Exploring 3D Data and Beyond in a Low Data Regime
Creators: name: Cheraghian, Ali
Publisher: name: The Australian National University
Year: 2022

description: 3D object classification of point clouds is an essential task as laser scanners, or other depth sensors, producing point clouds are now a commodity on, e.g., autonomous vehicles, surveying vehicles, service robots, and drones. There have been fewer advances using deep learning methods in the area of point clouds compared to 2D images and videos, partially because the data in a point cloud are typically unordered as opposed to the pixels in a 2D image, which implies standard deep learning architectures are not suitable. Additionally, we identify there is a shortcoming of labelled 3D data in many computer vision tasks, as collecting 3D data is significantly more costly and difficult. This implies using zero- or few-shot learning approaches, where some classes have not been observed often or at all during training. As our first objective, we study the problem of 3D object classification of point clouds in a supervised setting where there are labelled samples for each class in the dataset. To this end, we introduce the {3DCapsule}, which is a 3D extension of the recently introduced Capsule concept by Hinton et al. that makes it applicable to unordered point sets. The 3DCapsule is a drop-in replacement of the commonly used fully connected classifier. It is demonstrated that when the 3DCapsule is applied to contemporary 3D point set classification architectures, it consistently shows an improvement, in particular when subjected to noisy data. We then turn our attention to the problem of 3D object classification of point clouds in a Zero-shot Learning (ZSL) setting, where there are no labelled data for some classes. Several recent 3D point cloud recognition algorithms are adapted to the ZSL setting with some necessary changes to their respective architectures. To the best of our knowledge, at the time, this was the first attempt to classify unseen 3D point cloud objects in a ZSL setting. A standard protocol (which includes the choice of datasets and determines the seen/unseen split) to evaluate such systems is also proposed. In the next contribution, we address the hubness problem on 3D point cloud data, which is when a model is biased to predict only a few particular labels for most of the test instances. To this end, we propose a loss function which is useful for both Zero-Shot and Generalized Zero-Shot Learning. Besides, we tackle 3D object classification of point clouds in a different setting, called the transductive setting, wherein the test samples are allowed to be observed during the training stage but then as unlabelled data. We extend, for the first time, transductive Zero-Shot Learning (ZSL) and Generalized Zero-Shot Learning (GZSL) approaches to the domain of 3D point cloud classification by developing a novel triplet loss that takes advantage of the unlabeled test data. While designed for the task of 3D point cloud classification, the method is also shown to be applicable to the more common use-case of 2D image classification. Lastly, we study the Generalized Zero-Shot Learning (GZSL) problem in the 2D image domain. However, we also demonstrate that our proposed method is applicable to 3D point cloud data. We propose using a mixture of subspaces which represents input features and semantic information in a way that reduces the imbalance between seen and unseen prediction scores. Subspaces define the cluster structure of the visual domain and help describe the visual and semantic domain considering the overall distribution of the data.