PTC-Net: Point-Wise Transformer with Sparse Convolution Network for Place Recognition

In the point-cloud-based place recognition area, the existing hybrid architectures combining both convolutional networks and transformers have shown promising performance. They mainly apply the voxel-wise transformer after the sparse convolution (SPConv). However, they can induce information loss by the sparse voxelization and further result in loss propagation to the transformer, significantly degrading the performance of the network, especially in outdoor scenes with complex geometric structures and multiple small objects. To address this issue, we propose a novel Point-wise Transformer with sparse Convolution (PTC). Specifically, SPConv is applied to the sparsely voxelized point cloud to extract local features, which are then converted to the point-based representation via a feature transformation unit (FTU). As such, our PTC can apply a transformer model based on the point-wise representation rather than on the voxel-wise one. To enhance the ability to capture long-range features and reduce the computational complexity of the transformer, we propose a two-step transformer, each with different grouping strategies. Meanwhile, in both steps, the attention matrix is computed with much fewer points by grouping a single point cloud into different attention domains. The experiments show that the PTC-Net can achieve state-of-the-art (SOTA) performance, with an improvement of 3.6% on average recall@1. Furthermore, to demonstrate the effectiveness of the PTC, we introduce an extremely light-weight version, PTC-Net-L, with only one PTC layer and half initial channel dimensions, also achieving SOTA performance in terms of the average recall rate and running time with only 0.08 M parameters.