NeurIPS 2020

Improving Inference for Neural Image Compression

Review 1

Summary and Contributions: 1. Authors propose Stochastic Gumbel Annealing (SGA) method - a novel way of relaxing objectives involving discretized representations which results in improved optimization. Significance: High 2. Authors propose a scheme for using bits-back coding in the context of lossy compression with hierarchical autoencoders which doesn't yield large empirical improvements but is nevertheless an interesting methodological contribution. Significance: Medium 3. Authors perform a clean and comprehensive empirical investigation of the influence of different methods of optimizing the quantized representation at test/compression time, and a good ablation study of their proposed method. Significance: Medium/High. To my best knowledge and judgement, all 3 of them are novel and valuable. POST REBUTTAL: Thanks to the authors for quite thoroughly addressing my questions, I think it's a good submission, and I keep my score fixed at 7.

Strengths: Described in the contributions section.

Weaknesses: I have some concerns about the computational cost of the methods outlined above but it doesn't decrease the value of the work.

Correctness: To my best knowledge the claims and method is correct and the empirical methodology follows standard practice in the subfield.

Clarity: The paper is well organized and the exposition is clear. The paper provides a good structure of thinking about the problem of inference for compression by breaking down the sources of approximation gap and addressing each aspect individually. To me, this clarity becomes another contribution of the paper. Supplementary materials provide a good amount of implementation and experimental details, code was provided.

Relation to Prior Work: I think that the results are significant to the community interested in learning discrete representations, and of interest to wider NeurIPS community. To my best knowledge and judgement, all 3 of the contributions mentioned above are novel and valuable.

Reproducibility: Yes

Additional Feedback: Questions to authors: - This paper uses SGA only at test/compression time. Can SGA be used at training time as well? If so, would you expect that training the network with SGA would change the representation learned to yield even better results than those offered in this paper? Can you comment on that? - Could you provide some approximate timing comparison between different models? Particularly M1-3 A5 A6, although the more the better. I think the paper deserves some discussion on the trade-off between the amount of computation performed and the RD performance. I know it's such a cliche point to raise, but it's surely of interest to many. Your work is clearly valuable even if the computational considerations do not favour practical use of iterative inference or BB at test time, and any further information on this aspect won't affect my score negatively. Clarity: - fix the sentence l129-130 - mention an illustration of SGA relaxed objective in Section S1 in the main text, it's a good illustrative example

Review 2

Summary and Contributions: This paper improves the learned lossy image compression by enhancing the inference stage. Specifically, it introduces a new quantization method and an improved entropy coding model.

Strengths: - The ideas to improve the inference of learned image compression is very interesting. - The proposed methods are straightforward and sound. - The experimental results are extensive and comprehensive.

Weaknesses: - The proposed methods are based on previous iterative compression [Campos et al., 2019] and may lack of novelty. - Besides, the improvements from bitsback coding are relatively marginal, which means the main contribution comes from the quantization and novelty may be not sufficient for neurips.

Correctness: The claims are correct and the evaluations are comprehensive.

Clarity: The paper is well written and easy to follow.

Relation to Prior Work: Provide provides the differences between the proposed method and [Campos et al., 2019] in the implementation stage.

Reproducibility: Yes

Additional Feedback: - Is it possible to implemnt the proposed method based on the M4(context+hyperprior)? It would be better to provide the experimental results. - It is encourage to compare the proposed with the recent work in ECCV20[R1], where the whole encoder is updated for better RD performance. Is it possible to use this method to improve the inference for image compression? [R1] Guo Lu, Chunlei Cai, Xiaoyun Zhang, Li Chen, Wanli Ouyang, Dong Xu, and Zhiyong Gao. "Content Adaptive and Error Propagation Aware Deep Video Compression." arXiv preprint arXiv:2003.11282 (2020).

Review 3

Summary and Contributions: This paper proposed several improved techniques for lossy compression with a deep latent variable model, i.e. a hierarchical variational autoencoder. The authors conduct a comprehensive analysis on the amortization gap and the discretization gap which could be the key bottleneck on bridging the variational inference and lossy compression. And the proposed SGA and lossy bits back coding have achieved impressive empirical results on the compression benchmarks. POST REBUTTAL: Thanks to authors for addressing my concerns and provide new sensitive analysis results. I think the paper is above the acceptance threshold and I vote for acceptance.

Strengths: Exploring the potential of the generative model in compression is an important research direction. The proposed method is well motivated. The introduced SGA method is novel, which could optimize the rate-distortion objective with a small discretization gap. Though bits-back coding has been explored in the lossless compression scenario with latent variable models, it is nice to see the attempt in lossy compression settings. The experiment settings are extensive and the results are solid and impressive. And the overall writing is sound.

Weaknesses: The paper introduces two components in the paper. The stochastic Gumbel Annealing (SGA) tends to approximate the discretization with an annealing scheme. The main concern lies in how the model performance is influenced by the choice of annealing scheme. It would be better to add more details and sensitive analysis to show the effect of temperature annealing. The lossy bits back coding part may also lack some details: 1. How the entropy coding is implemented, i.e., which exact coding scheme is used? 2. The bits back coding saves the bitlength by getting the uniform bits during sampling of latent variable back at decoder part which could be used to transmit the so-called "side information". My question is how the side information is designed? If the side information refers to the encoded results of the previous image, then the benefit is related to the sequence length. Then how the sequence length is selected. If not, then what is the exact side information encoded and how the comparison between the proposed methods and other baseline is conducted. Besides, the benefit of lossy bits back coding seems to be marginal as shown in Figure 3. Another question is whether the proposed procedure is only compatible with the hierarchical vae? Maybe the lossy bits back coding is not easy to apply in the vanilla vae, but it will be a benefit to show the performance of SGA in a vanilla vae setting.

Correctness: yes

Clarity: yes

Relation to Prior Work: yes

Reproducibility: Yes

Additional Feedback: The SGA need the gumbel trick to backpropagate the gradient, it may be helpful to try some low variance estimator such as VIMCO[1]. [1]Variational inference for monte carlo objectives

Review 4

Summary and Contributions: In this paper, a new inference scheme is proposed to improve the performance of lossy image compression with deep latent variable models. The proposed scheme only changes inference at test time, therefore existing model structures and other innovations can be easily combined. The proposed method results in improved amortization, improved discretization and improved entropy coding.

Strengths: 1. Overall, the paper is well written. The proposed inference scheme is well described. 2. Both qualitative and quantitative comparisons are provided and showed that the proposed method improved compression quality. 3. A good ablation study is provided to justify the effectiveness of each part of the model.

Weaknesses: 1. In all the illustration and the equations, there is a clear definition of two hierarchies in the latent space: latent variable y and hyperlatents z. It would be more convincing if the author could provide some reasoning behind the choice of the latent structure. I am not sure why the latent structure is restricted to a 2-layer structure. Recent progress in VAE community including ladderVAE and structuredVAE shows that either a ladder structure with many hierarchies or a BN as the latent structure leads to better modelling performance. I would like the authors to explain if the proposed method can be applies to these innovations as well. 2. The model combines several existing techniques together, including iterative inference, Gumbel annealing, etc. Although the combination of these techniques seem to be working for the compression task, the overall novelty seems to be incremental.

Correctness: Yes.

Clarity: Yes.

Relation to Prior Work: Yes, the authors specifically discussed the difference between this method with related works. However, as mentioned before, this paper seems to be combining the innovations from several other papers.

Reproducibility: Yes

Additional Feedback: After the rebuttal, I believe the authors have addressed most of the concerns. So I will stay on the positive side to suggest accept the paper.