One-Dimensional Directional Unified Transform For Intra Coding

In this paper, we present the One-Dimensional Directional Unified Transform (1DDUT) for intra coding. Intra prediction in H.264/AVC has eight different directional prediction modes and a DC prediction mode. Since the statistical characteristics of prediction residuals are different in each prediction mode, coding efficiency can be improved by applying the different optimal transform for each prediction mode. However, in order to avoid the increase of the complexity, unification of the transforms is desired. Therefore we classify the characteristics of the prediction residuals along the vertical or the horizontal direction, where each direction has two classes and their characteristics are similar to each other between directions. It is sufficient to use two kinds of 1-D transforms that are designed based on the unified characteristics. One is the well-known Discrete Cosine Transform, and the other is a predetermined 1-D transform based on the Karhunen-Loeve Transform method. 1DDUT switches DCT and the predetermined 1-D transform according to the intra prediction modes. 1DDUT can achieve 8.71% bitrate reduction on average with negligible complexity increase compared to the DCT-like transform used in H.264/AVC.