econSG: Efficient and Multi-view Consistent Open-Vocabulary 3D Semantic Gaussians

Authors: Can Zhang, Gim Hee Lee
Primary area: applications to computer vision, audio, language, and other modalities
Keywords: 3d scene understanding, gaussian splatting, open-vocabulary 3d semantic

Abstract:
The primary focus of most recent works on open-vocabulary neural fields is extracting precise semantic features
from the VLMs and then consolidating them efficiently into a multi-view consistent 3D neural fields
representation. However, most existing works over-trusted SAM to regularize image-level CLIP without any further refinement. Moreover, several existing works improved efficiency by dimensionality reduction of semantic features from 2D VLMs before fusing with 3DGS semantic fields, which inevitably leads to multi-view inconsistency. In this work, we propose econSG for open-vocabulary semantic segmentation with 3DGS. Our econSG consists of: 1) A Confidence-region Guided Regularization (CRR) that mutually refines SAM and CLIP to get the best of both worlds for precise semantic features with complete and precise boundaries. 2) A low dimensional contextual space to enforce 3D multi-view consistency while improving computational efficiency by fusing backprojected multi-view 2D features and follow by dimensional reduction directly on the fused 3D features instead of operating on each 2D view separately. Our econSG show state-of-the-art performance on four benchmark datasets compared to the existing methods. Furthermore, we are also the most efficient training among all the methods.

Decision: accept (poster)
Reviewer scores: 3, 6, 6, 6

Reviews and discussion:
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creation date:
1730006070837

thread_type:
official_review

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creator: reviewer_zwwm

<content>
summary:
This paper introduces econSG, a novel 3D semantic Gaussian model that tackles the challenges of extracting precise semantic features and maintaining multi-view consistency in open-vocabulary neural fields. The proposed method incorporates Confidence-region Guided Regularization (CRR) and a low-dimensional contextual space, resulting in state-of-the-art performance on four benchmark datasets. Notably, econSG also exhibits superior efficiency in training and Inference compared to existing methods.

strengths:
+ The paper introduces CRR as a novel approach to refine semantic features from VLMs, specifically focusing on achieving precise boundaries and completeness. The mutual refinement strategy between OpenSeg and SAM is a significant innovation that addresses the limitations of existing methods in capturing accurate and complete semantic information.
+ By constructing a low-dimensional 3D contextual space, econSG effectively integrates 2D features from different views, improving the consistency and accuracy of 3D scene representation.
+ EconSG significantly reduces computational complexity and improves training and querying efficiency by mapping high-dimensional features to a low-dimensional space using a pre-trained autoencoder.
+ The paper demonstrates a notable  improvement in performance, achieving state-of-the-art results on four benchmark datasets.

weaknesses:
- Despite mentioning an ablation study on CRR, the paper does not include the corresponding experimental outcomes. This gap is detrimental to evaluating the specific impact of each CRR component on the overall effectiveness of the method.
- Figure 2 shows that the method proposed in this article tends to segment large objects and directly ignores small objects?

questions:
Since the article repeatedly emphasizes the multi-view consistency, I suggest adding some introductions to related research in the related work or introduction.
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creation date:
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thread_type:
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<thread_object>
creator: reviewer_c9fh

<content>
summary:
This paper aims to refine visual foundation models' output and solve the multi-view inconsistency problem in 3D rendering. Specifically, they proposed a confidence-region guided regularization and back projected 2D features into a latent space shared with 3D features. Rendering is performed via alpha blending, supervised by semantic information, 2D features, and color information.

strengths:
1. Experiment results seem to be good.
2. Incorporating visual foundation models into this field for multi-modal learning is interesting.

weaknesses:
Currently, I vote for 5, marginally below the acceptance threshold, for the reasons below:

1. My primary concern is the lack of explanation on the open-vocabulary experiment settings. In section 5.1, this manuscript does not clarify how their sampling method ensures an open-vocabulary setting or specify which classes are used in training. It will be helpful to provide more explanation on experiments. Additionally, in Table 1, only a few classes are listed for the class-level comparisons. From my current knowledge, they adopted ‘20 different semantic class categories’ in Scannet. It will be beneficial to include the open-vocabulary experiment results in more classes.

2. Writing. Some parts of the manuscript will benefit from fine-tuning for brevity, i.e., ‘The parallel rapid developments of neural 3D scene representation and large multi-modality foundation models naturally lead to research on open-vocabulary 3D scene understanding by leveraging the neural rendering capability of neural fields to align the visual-language models to 3D scenes’. Condensing long sentences can better retain reader’s attention on the main point. Another small note: it seems that there is a typo in section 4.1 in the text feature’s notation T. 

3. Back projecting 2D information into a shared latent space with 3D is not entirely novel. It would be helpful to provide more explanation on their unique contribution for low-dimensional 3D contextual space.

questions:
Please refer to the weakness part.
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<thread>
creation date:
1730275270014

thread_type:
official_review

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creator: reviewer_mv8f

<content>
summary:
This paper introduces econSG, a new approach for open-vocabulary semantic segmentation within 3D Gaussian Splatting (3DGS), addressing challenges with consistency and efficiency in recent methods. Most current open-vocabulary neural fields rely heavily on Segment Anything Model (SAM) to regularize image-level features extracted from CLIP, often without additional refinement, which can lead to inconsistencies across multiple views. Additionally, some methods use dimensionality reduction on 2D semantic features before fusing them with 3D semantic fields, which compromises multi-view consistency.

EconSG overcomes these issues through two main innovations:

1. Confidence-region Guided Regularization (CRR): This approach mutually refines SAM and CLIP features to achieve complete and precise semantic boundaries with enhanced accuracy.
2. Low-dimensional Contextual Space: By fusing multi-view 2D features into a single 3D representation before dimensionality reduction, econSG enhances multi-view consistency and computational efficiency.

strengths:
This paper clearly explains its motivation and methodology, with both the algorithmic process and pipeline illustrations presented in a way that is very clear to the reviewers.

weaknesses:
1. I believe the second contribution Low-dimensional Contextual Space lacks novelty. Similar idea of mapping high-dimensional features into a low-dimensional space in this task can be seen in previous work like one of the baselines, LangSplat[1]. Even if the authors claim that the contribution is to increase the inference speed by changing the space of rendering, recent work like FastLGS: Speeding up Language Embedded Gaussians with Feature Grid Mapping[2] also proposes the same idea of directly rendering low dimensional semantic features.
2. The qualitative and quantitative experiments are not well-aligned. The baselines that appeared in the quantitative tables are not mentioned in the qualitative comparisons, which reduces the reliability of data in the tables. If the author can provide qualitative results that can align with the quantitative results and more qualitative results that display your 3D consistency, this work can be more convincing.

[1] Qin, M., Li, W., Zhou, J., Wang, H., & Pfister, H. (2024). Langsplat: 3d language gaussian splatting. In Proceedings of the IEEE/CVF Conference on Computer Vision and Pattern Recognition (pp. 20051-20060).
[2] Ji, Yuzhou, et al. "FastLGS: Speeding up Language Embedded Gaussians with Feature Grid Mapping." arXiv preprint arXiv:2406.01916 (2024).

questions:
I'm confused with the 3D consistency brought by the CRR module.
- For step b. From my understanding, the consistency here highly depends on the accuracy of the COLMAP depth maps. I understand that average pooling can partially mitigate issues like complex backgrounds or occlusion in individual views, resulting in more stable, 3D-consistent semantic features. But if some view features are affected by occlusion, noise, or low confidence, simple averaging may dilute the features, potentially affecting consistency. Moreover, if the features from different views have large discrepancies, average pooling alone may not fully eliminate these differences. 
- For step c. If a reprojected point is occluded in most of the views, I believe your majority voting strategy may assign a label of the occlusion  to that point instead of the correct label of that point.
Can you resolve my confusion about the two points mentioned above? Please elaborate on how you ensure your 3D consistency and how your major voting strategy is implemented. Thanks.
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<thread>
creation date:
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thread_type:
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creator: reviewer_9xdx

<content>
summary:
This paper presents a novel approach for open-vocabulary neural field segmentation. Current methods face two key challenges: 1) they rely too heavily on the inconsistent 2D features from 2D VLMs, and 2) they are computationally expensive due to the need for high-dimensional feature rendering. To address these, the authors propose a Confidence-region Guided Regularization technique to achieve precise boundaries and multi-view consistency, as well as a 3D autoencoder to project high-dimensional features into a lower-dimensional space. The proposed method is evaluated against the current state-of-the-art across multiple benchmarks.

strengths:
1. The paper is well-motivated, addressing the issues of 2D feature inconsistency and the high computational cost associated with high-dimensional features.
2. The paper provides extensive benchmarking against current state-of-the-art methods.

weaknesses:
1. The implementation details of the proposed 3D autoencoder are unclear. Could you clarify the architecture of the 3D autoencoder (e.g., is it an MLP or another type of model), and specify its number of parameters? Additionally, how is the autoencoder trained—is it trained per scene? What is the duration of the training process?
2. During inference, text features are also projected into a low-dimensional space for querying. However, since the autoencoder is not trained on a rich set of diverse text features, information loss is likely during encoding, which may impair the model's ability to handle open-world queries. The authors should conduct experiments to assess the autoencoder's impact on text embeddings by randomly selecting open-world terms and evaluating the feature correlation before and after encoding.
3. The proposed method requires a set of text queries as input. How are these text queries obtained? Do they need to comprehensively include all objects present in the scene? In the experiments, are the open-vocabulary test queries the same as the input text queries? If so, this could bias comparisons with LangSplat, which does not see the text queries prior to training.
4. Does the proposed method support multi-scale segmentation similar to LangSplat?

questions:
Please see weaknesses.
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Metareview:
metareview:
This paper targets 3D open vocabulary semantic segmentation. Different from existing methods, it proposes CRR to refine SAM and CLIP to get more accurate semantic features, moreover, it proposes a low dimension contextual space to reduce the dimension of semantic features so as to improve the computational efficiency. Experiments show that the proposed method achieves state-of-the-art results.

Reviewers acknowledge that the paper is well motivated, the CRR and the low dimension contextual space is novel and thorough experiments demonstrate the effectiveness of the proposed method. As mentioned by several reviewers, the paper lacks some implementation details and the paper needs to be polished for better understanding. Overall, this paper meets the bar for publication at ICLR.