The HEVC standard incorporates numerous improvements over AVC, including a new prediction block structure, and updates to the toolkit that include intra-prediction, inverse transforms, motion compensation, loop filtering and entropy coding. A major difference from MPEG-2 and AVC is a new framework encompassing coding units (CUs), prediction units (PUs) and transform units (TUs). Coding units (CUs) define a sub-partitioning of a picture into arbitrary rectangular regions. The CU replaces the macroblock structure of previous video coding standards, and contains one or more prediction units and transform units, as shown in Figure 1. The PU is the elementary unit for intra- and inter-prediction, and the TU is the basic unit for transform and quantization.
Overall, this framework describes a treelike structure in which the individual branches can have different depths for different portions of a picture. Each frame is divided into largest coding units that can be recursively split into smaller CUs using a generic quad-tree segmentation structure, as shown in Figure 2. CUs can be further split into PUs and TUs. This new structure greatly reduces blocking artifacts, while at the same time providing a more efficient coding of picture-detail regions.
MPEG-2 intra-prediction employs fixed blocks for transform coding and motion compensation. AVC went beyond this by allowing multiple block sizes. HEVC also divides the picture into coding tree blocks, which are 64 x 64-, 32 x 32-,16 x16-, or 8 x 8-pixel regions. But these coding units can now be hierarchically subdivided all the way down to 4 x 4-sized units. In addition, an internal bit-depth increase allows encoding of video pictures by processing them as having a color depth higher than eight bits.
HEVC also specifies 33 different intra-prediction directions, as well as planar and DC modes, which reconstruct smooth regions or directional structures, respectively, in a way that hides artifacts better.