Air-writing recognition using reverse time ordered stroke context

•A novel technique is proposed for recognizing any air-written trajectories without using any pen-starting-lift signal.•A backward time order stroke recognizer is implemented to characterize a writing trajectory in a backward manner. It can effectively remove redundant trajectory data.•A novel hierarchical arrangement technique that presents an air-written English alphabet tier by tier can well handle both the multiplicity and confusion problems. Air-writing is a new human and smart device communication approach, permits users to write inputs in a natural and relentless way. This touch-less way can prevent users fromvirus infection such as COVID-19. Compared with othermethods, air writing ismore challenging due to its unique characteristics such as redundant lifting strokes, multiplicity (different writing styles from various users), and confusion (different character types written in air are similar). Without the need of any starting trigger, a novel reverse time-ordered algorithm is proposed in this paper toefficiently filter out unnecessary lifting strokes, and thus simplifies the matching procedure. As to the second and third issues, a tiered arrangement structure is proposed by sampling the air-writing results with various sampling rates to solvethe multiplicity and confusion problems. Analyzed with other recently proposed air writing algorithms, the proposed approach reaches satisfactory recognition accuracy (above 94%) without any starting triggers.