Enhancing Consistency-Based Image Generation via Adversarialy-Trained Classification and Energy-Based Discrimination

The recently introduced Consistency models pose an efficient alternative to diffusion algorithms, enabling rapid and good quality image synthesis. These methods overcome the slowness of diffusion models by directly mapping noise to data, while maintaining a (relatively) simpler training. Consistency models enable a fast one- or few-step generation, but they typically fall somewhat short in sample quality when compared to their diffusion origins. In this work we propose a novel and highly effective technique for post-processing Consistency-based generated images, enhancing their perceptual quality. Our approach utilizes a joint classifier-discriminator model, in which both portions are trained adversarially. While the classifier aims to grade an image based on its assignment to a designated class, the discriminator portion of the very same network leverages the softmax values to assess the proximity of the input image to the targeted data manifold, thereby serving as an Energy-based Model. By employing example-specific projected gradient iterations under the guidance of this joint machine, we refine synthesized images and achieve an improved FID scores on the ImageNet 64x64 dataset for both Consistency-Training and Consistency-Distillation techniques.

Paper

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Peer review

Reviewer zvWf5/10 · confidence 3/52024-07-10

Summary

The paper introduces a post-processing method using a joint classifier-discriminator model to imporve the generation quality of consistency models. This model is trained adversarially, with the robust classifier providing gradient based on class assignment and the discriminator score. This post-processing method consistently improves the quality of the generated images in both single-step and double-step cases, resulting in better FID scores for both consistency distillation and consistency training.

Strengths

- The method has connections, and it is possible to unify the Robust Classifier and Energy-based Models with consistency models. - The proposed method is efficient and easy to plug-and-play in consistency distillation and consistency training. - The paper is overall easy to read and well organized.

Weaknesses

- Although the method seems to be connected to the Robust Classifier and Energy-based Models, the authors did not give comprehensive theoretical derivation of the connections. The resulted loss is simply linear combination of binary CE and CE. - Is the proposed Robust Joint-Model specifically useful for consistency model? If so the authors may need to give analysis why this is the case. If not, it worth to study how the proposed method applicable with other generative models. - How does the radius $\epsilon$ affects the final performance? Also, how do the other hyper-parameters affect the model performance? The authors should give ablation studies regarding these parts. - From the empirical results in Table 3, the improvement of the final loss is marginal compared to using CE and BCE only sometimes. Some minors: - line 124: algorithm (missed) 1.

Questions

Please see the Weakness.

Rating

5

Confidence

3

Soundness

3

Presentation

2

Contribution

2

Limitations

The authors have discussed the limitations and potential negative societal impact of their work.

Reviewer JUbG6/10 · confidence 3/52024-07-11

Summary

This paper proposes a post-processing method to improve the generated image quality of consistency models. The method mainly includes a joint classifier-discriminator, which contains an adversarial training objective and a gradual training approach. Experimental results show the effectiveness of the proposed method.

Strengths

(++) The method can be seamlessly integrated into generative models, facilitating applications. (+) The writing is easy to follow.

Weaknesses

(--) The evaluations are limited. Only ImageNet at resolution 64x64 is utilized, which could not be representative since the development trend is high-resolution generation. Datasets that contain larger images should be considered, such as LSUN and FFHQ with resolution larger than 256x256. Meanwhile, more metrics, such as the precision and recall, are also worth considering for better evaluation about the data distribution. Minor issue: 1. A typo at line 55: We show a ... improvements ... 2. The case rules of the journal or conference names in the reference are inconsistent sometimes.

Questions

Please refer to Weaknesses.

Rating

6

Confidence

3

Soundness

3

Presentation

3

Contribution

3

Limitations

The limitations have been well discussed.

Reviewer 4wrH7/10 · confidence 3/52024-07-12

Summary

The paper proposes a method to mitigate the quality degradation of images generated by consistency-based generative models compared to diffusion models. The main idea is to adapt the generated image based on the logits of an adversarial real/synthetic image discriminator to maximize the image resemblance to real images. A single classifier is trained jointly for both discrimnation between real/synthesized images and for assignment to designated categories. Optimization in the generative model's output space makes the method more efficient than methods based on updating the model parameters. The method is evaluated on the ImageNet 64x64 dataset, showing performance gains in FID and IS over consistency-based models.

Strengths

1. The method is novel, simple (easy to understand and implement) and outperforms baselines on the evaluation dataset. It has the potential to inspire further research in this direction. 2. The paper is easy to follow, all the methodology is well justified and well explained, design choices such as the loss function are supported by ablations 3. Both CT and CD, with different number of inference steps are evaluated

Weaknesses

1. Evaluation on a single dataset with small resolution is not sufficient to judge the performance of genrative methods. 2. Very limited qualitative results See questions for more details on both.

Questions

### Weakness 1: 1. Other datasets such as ImageNet with higher resolution, COCO or flicker faces would help to judge the method’s performance and generalizability. ### Weakness 2: 2. While the method outperforms baselines in quantitative evaluation, more visualizations should be provided, especially since the method is about generative models. I would recommend including a dataset like Flicker Faces for visualizations since human perception is sensitive to artefacts in the human face. ### Typos.: 3. Line 93 - 'in two main ways []' - missing citations 4. Line 124 – something went wrong at the beginning of the line, '1' should be 'Alg. 1' for better readability, also maybe the sentence should end after Alg. 1? ### Presentation: 5. While the paper is clearly written, it would be helpful to have a figure overview of the pipeline, rather than just equations.

Rating

7

Confidence

3

Soundness

3

Presentation

3

Contribution

3

Limitations

Limitations are addressed in a dedicated section.

Reviewer Jjs55/10 · confidence 4/52024-07-14

Summary

The manuscript proposes a method for refining consistency-based image generation using a joint-trained classifier-discriminator. The method helps to improve the quality of the fast-sampling consistency models.

Strengths

1. The paper is well written 2. the performance is significant

Weaknesses

1. The motivation of the work is not clear. The main motivation of the work is to postprocess generated images to improve perceptual quality (line 36-38). In lines 39 - 41, the authors mention the adversarial attacks as the main reason why the perceptual quality is low. There is a gap between these two problems. I agree that an adversarial problem would cause poor alignment, but this phenomenon happens to other models as well, not only the consistency-based model. 2. Furthermore, the proposed method does not only focus on the robustness problem (only in section 3.1.2). The motivation behind joint classifier-discriminator is not clear. Although this scheme is very popular in many generative models, it is not easy to point out its motivation in the context of the manuscript. From line 170 to 175, there is a mention about the drawback of EBMs, yet it is not clear how classifier-discriminator training would solve it. 3. The analysis is lacked. From line 170 to 175, the authors mention about the sensitivity of EBM, instability and poor generalization. However, no evidence is provided. 4. The qualitative improvement is limited and hard to observe. This raises the question whether the method is effective or not.

Questions

see weaknesses.

Rating

5

Confidence

4

Soundness

3

Presentation

3

Contribution

2

Limitations

N/A

Reviewer JUbG2024-08-10

Thank you for your response. I recommend including these results in the paper, or the Appendix if they exceed the page limit. There are no more questions.

Reviewer zvWf2024-08-10

I appreciate the time and effort the authors have taken to address the concerns. The clarifications have addressed my questions and I have increased my score. Please make sure to incorporate these discussions in the final revision.

Reviewer 4wrH2024-08-11

Thank you for the response and effort! I am increasing my score based on the additional experiments. My biggest remaining concern is the qualitative results; they mostly seem limited to color enhancements with maybe the exception of the robe image in Figure 1. I would still consider qualitative results with at least animal faces.

Reviewer Jjs52024-08-12

Thanks for the response

The rebuttal has addressed my concerns. I will increase the score. Best regards,

Program Chairsdecision2024-09-25

Decision

Accept (poster)

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