Prediction Coding - Advanced Video Coding Systems - page 57

In AVS2, the selection of intra or interprediction is decided at CU level with
rate distortion cost criteria. Before that, CU is split into PU partitions to do intra or
interprediction. According to the intra or interprediction type, different partitionsmay
be used. For intraprediction, a directional prediction is used and each partition selects
the best prediction mode with minimal rate distortion cost. And for interprediction,
motion search is done for each partition to get a motion vector. The motion vectors
are predicted coded.

4.2 Intraprediction

4.2.1 Luma Prediction

First, for the partition size of luma prediction, assuming the size of coding unit is
2N × 2N, the size of luma prediction block can be either 2N × 2N, N × N, 2N × 0.5N

or 0.5N 2N, as shown in Fig. 4.3 , where N can be 32, 16, 8, 4. Specifically, non-

square partitioning, including 2N

×

× 0.5N or 0.5N × 2N is only used for 32 × 32 and

16 16 CU (Yu et al. 2013 ). The encoder typically selects one of those partitioning

that minimize the rate distortion cost.
Given one prediction block, there are 33 prediction modes to get the prediction
samples. Table 4.1 tabulates the intra luma prediction modes in AVS2 where mode
index 0, 1 and 2 correspond to DCmode, plane mode, and bilinear mode respectively
(Piao et al. 2014 ). The other modes are called angular predictionmodes, amongwhich
the modes indexed with 12 and 24 are two special prediction angles (i.e. 0 ⓦ and 90 ⓦ )
named vertical and horizontal prediction specifically, as shown in Fig. 4.4 .

×

Fig. 4.3 Prediction block partitioning for luma

Table 4.1 Luma
intraprediction modes

Mode index Prediction modes

0 DC

1 Plane

2 Bilinear

3-11,13-23, 25-32 Angular

12 Vertical

24 Horizontal