This report describes work on the temporal segmentation of grasping task sequences based on human hand motion. The segmentation process results in the identification of motion breakpoints separating the different constituent phases of the grasping task. A grasping task is composed of three basic phases: pregrasp phase, static grasp phase, and manipulation phase. The pregrasp phase is the initial stage of the grasping task prior to the establishment of a stable grasp (static grasp phase) involved in the task. The manipulation phase refers to the purposeful hand-object interaction performed to achieve a goal in the task. In the pregrasp phase, the trajectory of the hand and the movement of the fingers follow an established pattern. Specifically, it comprises two parallel and simultaneous components: the hand transportation, and the hand preshape.

We show that by analyzing the fingertip polygon area (which is an indication of the hand preshape) and the speed of hand movement (which is an indication of the hand transportation), we can divide a task into meaningful action segments such as approach object (which corresponds to the pregrasp phase), grasp object, manipulate object, place object, and depart (a special case of the pregrasp phase which signals the termination of the task). We introduce a measure called the volume sweep rate, which is the product of the fingertip polygon area and the hand speed. The profile of this measure is also used in the determination of the task breakpoints.

The temporal task segmentation process is important as it serves as a preprocessing step to the characterization of the task phases. Once the breakpoints have been identified, steps to recognize the grasp and extract the object motion can then be carried out.