor of a stochastic posterior (with each VQ-VAE layer learning to reconstruct the full distribution of the layer below, rather than a sample) is interesting and definitely worth exploring. Empirical results seem to be relatively impressive (at least for CelebA). This work is part of an active research area that is interesting e.g. for the compression and (deep) generative modeling communities. It is therefore relevant for NeurIPS.

Weaknesses: I believe the contributions might be a bit overblown. In particular, in the first bullet point the authors mention an "analysis as to why probabilistic quantized hierarchies are particularly well-suited to optimising the perception-rate tradeoff when performing extreme lossy compression", but there doesn't seem to be a thorough and conclusive analysis on this matter. Points (2, 3) and (4, 5) are in my opinion spread too thin and should be combined. I find the model description somewhat lacking in clarity. I think everything would be much more understandable with a simple, but more structured, exposition (with equations) of the model, and possibly also of VQ-VAE, as this work heavily builds on it. Section 4 is in my opinion quite unclear. First, details of HQA are mentioned before it is introduced. Then, the toy experiment is only very briefly explained, and definitely needs more details (which are not in the Appendix as far as I can see). It is hard to assess the correctness and significance of the argument put forward by the authors, without a thorough understanding of the setup. Third, the structure and writing of this section is confusing, especially Section 4.3. Finally, I don't get how it's fair to compare the toy 2-layer HQA with a simple VQ-VAE with 2-code latents. The authors ask whether it's possible to model the toy density correctly by using 2 codes only, but then solve the problem by stacking 2 VQ-VAEs, of which the bottom one has in fact 4 codes. I'm sure this is a misunderstanding on my part, but I think it means that the argument is not explained effectively enough. The empirical evaluation could be more extensive, for example regarding ablation studies, and other widely used datasets such as ImageNet or FFHQ. The concept of rate-perception tradeoff, and its relation to the rate-distortion tradeoff, could be explained in more detail and briefly discussed in the experimental section. There is significant overlap with previous work, which raises questions about the potential influence of this work on the community, especially since most of the novelty seems to come from introducing stochasticity. This is in general a nice idea, although the novelty is limited. But I believe the paper needs restructuring, a clearer exposition of arguments and methods, and maybe, if possible, even an expanded experimental section. === EDIT after rebuttal: Thanks for the clarifications. I'm raising my score, but I still think this work is not particularly novel and, perhaps even more importantly, the ideas&model should be presented more clearly. In my opinion this would allow this paper to have a broader impact than just the learned image compression community. Regarding additional experiments: that would of course be a plus, but it's not my main concern.

Correctness: Claims, method, and empirical methodology seem correct.

Clarity: The paper is generally well written. My clarity concerns are related to the technical presentation only.

Relation to Prior Work: Closely related work is discussed in enough depth. However, works that are more loosely related to this are not mentioned. For example, there is a very rich literature on hierarchical VAEs, and on VAEs with discrete latent variables, and it would be interesting to read arguments for/against using these other approaches.

Reproducibility: Yes

Additional Feedback:

Review 2

Summary and Contributions: The paper expands on prior work on vector-quantized VAEs (VQVAE) and hierarchical autoregressive image models (De Fauw, 2019) by presenting a new compression scheme called Hierarchical Quantized Autoencoders (HQA) with a novel loss objective in comparison to VQ-VAEs. The proposed model introduces a hierarchical latent structure and replaces the deterministic posterior of VQ-VAE with Gumbel Softmax relaxation. It can produce high perceptual quality despite high compression and without relying on autoregressive decoders used by VQVAE and VQVAE2. In the low-rate lossy compression context, the method outperforms the baselines with FID and downstream classification task in MNIST, and visually in Celeba 64x64.

Strengths: The formulation as a whole is novel and seems theoretically solid. The significance of the work seems most obvious in the context of very high compression rate. In CelebA 64x64, the quality of reconstructions degrades gracefully when compression increases, unlike with the other models (as seen in Table 2). The proposed hierarchical latent approach seems sensible and could be more robust and less intensive at evaluation-time than the approach taken in VQ-VAE, especially as it does not require an autoregressive decoder. These ideas seem relevant in the continuing research for improving the vector quantized generative models.

Weaknesses: My biggest concern is about giving the primary (non-ablation) results only on a single dataset (CelebA) and the use of only low-resolution images (32x32 and 64x64) across the experiments. VQ-VAE, VQ-VAE2, and HAM all present results also on 256x256 resolution. While it is plausible that the proposed method scales gracefully to the larger resolutions, in generative models this cannot be taken for granted, as it can e.g. lead to highly unstable behaviour or infeasible computational complexity (in absence of any discussion to the contrary by the authors). Because the closest prior work does show those higher resolutions, this omission makes the paper feel incomplete. Although the performance of the model in the context of very high compression (i.e. low rate) appears good in 64x64 images, it is not obvious how these benefits scale to the larger resolutions. While the paper emphasizes the compression view of generative modelling, it would be useful to see random sampling performance and FIDs also for random samples, not only for reconstructions. In absence of that, it is harder to put the paper fully into the context of previous models, which nearly always provide these figures. (To be clear, I do not find it a problem if the FIDs for this model are not good, but it is a fairly simple thing to measure for the sake of completeness.) Due to the resolution limitations and scarce details about the computational requirements, I cannot honestly judge whether the proposed method is as relevant as the authors imply. EDIT after rebuttal: In the light of the rebuttal and the discussions with other reviewers, I concede that the resolution question, though highly relevant, does not have to be a show-stopper. Hence I have upgraded my score.

Correctness: The theoretical exposition seems correct, but the empirical justification for the claims, though acceptable as such, seems too limited. In this reviewer's opinion, a single dataset used only up to 64x64 resolution is not enough for the primary experiments in this context. Given that FID is famously dependent on sample size, I find the authors should definitely mention it in the context of FID calculations. (However, I base my initial estimate on the assumption that the sample size was sufficient.) The discussion of mode-covering in Sec 4 is interesting and seem to imply a fundamental improvement over the VQVAE approach. However, the claim (L220-222) that "the HQA reconstructions display higher perceptual quality than both VQ-VAE and HAMs at all compression rates" seems false, at least as far as Table 2 is concerned. The difference compression rates higher than 43x seem obvious, but I am hard-pressed to see differences at the ones before that (at the left). There could be a difference, but this is not obvious at the level of visual comparison. Moreover, I would not consider two sample faces sufficient for that kind of evaluation. E.g. HAMs seems to fail considerably for 683x and onwards, but it would be illustrative to see this analyzed for at least 10 random inputs for each model (e.g. in the supplement), if you seek to make a strong qualitative argument.

Clarity: The writing was mostly clear, but there are some parts of the paper that were somewhat confusing, especially when referring to 'Mode-covering' in Sec 4.3 and earlier. Minor: having section 3 only consist of subsection 3.1 is bad style.

Relation to Prior Work: The relationship to HAM could be made clearer. L96-98 sums it up, but this seems to me like the most relevant reference, and the relationship should be elaborated more.

Reproducibility: Yes

Additional Feedback: I was somewhat divided about this paper, since I find the ideas (and the experiment in Table 2) largely compelling, but I am disappointed to see the primary evaluation only on CelebA 64x64, and ablation only for MNIST. (MNIST evaluation might be acceptable for the ablation study, if there were more primary experiments on higher resolution datasets.) Therefore, I must reluctantly give lower score than I would like, and to recommend the authors to expand the paper by another high-resolution dataset. I assume some of the experiments here may be computationally intensive, though for the HQA model, the computational burden should be less than with the autoregressive decoders, correct? However, I see no mention of the computational requirements for the training and evaluation. Also, if HQA is considerably faster to evaluate than e.g. VQ-VAE, as one would expect, I believe you could emphasize this more. Note on reproducibility: The provided source code seems to support only the MNIST experiments. Will you be releasing the whole source code?

Review 3

Summary and Contributions: The paper improves over VQ-VAE by learning a hierarchy of discrete latent variables. Contrary to VQ-VAE they don't need to learn autoregressive priors over the latent to achieve good reconstructions even at low bitrates. The main contribution of the paper is to propose to use a stochastic posterior for the latent and a new probabilistic loss. They show experimentally that the visual quality of the reconstructions of their methods is superior to other discrete auto-encoders.

Strengths: The experiments are convincing, showing that the proposed approach enables better reconstruction according to rFID, especially at low bitrates. The proposed approach seems like a good step in the direction of learning low bitrates representation of images.

Weaknesses: The novelty of the approach is quite limited. The approach is close to VQ-VAE, the main difference being the stochastic posterior (already proposed in the literature as mentioned in the paper) and the probabilistic loss. It's not clear in the paper to which extent the probabilistic loss is important. The paper claim that it helps stabilize training on CelebA, but without any numbers or figure supporting this statement.

Correctness: The proposed approach although not supported by a strong theoretical grounding, is to some extent "close" to some ELBO objective as shown in the appendix of the paper. The empirical methodology is correct and supports the validity of the approach. However it would have been nice to have some confidence intervals about the results provided.

Clarity: The paper is overall well written and quite clear. However, I find section 4 not very informative.

Relation to Prior Work: The authors clearly discussed the related work, however the proposed approach is equivalent to fitting a hierarchical prior over the latent variables, in that sense it's related to autoregressive models that try to fit an autoregressive prior over the latent. I think this could be discussed a bit more.

Reproducibility: Yes

Additional Feedback: === Edit after rebuttal === I took into account the author response. I will maintain my score as I think the paper could still be improved but I think it meets the bar for acceptance. I encourage the author to clarify section 4 and the contributions in the next version and include results on higher resolution as mentioned by other reviewers.

Review 4

Summary and Contributions: This paper proposes to use a hierarchy of VQ-VAEs for lossy image compression. A stochastic posterior and adjusted training objective are introduced to better fit such a model to this task. Experiments show good performance at the challenging low bitrate regime.

Strengths: - The scheme proposed in this paper effectively tackles the challenging problem of maintaining realism and high-level semantic content in the scenario of (extreme) low-rate image compression. - Using a VQ-VAE stack jointly with probabilistic quantization seems to be an elegant method for remedying the mode covering / mode dropping behavior which is often present in image compression / reconstruction tasks. The authors present a very good explanation of how a probabilistic quantized hierarchy can alleviate these problems (section 4). - The proposed method performs better than the baselines both on common metrics and in qualitative visual comparisons. In particular, this model excels at the low bitrate regime, where obtaining realistic reconstructions is very challenging. - This paper can have a significant impact on the sub-community working on these tasks.

Weaknesses: - I think comparisons to prior work on generative lossy image compression (not only to prior work on hierarchical stacks of VQ-VAEs) are missing. In particular, a comparison to ref [39] (for which online code is available) could be interesting. - This paper builds on prior work which proposed similar concepts. The authors indeed discuss these works and the added contribution well, yet this somewhat limits the novelty.

Correctness: Yes.

Clarity: Very well written.

Relation to Prior Work: Yes.

Reproducibility: Yes

Additional Feedback: ===== Edit after rebuttal ===== After reading all the reviews and the author feedback, I think this is a very good paper and recommend it be accepted. The author feedback on the main points raised in the reviews is convincing. I would encourage the authors in any final revision: (a) To clarify section 4. (b) To include experiments at higher resolution.