S$^{2}$ Loop: A Lightweight Spectral-Spatio Loop Closure Detector for Resource-Constrained Platforms

Abstract

Visual loop closure detection is an essential backend task for long-term vSLAM applications. However, prior works cannot simultaneously meet the requirements of high recall and low computing and memory overhead, which prohibits their applicability to resource-constrained platforms. In this work we propose S <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula> Loop, a novel spectral-spatio loop detector that explores an efficient loop searching method based on spectral image analysis and direct image alignment. S <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula> Loop digests an image by its Fourier spectrum which provides a compact image hash value for fast candidate selection. The spectra also enable a convenient method of image alignment based on Fourier phase correction. The aligned images are further matched by an optical-flow process at the pixel granularity, which precisely estimates the frame dissimilarity against the turbulence caused by the perspective warping of random 3D scenes. According to our benchmark experiments on various datasets, S <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula> Loop shows two orders of magnitude of less computing and memory overhead with comparable precision and recall over the prior DBoW method on a desktop PC. It is also much faster than the prior Fern method that has low computing complexity, and exhibits a much higher recall. While querying loops in 1000 runtime keyframes, S <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula> Loop deployed onto a low-end STM32F4 MCU achieves 64fps.