The unary and binary feature extensions. An illustration of the unary and binary feature extensions for the proposed Type I Kernels. Here local features (‘+’s) of an action is contained in a space-time volume displayed as a green cube. Besides residing in this local coordinate, each feature also has a descriptor that is further mapped to the histogram of codewords representation using the bag-of-words model. This figure displays two examples of the proposed Type I Kernels. (a) presents a unary extension where the local volume is further partitioned into concentric layers. (b) demonstrates a binary extension scheme that considers the distance and codeword entries of feature pairs. Note other extension schemes are also possible.