# A Vehicle-Infrastructure Cooperative Perception Network Based on Multi-Scale Dynamic Feature Fusion
## 3 Method
### Multi-Scale Dynamic Feature Fusion
#### 3.3.2 Multi-Source Dynamic Interaction Module  
The Multi-Source Dynamic Interaction Module (MSDI) is a crucial component of the Multi-Scale Dynamic Feature Fusion Module, facilitating deep interactions between vehicle and infrastructure features at each scale. As illustrated in Figure 2, this module consists of two parallel sub-modules: the Spatial Multi-Source Dynamic Interaction Module (SMSDI) and the Channel Multi-Source Dynamic Interaction Module (CMSDI). The SMSDI captures correlations between features in the spatial dimension, while the CMSDI focuses on channel-wise correlations, as depicted in Figure 3.  
The input to the SMSDI consists of vehicle and infrastructure point cloud features. The SMSDI generates a spatial priority map through a spatial priority generator, which indicates the importance of different spatial locations in the feature map. This allows the system to concentrate on critical spatial areas, thereby reducing interference from irrelevant background information. As shown in Figure 3, the SMSDI first processes vehicle and infrastructure features through the spatial priority generator. It then obtains spatial priority maps via convolution operations, enhancing the robustness of these maps, given by  
\[P^{V}_{i,S},P^{ID}_{i,S}=\text{Conv}\Big{(}\Phi_{gen,S}(F^{V}_{i},F^{ID}_{i}) \Big{)}, \tag{3}\]  
where \(P^{V}_{i,S}\) and \(P^{ID}_{i,S}\in\mathbb{R}^{H_{i}\times W_{i}\times 1}\) represent the spatial priority maps for vehicle and infrastructure features, respectively, and \(\Phi_{gen,S}\) denotes the spatial priority generator following a detection decoder from PointPillars. Subsequently, we concatenate the spatial priority maps for vehicle and infrastructure features to ensure alignment at corresponding spatial locations. A Softmax function is then applied at each location to dynamically adjust the weights, thereby reinforcing the association between vehicle and infrastructure features, as detailed below:  
\[W^{V}_{i,S}=\psi\Big{(}P^{V}_{i,S}\Big{)}=\frac{e^{P^{V}_{i,S}}}{e^{P^{ID}_{i,S}}+e^{P^{V}_{i,S}}}, \tag{4}\]where \(W_{i,S}^{V}\in\mathbb{R}^{H_{i}\times W_{i}\times 1}\) denotes the spatial priority map after interaction, with higher values indicating greater importance at the corresponding locations. Likewise, \(W_{i,S}^{ID}=\psi\Big{(}P_{i,S}^{ID}\Big{)}\in\mathbb{R}^{H_{i}\times W_{i} \times 1}\) represents the processed spatial priority map for infrastructure features. The spatial priority maps obtained after interaction are applied to the vehicle and infrastructure features, enabling each point to be re-weighted based on its spatial location's priority, thus enhancing feature expressiveness. Finally, the weighted vehicle and infrastructure features are combined to produce the final spatial dynamic interaction features:  
\[F_{i}^{S}=W_{i,S}^{ID}\odot F_{i}^{ID}+W_{i,S}^{V}\odot F_{i}^{V}, \tag{5}\]  
where \(F_{i}^{S}\in\mathbb{R}^{H_{i}\times W_{i}\times C_{i}}\) represents the spatial dynamic interaction feature, fully integrating perception information from both vehicles and infrastructure. CMSDI operates similarly to SMSDI but focuses on dynamic interactions in the channel dimension. Instead of spatial priority maps, it generates channel priority maps \(P_{i,C}^{V}\) and \(P_{i,C}^{ID}\in\mathbb{R}^{1\times 1\times C_{i}}\) through the channel priority generator \(\Phi_{gen,C}\), which consists of parallel max pooling and average pooling operations. These maps are then normalized via Softmax (as in Equation (4)) to produce channel weights, \(W_{i,C}^{V}\) and \(W_{i,C}^{ID}\), which re-scale features along the channel dimension. The output is fused as follows:  
\[F_{i}^{C}=W_{i,C}^{ID}\odot F_{i}^{ID}+W_{i,C}^{V}\odot F_{i}^{V}, \tag{6}\]  
where \(F_{i}^{C}\in\mathbb{R}^{H_{i}\times W_{i}\times C_{i}}\) denotes the channel dynamic interaction feature, representing a deep integration of vehicle and infrastructure features at the channel level. Figure 3: Structures of the SMSDI and CMSDI.