2022-07-04 更新
Rethinking Unsupervised Domain Adaptation for Semantic Segmentation
Authors:Zhijie Wang, Masanori Suganuma, Takayuki Okatani
Unsupervised domain adaptation (UDA) adapts a model trained on one domain to a novel domain using only unlabeled data. So many studies have been conducted, especially for semantic segmentation due to its high annotation cost. The existing studies stick to the basic assumption that no labeled sample is available for the new domain. However, this assumption has several issues. First, it is pretty unrealistic, considering the standard practice of ML to confirm the model’s performance before its deployment; the confirmation needs labeled data. Second, any UDA method will have a few hyper-parameters, needing a certain amount of labeled data. To rectify this misalignment with reality, we rethink UDA from a data-centric point of view. Specifically, we start with the assumption that we do have access to a minimum level of labeled data. Then, we ask how many labeled samples are necessary for finding satisfactory hyper-parameters of existing UDA methods. How well does it work if we use the same data to train the model, e.g., finetuning? We conduct experiments to answer these questions with popular scenarios, {GTA5, SYNTHIA}$\rightarrow$Cityscapes. Our findings are as follows: i) for some UDA methods, good hyper-parameters can be found with only a few labeled samples (i.e., images), e.g., five, but this does not apply to others, and ii) finetuning outperforms most existing UDA methods with only ten labeled images.
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PolarFormer: Multi-camera 3D Object Detection with Polar Transformers
Authors:Yanqin Jiang, Li Zhang, Zhenwei Miao, Xiatian Zhu, Jin Gao, Weiming Hu, Yu-Gang Jiang
3D object detection in autonomous driving aims to reason “what” and “where” the objects of interest present in a 3D world. Following the conventional wisdom of previous 2D object detection, existing methods often adopt the canonical Cartesian coordinate system with perpendicular axis. However, we conjugate that this does not fit the nature of the ego car’s perspective, as each onboard camera perceives the world in shape of wedge intrinsic to the imaging geometry with radical (non-perpendicular) axis. Hence, in this paper we advocate the exploitation of the Polar coordinate system and propose a new Polar Transformer (PolarFormer) for more accurate 3D object detection in the bird’s-eye-view (BEV) taking as input only multi-camera 2D images. Specifically, we design a cross attention based Polar detection head without restriction to the shape of input structure to deal with irregular Polar grids. For tackling the unconstrained object scale variations along Polar’s distance dimension, we further introduce a multi-scalePolar representation learning strategy. As a result, our model can make best use of the Polar representation rasterized via attending to the corresponding image observation in a sequence-to-sequence fashion subject to the geometric constraints. Thorough experiments on the nuScenes dataset demonstrate that our PolarFormer outperforms significantly state-of-the-art 3D object detection alternatives, as well as yielding competitive performance on BEV semantic segmentation task.
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