A Fast Mode Decision Algorithm and Its Hardware Design for H.264/AVC Intra Prediction - Advances in Image and Graphics Technologies - page 50

A MB has high texture complexity while TC th >1, and I4MB is selected as the pre-
diction block size; otherwise, I16MB is chosen.

2.2 Mode Decision for Chroma 8×8 Block Intra Prediction

In the intra prediction mode decision algorithm, the conventional calculation times of
Rate-Distortion-Optimization is 4×(9×16+4)=592, the prediction mode of chroma
block is used as the large exterior loop. By reducing the number of chroma candidate
prediction modes, a significant computational saving can be achieved.
The texture complexity of chroma blocks on the main prediction directions are cal-
culated by above equations (1)(5). And m is the size of chroma blocks. Based on
the texture complexity calculation of the 8×8 chroma blocks, the main candidate pre-
diction modes can be determined as shown in table 1.

Table 1. Mode decision for Chroma 8×8 blocks

The Threshold of TC th The minimum value Candidate Modes of Chroma 8×8 Block

TC vertical mode 2 (Vertical)

TC horizontal mode 1(Horizontal)

TC th >1

TC angle mode 3 (Plane)

mode 2 ,0DC

TC vertical

mode 1 ,0DC

TC horizontal

TC th <1

mode 3 ,0DC

TC angle

2.3 Mode Decision for Luma 4×4 Block Intra Prediction

In the mode decision of intra prediction, a macroblock can be divided into one 16×16
block or sixteen 4×4 blocks or four 8×8 blocks, which are used for executing the
mode decision. Therefore, the mode decision of 8×8 blocks are correlated with the
4×4 blocks, and the main candidate prediction modes of the 4×4 luma blocks can be
determined depending on the prediction mode of 8×8 block as described in table 2.

Table 2. Intra prediction mode decision of luma 4×4 blocks

prediction mode of Chroma 8×8 block Candidate modes of luma 4×4 macroblock

Mode 0 Main candidate modes 0, 1, 2, 4

Mode 1 Main candidate modes 1, 2, 6, 8

Mode 2 Main candidate modes 0, 2, 5, 7

Mode 3 Main candidate modes 0, 1, 2, 3

2.4 Mode Decision for Luma 16×16 Blocks

Due to the similarity of the prediction modes between luma 16×16 blocks and chroma
8×8 blocks, the 16×16 intra prediction mode decision are shown in table 3.