Timely and accurately extracting and assessing pavement cracks is crucial for intelligent transportation systems (ITS) to improve road maintenance and safety. In this paper, we present an automated framework for crack semantic segmentation and quantification using optical images. First, a unique boundary-guided real-time high-resolution network is proposed, termed as BulletNet, for crack semantic segmentation. BulletNet is a bullet-head structure that can retain crack details while ensuring real-time inference speed, in which a Cross-Scale Global Attention (CSGA) module is designed to enhance global feature representation and pixel-level relations, as well as a Boundary-Guided Fusion (BGF) module proposed to utilize boundary features to guide the fusion of crack details and contextual information. Second, a Pixel-level Crack Quantification (PCQ) algorithm is proposed for complex cracks, incorporating an Improved Discrete Skeleton Evolution (IDSE) method to optimize skeleton pruning for accurate crack length and a normal vector correction method to adjust propagation direction for precise crack width. Comprehensive experiments on three datasets showed that the proposed BulletNet surpassed the comparative models in terms of efficiency and performance, with average F₁-score, mIoU, and Frames per second (FPS) of 87.20%, 88.70%, and 125.53, respectively. In addition, tested on 200 images, the PCQ calculated the crack maximum widths and lengths with an average relative error of 6.96% and 4.62%, respectively. Finally, BulletNet was deployed on edge devices for field testing, and a system based on the PCQ algorithm was developed to validate the effectiveness of the entire framework.

Multimodal fusion unlocks the potential of point cloud semantic segmentation, thereby driving advancements in surface observation and visual perception tasks. Although light detection and ranging (LiDAR) systems capture precise 3D structural geometry and optical images provide rich semantic and textural information, existing fusion methods struggle with limited cross-modal perception and insufficient information complementarity. To address these limitations, we propose a multi-stage LiDAR-image collaborative perception fusion network (MCPFNet) for point cloud semantic segmentation of urban scenes. At the middle fusion stage, the network incorporates an elevation-guided geometric-aware fusion module and a semantic-aware cross-attention fusion module to enable bidirectional feature injection between LiDAR and image modalities. In the late fusion stage, a bidirectional adaptive fusion module further refines semantic representations through gated weighting and bidirectional cross-attention mechanisms. Extensive experiments on three multimodal datasets with different resolutions, i.e., ISPRS Vaihingen, N3C-California, and UAVScenes, demonstrate that MCPFNet outperforms existing fusion methods, achieving mIoUs of 74.51%, 95.15%, and 62.76%, respectively. Hence, our multi-stage fusion and bidirectional interaction strategy is more reliable and accurate than existing methods in performing segmentation across diverse and complex urban scenes.