Efficient LoFTR: Semi-Dense Local Feature Matching with Sparse-Like Speed

We present a novel method for efficiently producing semi-dense matches across images.Previous detector-free matcher LoFTR has shown remarkable matching capability in handling large-viewpoint change and texture-poor scenarios but suffers from low efficiency.We revisit its design choices and derive multiple improvements for both efficiency and accuracy.One key observation is that performing the transformer over the entire feature map is redundant due to shared local information, therefore we propose an aggregated attention mechanism with adaptive token selection for efficiency.Furthermore, we find spatial bias exists in LoFTR's fine correlation module, which is adverse to matching accuracy.A novel two-stage correlation layer is proposed to achieve unbiased subpixel correspondences for accuracy improvement.Our efficiency optimized model is $\sim 2.5\times$ faster than LoFTR which can even surpass state-of-the-art efficient sparse matching pipeline SuperPoint + LightGlue. Moreover, extensive experiments show that our method can achieve higher accuracy compared with competitive semi-dense matchers, with considerable efficiency benefits.This opens up exciting prospects for large-scale or latency-sensitive applications such as image retrieval and 3D reconstruction.Our code will be released for reproducibility.