The $Q Super-Quick Recognizer is a 2-D gesture recognizer designed for rapid prototyping of gesture-based user interfaces, especially on low-power mobiles and wearables. It builds upon the $P Point-Cloud Recognizer but optimizes it to achieve a whopping 142x speedup, even while improving its accuracy slightly.
The official page of the algorithm.
Vatavu, R.-D., Anthony, L. and Wobbrock, J.O. (2018). $Q: A super-quick, articulation-invariant stroke-gesture recognizer for low-resource devices. Proceedings of the ACM Conference on Human-Computer Interaction with Mobile Devices and Services (MobileHCI '18). Barcelona, Spain (September 3-6, 2018). New York: ACM Press. Article No. 23. https://dl.acm.org/citation.cfm?id=3229434.3229465