Exploiting Local and Global Structure for Point Cloud Semantic Segmentation with Contextual Point Representations
In this paper, we propose one novel model for point cloud semantic segmentation, which exploits both the local and global structures within the point cloud based on the contextual point representations. Specifically, we enrich each point representation by performing one novel gated fusion on the point itself and its contextual points. Afterwards, based on the enriched representation, we propose one novel graph pointnet module, relying on the graph attention block to dynamically compose and update each point representation within the local point cloud structure. Finally, we resort to the spatial-wise and channel-wise attention strategies to exploit the point cloud global structure and thereby yield the resulting semantic label for each point. Extensive results on the public point cloud databases, namely the S3DIS and ScanNet datasets, demonstrate the effectiveness of our proposed model, outperforming the state-of-the-art approaches. Our code for this paper is available at https://github.com/fly519/ELGS.
The point cloud captured by 3D scanners has attracted more and more research interests, especially for the point cloud understanding tasks, including the 3D object classification qi2017pointnet ; qi2017pointnet++ ; li2018pointcnn ; liu2019rscnn , 3D object detection wang2015voting ; zhou2018voxelnet , and 3D semantic segmentation wu20153d ; qi2017pointnet ; qi2017pointnet++ ; wang2018sgpn ; li2018pointcnn . 3D semantic segmentation, aiming at providing class labels for each point in the 3D space, is a prevalent challenging problem. First, the points captured by the 3D scanners are usually sparse, which hinders the design of one effective and efficient deep model for semantic segmentation. Second, the points always appear unstructured and unordered. As such, the relationship between the points is hard to be captured and modeled.
As points are not in a regular format, some existing approaches first transform the point clouds into regular 3D voxel grids or collections of images, and then feed them into traditional convolutional neural network (CNN) to yield the resulting semantic segmentation wu20153d ; graham2014spatially ; wang2017cnn . Such a transformation process can somehow capture the structure information of the points and thereby exploit their relationships. However, such approaches, especially in the format of 3D volumetric data, require high memory and computation cost. Recently, another thread of deep learning architectures on point clouds, namely PointNet qi2017pointnet and PointNet++ qi2017pointnet++ , is proposed to handle the points in an efficient and effective way. Specifically, PointNet learns a spatial encoding of each point and then aggregates all individual point features as one global representation. However, PointNet does not consider the local structures. In order to further exploit the local structures, PointNet++ processes a set of points in a hierarchical manner. Specifically, the points are partitioned into overlapping local regions to capture the fine geometric structures. And the obtained local features are further aggregated into larger units to generate higher level features until the global representation is obtained. Although promising results have been achieved on the public datasets, there still remains some opening issues.
First, each point is characterized by its own coordinate information and extra attribute values, color, normal, reflectance, etc. Such representation only expresses the physical meaning of the point itself, which does not consider its neighbouring and contextual ones. Second, we argue that the local structures within point cloud are complicated, while the simple partitioning process in PointNet++ cannot effectively capture such complicated relationships. Third, each point labeling not only depends on its own representation, but also relates to the other points. Although the global representation is obtained in PointNet and PointNet++, the complicated global relationships within the point cloud have not been explicitly exploited and characterized.
In this paper, we propose one novel model for point cloud semantic segmentation. First, for each point, we construct one contextual representation by considering its neighboring points to enrich its semantic meaning by one novel gated fusion strategy. Based on the enriched semantic representations, we propose one novel graph pointnet module (GPM), which relies on one graph attention block (GAB) to compose and update the feature representation of each point within the local structure. Multiple GPMs can be stacked together to generate the compact representation of the point cloud. Finally, the global point cloud structure is exploited by the spatial-wise and channel-wise attention strategies to generate the semantic label for each point.
2 Related Work
Recently, deep models have demonstrated the feature learning abilities on computer vision tasks with regular data structure. However, due to the limitation of data representation method, there are still many challenges for 3D point cloud task, which is of irregular data structures. According to the 3D data representation methods, existing approaches can be roughly categorized as 3D voxel-based graham2014spatially ; wu20153d ; wang2017cnn ; Klokov2017 ; Le2018 , multiview-based shi2015deeppano ; pang20163d , and set-based approaches qi2017pointnet ; qi2017pointnet++ .
3D Voxel-based Approach. The 3D voxel-based methods first transform the point clouds into regular 3D voxel grids, and then the 3D CNN can be directly applied similarly as the image or video. Wu et al. wu20153d propose the full-voxels based 3D ShapeNets network to store and process 3D data. Due to the constraints of representation resolution, information loss are inevitable during the discretization process. Meanwhile, the memory and computational consumption are increases dramatically with respect to the resolution of voxel. Recently, Oct-Net Riegler2017 , Kd-Net Klokov2017 , and O-CNN wang2017cnn have been proposed to reduce the computational cost by skipping the operations on empty voxels.
Multiview-based Approach. The multiview-based methods need to render multiple images from the target point cloud based on different view angle settings. Afterwards, each image can be processed by the traditional 2D CNN operations shi2015deeppano . Recently, the multiview image CNN pang20163d has been applied to 3D shape segmentation, and has obtained satisfactory results. The multiview-based approaches help reducing the computational cost and running memory. However, converting the 3D point cloud into images also introduce information loss. And how to determine the number of views and how to allocate the view to better represent the 3D shape still remains as an intractable problem.
Set-based Approach. PointNet qi2017pointnet is the first set-based method, which learns the representation directly on the unordered and unstructured point clouds. PointNet++ qi2017pointnet++ relies on the hierarchical learning strategy to extend PointNet for capturing local structures information. PointCNN li2018pointcnn is further proposed to exploit the canonical order of points for local context information extraction.
Recently, there have been several attempts in the literature to model the point cloud as structured graphs. For example, Qi et al. Qi2017ICCV propose to build a -nearest neighbor directed graph on top of point cloud to boost the performance on the semantic segmentation task. SPGraph landrieu2018large is proposed to deal with large scale point clouds. The points are adaptively partitioned into geometrically homogeneous elements to build a superpoint graph, which is then fed into a graph convolutional network (GCN) for predicting the semantic labels. DGCNN dgcnn2019 relies on the edge convolution operation to dynamically capture the local shapes. RS-CNN liu2019rscnn extends regular grid CNN to irregular configuration, which encodes the geometric relation of points to achieve contextual shape-aware learning of point cloud. These approaches mainly focus on the point local relationship exploitation, and neglect the global relationship.
Unlike previous set-based methods that only consider the raw coordinate and attribute information of each single point, we pay more attentions on the spatial context information within neighbor points. Our proposed context representation is able to express more fine-grained structural information. We also rely on one novel graph pointnet module to compose and update each point representation within the local point cloud structure. Moreover, the point cloud global structure information is considered with the spatial-wise and channel-wise attention strategies.
The point cloud semantic segmentation aims to take the 3D point cloud as input and assign one semantic class label for each point. We propose one novel model for handling this point cloud semantic segmentation, as shown in Fig. 1. Specifically, our proposed network consists of three components, namely, the point enrichment, the feature representation, and the prediction. These three components fully couple together, ensuring an end-to-end training manner.
Point Enrichment. To make accurate class prediction for each point within the complicated point cloud structure, we need to not only consider the information of each point itself but also its neighboring or contextual points. Different from the existing approaches, relying on the information of each point itself, such as the geometry, color, etc., we propose one novel point enrichment layer to enrich each point representation by taking its neighboring or contextual points into consideration. With the incorporated contextual information, each point is able to sense the complicated point cloud structure information. As will be demonstrated in Sec. 4.4, the contextual information, enriching the semantic information of each point, can help boosting the final segmentation performance.
Feature Representation. With the enriched point representation, we resort to the conventional encoder-decoder architecture with lateral connections to learn the feature representation for each point. To further exploit local structure information of the point cloud, the GPM is employed in the encoder, which relies on the GAB to dynamically compose and update the feature representation of each point within its local regions. The decoder with lateral connections works on the compacted representation obtained from the encoder, to generate the semantic feature representation for each point.
Prediction. Based on the obtained semantic representations, we resort to both the channel-wise and spatial-wise attentions to further exploit the global structure of the point cloud. Afterwards, the semantic label is predicted for each point.
3.1 Point Enrichment
The raw representation of each point is usually its 3D position and associated attributes, such as color, reflectance, surface normal, etc. Existing approaches usually directly take such representation as input, neglecting its neighboring or contextual information, which is believed to play an essential role Roweis00nonlineardimensionality for characterizing the point cloud structure, especially from the local perspective. In this paper, besides the point itself, we incorporate its neighboring points as its contextual information to enrich the point semantic representation. With such incorporated contextual information, each point is aware of the complicated point cloud structure information.
As illustrated in Fig. 2, a point cloud consists of points, which can be represented as , with denoting the attribute values of the -th point, such as position coordinate, color, normal, etc. To characterize the contextual information for each point, -nearest neighbor set within the local region centered on -th point are selected and concatenated together, where the contextual representation of the given point is as follows:
For each point, we have two different representations, specifically the and . However, these two representations are of different dimensions and different characteristics. How to effectively fuse them together to produce one more representative feature for each point remains an open issue. In this paper, we propose a novel gated fusion strategy. We first feed into one fully-connected (FC) layer to obtain a new feature vector . Afterwards, the gated fusion operation is performed:
where and are the learnable parameters. is the non-linear sigmoid function. is the element-wise multiplication. The gated fusion aims to mutually absorb useful and meaningful information of and . And the interactions between and are updated, yielding and . As such, the -th point representation is then enriched by concatenating them together as . For easing the following introduction, we will re-use to denote the enriched representation of the -th point.
3.2 Feature Representation
Based on the enriched point representation, we rely on one traditional encoder-decoder architecture with lateral connections to learn the feature representation of each point.
Although the enriched point representation has somewhat considered the local structure information, the complicated relationships within points, especially from the local perspective need to be further exploited. In order to tackle this challenge, we propose one novel GPM in the encoder, which aims to learn the composition ability between points and thereby more effectively capture the local structural information within the point cloud.
Graph Pointnet Module.
Same as qi2017pointnet++ , we first use the sampling and grouping layers to divide the point set into several local groups. Within each group, the GPM is used to exploit the local relationships between points, and thereby update the point representation by aggregating the point information within the local structure.
As illustrated in Fig. 3, the proposed GPM consists of one multi-layer perceptron (MLP) and GAB. The MLP in conventional PointNet qi2017pointnet and PointNet++ qi2017pointnet++ independently performs on each point to mine the information within the point itself, while neglects the correlations and relationships among the points. In order to more comprehensively exploit the point relationship, we rely on the GAB to aggregate the neighboring point representations and thereby updated the point representation.
For each obtained local structure obtained by the sampling and grouping layers, GAB velivckovic2017graph first defines one fully connected undirected graph to measure the similarities between any two points with such local structures. Given the output feature map of the MLP layer in the GPM module, we first linearly project each point to one common space through a FC layer to obtain new feature map . The similarity between point and point is measured as follows:
Afterwards, we calculate the influence factor of point on point :
where is regarded as the normalized attentive weight, representing how point relates to point . The representation of each point is updated by attentively aggregating the point representations with reference to :
It can be observed that the GAB dynamically updates the local feature representation by referring to the similarities between points and captures their relationships. Moreover, in order to preserve the original information, the point feature after MLP is concatenated with the updated one via one skip connection through a gated fusion operation, as shown in Fig. 3.
Please note that we can stack multiple GPMs, as shown in Fig. 3, to further exploit the complicated non-linear relationships within each local structure. Afterwards, one max pooling layer is used to aggregate the feature map into a one-dimensional feature vector, which not only lowers the dimensionality of the representation, thus making it possible to quickly generate compact representation of the point cloud, but also help filtering out the unreliable noises.
For decoder, we use the same architecture as qi2017pointnet++ . Specifically, we progressively upsample the compact feature obtained from the encoder until the original resolution. Please note that for preserving the information generated in the encoder as much as possible, lateral connections are also used.
After performing the feature representation, rich semantic representation for each point is obtained. Note that our previous operations, including contextual representation and feature representation, only mine the point local relationships. However, the global information is also important, which needs to be considered when determining the label for each individual point. For the semantic segmentation task, two points departing greatly in space may belong to the same semantic category, which can be jointly considered to mutually enhance their feature representations. Moreover, for high-dimensional feature representations, the inter-dependencies between feature channels also exist. As such, in order to capture the global context information for each point, we introduce two attention modules, namely spatial-wise and channel-wise attentions fu2018dual for modeling the global relationships between points.
Spatial-wise Attention. To model rich global contextual relationships among points, the spatial-wise attention module is employed to adaptively aggregate spatial contexts of local features. Given the feature map from the decoder, we first feed it into two FC layers to obtain two new feature maps and , respectively, where . is the number of points and is number of feature channel. The normalized spatial-wise attentive weight measures the influence factor of point on point as follows:
Afterwards, the feature map is fed into another FC layer to generate a new feature map . The output feature map after spatial-wise attention is obtained:
As such, the global spatial structure information is attentively aggregated with each point representation.
Channel-wise Attention. The channel-wise attention performs similarly with the spatial-wise attention, with the channel attention map explicitly modeling the interdependencies between channels and thereby boosting the feature discriminability. Similar as the spatial-wise attention module, the output feature map is obtained by aggregating the global channel structure information with each channel representation.
After summing the feature maps and , the semantic label for each point can be obtained with one additional FC layer. With such attention processes from the global perspective, the feature representation of each point is updated. As such, the complicated relationships between the points can be comprehensively exploited, yielding more accurate segmentation results.
4.1 Experiment Setting
Dataset. To evaluate the performance of proposed model and compare with state-of-the-art, we conduct experiments on two public available datasets, the Stanford 3D Indoor Semantics (S3DIS) Dataset armeni20163d and ScanNet Datasetdai2017scannet . The S3DIS dataset comes from real scan of the indoor environment, including 3D scans of Matterport scanners from 6 areas. There are 271 rooms divided by room. ScanNet is a point cloud dataset with scanned indoor scenes. It has 22 categories of semantic tags, with 1513 scenes. ScanNet contains a wide variety of spaces. Each point is annotated with an instance-level semantic category label.
|6 fold||PointNet qi2017pointnet||78.5||47.6|
|Engelmann et al. engelmann2017exploring||81.1||49.7|
Implementation Details. The number of neighboring points in contextual representation is set as 3, where the farthest distance for neighboring point is fixed to 0.06. For feature extraction, a four-layer encoder is used, where the spatial scale of each layer is set as 1024, 256, 64, and 16, respectively. The GPM is enabled in the first two layers of the encoder, to exploit the local relationships between points. The maximum training epochs for S3DIS and ScanNet are set as 120 and 500, respectively.
Evaluation Metric. Two widely used metrics, namely overall accuracy (OA) and mean intersection of union (mIoU), are used to measure the semantic segmentation performance. OA is the prediction accuracy of all points. IoU measures the ratio of the area of overlap to the area of union between the ground truth and segmentation result. mIoU is the average of IoU over all categories.
Competitor Methods. For S3DIS dataset, we compare our method with PointNet qi2017pointnet , PointNet++ qi2017pointnet++ , SEGCloud tchapmi2017segcloud , RSNet huang2018recurrent , SPGraph landrieu2018large , SGPN wang2018sgpn , Engelmann et al. engelmann2017exploring , A-SCN xie2018attentional and DGCNN dgcnn2019 . For ScanNet dataset, we compare with 3DCNN dai2017scannet , PointNet qi2017pointnet , PointNet++ qi2017pointnet++ , RSNet huang2018recurrent and PointCNN li2018pointcnn .
|Area5||PointNet qi2017pointnet in landrieu2018large||88.80||97.33||69.80||0.05||3.92||46.26||10.76||52.61||58.93||40.28||5.85||26.38||33.22|
|6fold||PointNet qi2017pointnet in engelmann2017exploring||88.0||88.7||69.3||42.4||23.1||47.5||51.6||42.0||54.1||38.2||9.6||29.4||35.2|
|Engelmann et al. engelmann2017exploring||90.3||92.1||67.9||44.7||24.2||52.3||51.2||47.4||58.1||39.0||6.9||30.0||41.9|
4.2 S3DIS Semantic Segmentation
We perform semantic segmentation experiments on the S3DIS dataset to evaluate our performance in indoor real-world scene scans and perform ablation experiments on this dataset. Same as the experimental setup in PointNet qi2017pointnet , we divide each room evenly into several cube, with each uniformly sampleing 4096 points.
Same as qi2017pointnet ; engelmann2017exploring ; landrieu2018large , we perform 6-fold cross validation with micro-averaging. In order to compare with more methods, we also report the performance on the fifth fold only (Area 5). The OA and mIoU results are summarized in Table 1. From the results we can see that our algorithm performs better than other competitor methods in terms of both OA and mIoU metrics.
Besides, the IoU values of each category are summarized in Table 2, it can be observed that our proposed method achieves the best performance for several categories. For simple shapes such as “floor” and “ceiling”, each model performs well, with our approach performing better. This is mainly due to that the prediction layer of our propose method incorporates the global structure information between points, which enhances the point representation in the flat area. For categories with complex local structure, such as “chair” and “bookcase”, our model shows the best performance, since we consider the contextual representation to enhance the relationship between each point and its neighbors, and use the GPM module to exploit the local structure information. However, the “window” and “board” categories are more difficult to distinguish from the “wall”, as they are close to the “wall” in position and appear similarly. The key to distinguishing them is to find subtle shape differences and detect the edges. It can be observed that our model performs well on the “window” and “board” categories. In order to further demonstrate the effectiveness of our model, some qualitative examples from S3DIS dataset are provided in Fig. 4 and Fig. 5, demonstrating that our model can yield more accurate segmentation results.
4.3 ScanNet Semantic Segmentation
For the ScanNet dataset, the number of scenes trained and tested is 1201 and 312, same as qi2017pointnet++ ; li2018pointcnn . We only use its XYZ coordinate information. The results are illustrated in Table 3. Compared with other competitive methods, our proposed model achieves better performance in terms of both the OA and mIoU metrics.
4.4 Ablation Study
To validate the contribution of each module in our framework, we conduct ablation studies to demonstrate their effectiveness. Detailed experimental results are provided in Table 4.
|Ours(CR with concatenation)||88.21||59.14|
Contextual Representation Module. After removing the contextual representation module in the input layer (denoted as w/o CR), we can see that the mIoU value dropped from 60.06 to 56.15, as shown in Table 4. Based on the results of each category in Table 5, some categories have significant drops in IoU, such as “column”, “sofa”, and “door”. The contextual representation can enhance the point feature of the categories with complex local structures. We also replace the gating operation in the contextual representation with a simple concatenation operation. Due to the inequality of the two kinds of information, the OA and mIoU decreases. Thus, the proposed gating operation is useful for fusing the information of the point itself and its neighborhood.
Graph Pointnet Module. The segmentation performance of our model without GPM module (denoted as w/o GPM) also significantly drops, which indicates that both the proposed GPM and CR are important for performance improvement. Specifically, without GPM, the mIoU of the categories, such as “column” and “sofa” drops significantly.
Attention Module. Removing the attention module (denoted as w/o AM) decreases both OA and mIoU. Moreover, the performances on categories with large flat area, such as “ceiling”, “floor”, “wall”, and “window”, significantly drop. As aforementioned, the attention module aims to mine the global relationship between points. Two points within the same category may with large spatial distance. With the attention module, the features of these points are mutually aggregated.
We further incorporate the proposed CR, AM, and GPM together with DGCNN dgcnn2019 for point cloud semantic segmentation, with the performances illustrated in Table 6. It can be observed that CR, AM, and GPM can help improving the performances, demonstrating the effectiveness of each module.
|Model||Time (ms)||Size (M)|
Model Complexity. Table 7 illustrates the model complexity comparisons. The sample sizes for all the models are fixed as 4096. It can be observed that the inference time of our model (28ms) is less than the other competitor models, except for PointNet (5.3ms) and PointNet++ (24ms). And the model size seems to be identical with other models except PointCNN, which presents the largest model.
Robustness under Noise. We further demonstrate the robustness of our proposed model with respect to PointNet++. As for scaling, when the scaling ratio are , the OA of our proposed model and PointNet++ on segmentation task decreases by and , respectively. As for rotation, when the rotation angle is , the OA of our proposed model and PointNet++ on segmentation task decreases by and , respectively. As such, our model is more robust to scaling while less robust to rotation.
In this paper, we proposed one novel network for point cloud semantic segmentation. Different with existing approaches, we enrich each point representation by incorporating its neighboring and contextual points. Moreover, we proposed one novel graph pointnet module to exploit the point cloud local structure, and rely on the spatial-wise and channel-wise attention strategies to exploit the point cloud global structure. Extensive experiments on two public point cloud semantic segmentation datasets demonstrating the superiority of our proposed model.
This work was supported in part by the National Natural Science Foundation of China (Grant 61871270 and Grant 61672443), in part by the Natural Science Foundation of SZU (grant no. 827000144) and in part by the National Engineering Laboratory for Big Data System Computing Technology of China.
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