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Current research on visual action/activity analysis has mostly exploited appearance-based static feature descriptions, plus statistics of short-range motion fields. The deliberate ignorance of dense, long-duration motion trajectories as features is largely due to the lack of mature mechanism for efficient extraction and quantitative representation of visual trajectories. In this paper, we propose a novel scheme for extraction and representation of dense, long-duration trajectories from video sequences, and demonstrate its ability to handle video sequences containing occlusions, camera motions, and nonrigid deformations. Moreover, we test the scheme on the KTH action recognition dataset, and show its promise as a scheme for general purpose long-duration motion description in realistic video sequences.