Abstract: The optical flow method applied at engineering sites is easily affected by illumination changes, leading to inaccurate feature point detection, matching errors, and a low tracking success rate. To improve the accuracy of structural displacement identification, a structural displacement measurement method integrating improved oriented FAST and rotated BRIEF (ORB) feature points with the Lucas-Kanade pyramid optical flow algorithm is proposed. The method comprises two modules: the feature detection layer and the displacement solution layer. At the feature detection layer, an adaptive threshold model for FAST feature points is constructed. By dynamically adjusting the threshold, the false detection and mismatching of feature points caused by fixed thresholds are reduced. In addition, by analyzing the edge response values of the Hessian matrix, edge pseudo-feature points are precisely removed. An improved non-maximum suppression algorithm is introduced to perform spatially uniform filtering of feature points, thereby enhancing the stability and spatial distribution rationality of the feature point set. At the displacement solution layer, based on the optimized feature point set, a multi-scale hierarchical pyramid optical flow tracking architecture is designed. An iterative computation strategy from coarse to fine is employed to optimize motion vectors layer by layer, effectively addressing tracking failures under large-scale displacement conditions. Experimental results show that: (1) under complex illumination conditions, the feature point matching accuracy of the proposed method reaches 76.0%, outperforming algorithms such as speeded-up robust features, scale-invariant feature transform, and traditional ORB in both accuracy and computational efficiency; and (2) the displacement error is less than 4%. While maintaining accuracy comparable to that of the digital image correlation algorithm, the proposed method is more efficient. (3) Field measurements during shield-launching shaft construction indicate that the results of this method align well with manual measurements, with displacement differences within ±1.5 mm, demonstrating its reliability.

Key words: optical flow method, displacement monitoring, oriented FAST and rotated BRIEF feature points, Lucas-Kanade pyramid, shield-launching shafts