17. The gesture recognition engine of claim 13 comprising an input receiving state data from a system controlled by a natural user interface of which the gesture recognition engine is a part.
13. A gesture recognition engine comprising: a processor arranged to receive an input stream of image frames depicting an articulating entity; an instance level representation engine arranged to compute instance level features from individual ones of the image frames and to send the computed features to a trained machine learning system; and the processor arranged to receive in response at least one gesture class from a plurality of possible gesture classes; wherein the trained machine learning system comprises at least one multiple decision boundary classifier comprising a plurality of decision boundaries arranged to divide an instance level feature space into a plurality of regions, at least one region being associated with a gesture primitive; and wherein the instance level features are calculated from one or more instances where each instance is a temporally non-sequential subset of frames depicting a complete gesture of the gesture class made by the articulated entity.