Improved spatial filtering velocimetry and its application in granular flow measurement*Project supported by the National Natural Science Foundation of China (Grant No. 11902190), the Construction Project of Shanghai Key Laboratory of Molecular Imaging (Grant No. 18DZ2260400), and the Fund from the Shanghai Municipal Education Commission, China (Class II Plateau Disciplinary Construction Program of Medical Technology of SUMHS, 2018-2020). (July 2020)