  Fine-grained 3D shape classification is important for shape understanding and
analysis, which poses a challenging research problem. However, the studies on
the fine-grained 3D shape classification have rarely been explored, due to the
lack of fine-grained 3D shape benchmarks. To address this issue, we first
introduce a new 3D shape dataset (named FG3D dataset) with fine-grained class
labels, which consists of three categories including airplane, car and chair.
Each category consists of several subcategories at a fine-grained level.
According to our experiments under this fine-grained dataset, we find that
state-of-the-art methods are significantly limited by the small variance among
subcategories in the same category. To resolve this problem, we further propose
a novel fine-grained 3D shape classification method named FG3D-Net to capture
the fine-grained local details of 3D shapes from multiple rendered views.
Specifically, we first train a Region Proposal Network (RPN) to detect the
generally semantic parts inside multiple views under the benchmark of generally
semantic part detection. Then, we design a hierarchical part-view attention
aggregation module to learn a global shape representation by aggregating
generally semantic part features, which preserves the local details of 3D
shapes. The part-view attention module hierarchically leverages part-level and
view-level attention to increase the discriminability of our features. The
part-level attention highlights the important parts in each view while the
view-level attention highlights the discriminative views among all the views of
the same object. In addition, we integrate a Recurrent Neural Network (RNN) to
capture the spatial relationships among sequential views from different
viewpoints. Our results under the fine-grained 3D shape dataset show that our
method outperforms other state-of-the-art methods.
