In one example, an apparatus for encoding video data includes a video encoder configured to calculate a residual block for a block of video data based on a predicted block formed using an intra-prediction mode, and transform the residual block using a transform mapped from the intra-prediction mode. In another example, an apparatus includes video encoder configured to receive an indication of a first intra-prediction mode in a first set of intra-prediction modes for a block of video data, determine 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, determine a directional transform to which the second intra-prediction mode is mapped, and apply the directional transform to residual data of the block.