Resolving Primitive-Sharing Ambiguity in Long-Tailed TLS-Based Industrial MEP Point Cloud Segmentation via Spatial Context Constraints

Abstract

In terrestrial laser scanning (TLS)-based mechanical, electrical, and plumbing (MEP) point cloud segmentation, safety-critical components such as reducers and valves are persistently misclassified, blocking reliable engineering knowledge extraction. This stems from a dual crisis--extreme class imbalance (215:1) compounded by geometric ambiguity, since most tail classes share cylindrical primitives with dominant head classes--that existing frequency-based re-weighting methods cannot resolve. We propose spatial context constraints that exploit neighborhood prediction consistency to disambiguate locally similar structures. Our approach extends Class-Balanced (CB) Loss with two architecture-agnostic mechanisms: Boundary-CB, an entropy-based constraint that emphasizes ambiguous boundaries and encodes an MEP assembly-topology prior, and Density-CB, a density-based constraint that compensates for scan-dependent variations and encodes TLS sensor-physics knowledge. Both operate at the loss level and require no backbone modifications; we evaluate them with ResPointNet++. On the Industrial3D dataset (612.7M labelled points from water treatment facilities), our method achieves 55.74% mIoU on the evaluation split, which also served as the validation split, exceeding the strongest of three backbone baselines (39.83-52.48% mIoU), with a 21.7% relative improvement on tail-class performance (29.59% vs. 24.32%) while preserving head-class accuracy (88.14%). Reducer improves from 0% to 21.12% IoU and valve by 24.3% relative. Evaluated per area, the gain comes from the area rich in these primitive-sharing components (tail mIoU 21.4% to 26.5%), while the other area's mIoU is 0.6 points lower. Spatial context constraints thus reduce primitive-sharing errors in the target MEP setting, supporting more reliable identification of safety-critical components for Digital Twin and Scan-to-BIM applications.

arXiv comment50 pages, 11 figures, 7 tables. v3: accepted manuscript, revised after peer review; published in Advanced Engineering Informatics. Supplementary material in ancillary files. Code: https://github.com/PointCloudYC/LongTail3D