Key structures extraction like points, short-lines or regions extraction is a big issue in computer vision. Many fields of application need large image acquisition and fast extraction of fine structures. Several methods have been proposed with different accuracies and execution times. In this study, we focus on situations where existing local feature extractors give not enough satisfying results concerning both accuracy and time processing. Especially, we focus on short-line extraction in local low-contrasted images. To this end, we propose a new Fast Local Analysis by threSHolding (FLASH) designed to process large images under hard time constraints. We apply FLASH on the field of concrete infrastructure monitoring where robots and UAVs(Unmanned Aerial Vehicles) are more and more used for automated defect detection (like cracks). For large concrete surfaces, there are several hard constraints such as the computational time and the reliability. Results show that the computations are faster than several existing algorithms without learning stage, and lead to an automated monitoring of infrastructures.