Juan Carlos Niebles Hongcheng Wang Fei-Fei Li 0002 Unsupervised Learning of Human Action Categories Using Spatial-Temporal Words. 299-318 2008 79 International Journal of Computer Vision 3 http://dx.doi.org/10.1007/s11263-007-0122-4 db/journals/ijcv/ijcv79.html#NieblesWF08