Prediction Coding - Advanced Video Coding Systems - page 65

Fig. 4.9 Neighboring blocks
used for mode prediction

4.2.3 Prediction Mode Coding

The luma prediction mode and chroma prediction mode need to be signaled in the
bitstream and transmitted to the decoder. Specifically, intra_luma_pred_mode and
intra_chroma_pred_mode in the AVS2 bitstream imply the prediction modes.
In AVS2, the neighboring left and up blocks are used as prediction for luma
predictionmode coding, as shown in Fig. 4.9 . And twomost probable modes (MPMs)
derived from the neighboring blocks are used. Assuming the prediction mode for
block A and B is ModeA and ModeB respectively (if the block is not available or
not intra coded, the default prediction mode is 0), the two MPM modes MPM0 and
MPM1 are derived as follows,

⊧
⊨

min

( ModeA , ModeB ) ,

if ModeA = ModeB

max ( ModeA , ModeB )

( MPM 0 , MPM 1 ) = .

⊩ ( 0 , 2 ) else if ModeA = ModeB = 0

( 0 , Mode ) otherwise

(4.36)

Assuming the prediction mode of the current block is ModeC. Then if ModeC is
equal to MPM0 or MPM1, 0 or 1 will be transmitted. Otherwise, if ModeC is less
than MPM0, intra _ luma _ pr ed _ mode is coded as ModeC

2; and if the value of
ModeC is larger than MPM0 and less than MPM1, intra l uma p red m ode is coded as
ModeC

+

1; for other cases intra _ luma _ pr ed _ mode is coded as ModeC. Inversely,
at the decoder side, if the decoded intra _ luma _ pr ed _ mode value is 0 or 1, it means
MPM0 and MPM1 is used. Otherwise if the decoded intra _ luma _ pr ed _ mode is
less than MPM0

+

2, the intra _ luma _ pr ed _ mode -2 is the true mode index; and if
the decoded intra _ luma _ pr ed _ mode is larger than MPM0 but less than MPM1,
the real prediction mode is intra _ luma _ pr ed _ mode -1; for the other cases, the
prediction mode is just intra _ luma _ pr ed _ mode .
For chroma prediction mode coding, the luma prediction mode of the first PU in
the CU is used as a prediction, but a mapping from luma prediction mode to chroma
prediction mode is needed. Specifically, if the luma prediction mode of the first PU is

+