StructSAM: Structure- and Spectrum-Preserving Token Merging for Segment Anything Models

SAM achieves strong segmentation but at high inference cost dominated by its ViT image encoder. Token merging accelerates ViTs without retraining, yet directly applying it to SAM is nontrivial: SAM mixes windowed and global attention and requires dense, prompt-conditioned features for precise boundary prediction. We systematically evaluate representative token-merging methods on the SAM family in a strict off-the-shelf setting and find that existing destination-selection heuristics erode boundaries and leak prompt information as merge rates increase. We propose \textbf{StructSAM}, a resolution-preserving framework that computes lightweight token-energy scores from first-order feature gradients, protects boundary and prompt regions via grid-based flatness screening, and merges flat-region tokens toward low-energy targets with explicit recovery. We further present a spectral graph coarsening analysis showing that score-guided merging yields bounded Laplacian spectral distortion relative to random or window-restricted baselines. Across five natural and medical benchmarks, StructSAM reduces encoder FLOPs by 25--30\% (up to 40\%+ with prompt-aware merging) with minor drops in mIoU/Dice, consistently outperforming recent merging techniques at the same compute.