1. A method of encoding video data, the method comprising: receiving an indication of a first intra-prediction mode in a first set of intra-prediction modes for a block of video data; determining a second intra-prediction mode from a second set of intra-prediction modes, smaller than the first set of intra-prediction modes, to which the first intra-prediction mode is mapped; determining a directional transform to which the second intra-prediction mode is mapped; and applying the directional transform to residual data of the block.