PointGPT: Auto-regressively Generative Pre-training from Point Clouds

Large language models (LLMs) based on the generative pre-training transformer (GPT) have demonstrated remarkable effectiveness across a diverse range of downstream tasks. Inspired by the advancements of the GPT, we present PointGPT, a novel approach that extends the concept of GPT to point clouds, addressing the challenges associated with disorder properties, low information density, and task gaps. Specifically, a point cloud auto-regressive generation task is proposed to pre-train transformer models. Our method partitions the input point cloud into multiple point patches and arranges them in an ordered sequence based on their spatial proximity. Then, an extractor-generator based transformer decoder, with a dual masking strategy, learns latent representations conditioned on the preceding point patches, aiming to predict the next one in an auto-regressive manner. Our scalable approach allows for learning high-capacity models that generalize well, achieving state-of-the-art performance on various downstream tasks. In particular, our approach achieves classification accuracies of 94.9% on the ModelNet40 dataset and 93.4% on the ScanObjectNN dataset, outperforming all other transformer models. Furthermore, our method also attains new state-of-the-art accuracies on all four few-shot learning benchmarks.

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Peer review

Reviewer LJmd5/10 · confidence 4/52023-06-30

Summary

This paper investigates self-supervised point cloud learning by introducing the GPT concept (e.g., point order) to the masked modeling framework. Specifically, the clustered point patches are arranged into ordered sequences based on spatial proximity. Then, the masked patches can be predicted without leaking their position (e.g., patch centers) in an auto-regressive manner. Experiments are conducted on object tasks.

Strengths

1. In my view, the most attractive point is the enlarged training set, which brings significant performance improvement over the competitors. (cf. Tab 1). This justifies the importance of the pretraining dataset for point clouds. 2. Significant performance on object tasks (e.g., shape classification), mainly due to the enlarged pertaining dataset. 3. The introduced relative direction prompt via point order is also reasonable, which avoids the position leak of patch reconstruction.

Weaknesses

1. Although the relative direction prompt via point order is highlighted, it brings marginal performance improvement (cf. Tab 4 (e)). However, the enlarged pretrained dataset contributes the most performance improvement and is understated. Therefore, in my view, a paper revision should be conducted. 2. The proposed dual masking strategy sets some attention values to zero, which is similar to the popular attention dropout (cf. attention is all your need). Please clarify their differences. 3. Besides the object-level tasks, more practical downstream tasks (e.g., detection and segmentation with scene input) are expected to be evaluated, especially when the scene datasets are used in the pretraining stage in this paper.

Questions

See the weakness

Rating

5: Borderline accept: Technically solid paper where reasons to accept outweigh reasons to reject, e.g., limited evaluation. Please use sparingly.

Confidence

4: You are confident in your assessment, but not absolutely certain. It is unlikely, but not impossible, that you did not understand some parts of the submission or that you are unfamiliar with some pieces of related work.

Soundness

3 good

Presentation

2 fair

Contribution

2 fair

Limitations

Non

Reviewer 4dwU6/10 · confidence 5/52023-07-04

Summary

This paper proposes PointGPT, a new self-supervised learning strategy for 3D representation learning. PointGPT follows the success of autoregressive pre-training paradigm in NLP and adapts it into 3D point clouds. With larger pre-training dataset and a post-pre-training stage, PointGPT achieves SOTA performance on different benchmarks.

Strengths

1. It's good to introduce GPT-style pre-training into 3D tasks. Although this idea is straightforward, this paper is the first attempt to my best knowledge. 2. The motivation of GPT pre-training in 3D is also reasonable and well clarified (avoid shape information leakage). 3. The writing, equations, and figures are easy to follow.

Weaknesses

1. The experiment tables should incorporate the comparison of learnable parameters during training. It's very important to know the parameters of 3D pre-training to better judge the contribution. 2. How about other methods with larger pre-training dataset and post-pre-training? This is also a necessary ablation study to the paper. 3. It's better to cite this related paper in 'Methods using cross-modal information and teacher models': Learning 3D Representations from 2D Pre-trained Models via Image-to-Point Masked Autoencoders, CVPR 2023

Questions

I think it's an interesting paper. Expect the author rebuttal to solve my concerns for increasing the rating.

Rating

6: Weak Accept: Technically solid, moderate-to-high impact paper, with no major concerns with respect to evaluation, resources, reproducibility, ethical considerations.

Confidence

5: You are absolutely certain about your assessment. You are very familiar with the related work and checked the math/other details carefully.

Soundness

3 good

Presentation

3 good

Contribution

3 good

Limitations

Yes

Reviewer 5U9K7/10 · confidence 5/52023-07-08

Summary

This paper propose an auto-regressively generative pre-training paradigm for point cloud feature encoding. By incorporating GPT, the disorder and low information density properties of point clouds are addressed. Besides GPT, a dual masking strategy is proposed to improve the pre-training performance. The proposed models achieves SOTA o downstream tasks.

Strengths

1. A novel GPT-style point cloud pre-training framework is proposed. 2. Arranging point patches via Morton-order curve is effective. 3. The performance of the PointGPT-L is extraordinary, significantly surpassing previous methods.

Weaknesses

See questions.

Questions

I think this is an excellent work. But I still have some questions: 1. Why PointGPT outperforms PointMAE and PointM2AE? I don’t understand what is ``overall object shape leakage`` in L109. In L163, PointGPT also utilizes position information to extract global structural information. 2. The dual masking strategy seems that further introduce MAE to PointGPT. What’s the performance when using the masking strategy only and removing the GPT loss? 3. The training of PointGPT-S is aligned to previous pretraining methods and PointGPT-S only presents minor improvement compared with Point-M2AE. The PointGPT-B and PointGPT-L are trained on larger datasets but the competitive methods Point-M2AE and Point-MAE are only trained on ShapeNet. Please provide an ablation study such as Point-M2AE+UHD+LHD to show that the large performance gain doesn’t trivially come from the larger training dataset.

Rating

7: Accept: Technically solid paper, with high impact on at least one sub-area, or moderate-to-high impact on more than one areas, with good-to-excellent evaluation, resources, reproducibility, and no unaddressed ethical considerations.

Confidence

5: You are absolutely certain about your assessment. You are very familiar with the related work and checked the math/other details carefully.

Soundness

4 excellent

Presentation

3 good

Contribution

4 excellent

Limitations

The Morton-order curve may not be the optimal order.

Reviewer EUQH5/10 · confidence 3/52023-07-13

Summary

This paper proposed a point clouds pretraining method named PointGPT, which extends the generative pretraining approach of NLP to point clouds. With point patch partitioning and sorting, point embeddings are feed into a transformer decoder for autoregressive prediction. Besides, a dual masking strategy is proposed to enhance the learned representation. PointGPT is evaluated on several point cloud classifiaction, few-shot classification and part segmentation datasets with promising results.

Strengths

- Extend generative pretraining into point clonds. pointGPT let us to rethink the feasibility of generative pretraining on point cloud tasks. - A complete framework with point sequencer and dual masking strategy. Re-arranging point clouds into a sequence of tokens like natural text and RGB images is challenging. PointGPT gives one possible solution using Morton code. The dual masking strategy appears to be a combination of masked modeling and autoregressive pretraining. - Promising results on a set of datasets including classfication, few-shot classification and part seggmentation.

Weaknesses

- Is it reasonable to use Morton code to sort unordered point clouds? As shown in Fig.2, PointGPT forces the current point $i$ to predict the coordinates of a point sorted by Morton code. However, the point $i$ should be able to predict any point adjacent to its 3D space, rather than a specified point. So what are the advantages of using Morton code sorting as the prediction target compared to conventional left-to-right and top-to-bottom prediction? - The real gain of PointGPT (PointGPT-S) compared with the previous masked modeling methods (Point-M2AE) is not significant. Although PointGPT-B/L show an improvement on these tasks, they use more parameters and more training data. - The overall contribution is limited. PointGPT is inspired by generative pretraining in NLP, but it doesn't show us how effective the generative pretraining approach is on point cloud tasks compared to previous methods.

Questions

See weaknesses.

Rating

5: Borderline accept: Technically solid paper where reasons to accept outweigh reasons to reject, e.g., limited evaluation. Please use sparingly.

Confidence

3: You are fairly confident in your assessment. It is possible that you did not understand some parts of the submission or that you are unfamiliar with some pieces of related work. Math/other details were not carefully checked.

Soundness

2 fair

Presentation

3 good

Contribution

2 fair

Limitations

Yes.

Reviewer 4dwU2023-08-17

Thanks for the rebuttal. It addresses most of my concerns. It looks like PointGPT has good parameter and training efficiency. I recommend the authors include these results (parameter comparison and -B results) in the revised version. I will raise the rating. Also, I expect the authors to cite and discuss two other related works in CVPR 2023. [1] Parameter is not all you need: Starting from non-parametric networks for 3d point cloud analysis. CVPR 2023 [2] Meta Architecture for Point Cloud Analysis. CVPR 2023

Authorsrebuttal2023-08-18

Thanks for your feedback!

Thanks for upgrading your score and providing valuable feedback! The submitted results will be integrated into the revised version, and these related papers will be cited and discussed in the revised article. **(1)** In the first paper, an innovative non-parametric network, Point-NN, is proposed. Building upon this, the Point-PN network is introduced, achieving promising performance across diverse tasks with only a few learnable parameters. **(2)** The second paper proposes a unified framework called PointMeta, exploring appropriate practices for each component to derive a fundamental building block named PointMetaBase. The proposed approach improves accuracy while reducing computational complexity. These approaches introduce effective frameworks that yield promising performance across a range of tasks. However, they still necessitate fully-supervised training from scratch. In contrast, PointGPT is able to acquire latent representations without relying on annotations, achieving enhanced generalization in downstream tasks. Notably, **our PointGPT consistently demonstrates superior performance** across the majority of benchmark evaluations. This verifies the effectiveness of our autoregressive generative pre-training approach for point clouds, affirming its capacity to enhance generalization and accuracy in downstream tasks. Thanks again for your insightful and constructive suggestions that improve paper quality! We will be happy to address any further questions or concerns about the work.

Reviewer LJmd2023-08-17

Thanks for the clarification. The effectiveness of the relative direction prompt and the proposed masking strategy is well received. Given that I am still concerned about the marginal improvement (i.e., about 0.5% Acc on average in Tab of Q1(2)) when fairly compared with the STOA (e.g., Point-M2AE), I will slightly raise my score to Borderline Accept.

Authorsrebuttal2023-08-18

Thanks for your feedback!

Thanks for considering our responses and raising the score! We will update our revised paper according to our discussions. Thanks again for your insightful and constructive suggestions that improve paper quality. We will be happy to address any further questions or concerns about the work.

Program Chairsdecision2023-09-21

Decision

Accept (poster)

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