Joint Collaborative Team on Video Coding (jct-vc) of itu-t sg16 wp3 and iso/iec jtc1/SC29/WG11

5Core experiments in Range Extensions (28)

5.1RCE1: Inter-component decorrelation methods (5)

5.1.1RCE1 summary and general discussion

JCTVC-O0035 RCE1: Summary Report of HEVC Range Extensions Core Experiment 1 on Inter-Component Decorrelation Methods [W.-S. Kim, T. Nguyen]

(Reviewed Thu 24th morning (GJS & JRO).)

HEVC Range Extensions (RExt) Core Experiment 1 (RCE1) was formed to study proposals on inter-component decorrelation methods as described in RCE1 description, JCTVC-N1121, where four Experiments are included to be tested. In this document, the results of four Experiments are summarized and their performance was studied.

For RGB coding, there is reportedly a crossover. YCbCr 4:4:4 coding works better in the low bit rate range, but there is a limit in quality. RGB coding usually consumes more bits due to correlation between color components.

Relevant equations and approximate error analysis:

Estimated aAchievable PSNR (dB):

	8 bit	10 bit
R	52.41	64.48
G	55.39	67.46
B	51.56	63.63
Average	53.48	65.55

Interpretation of Chroma Gain

• 
  Larger gain in chroma components in YCbCr 4:4:4
  
• 
  Applied 1.5xlambda_chroma to measure overall performance
  

Results of shifting chroma gain to measure equivalent luma gain:

	All Intra HE Main-tier
	Y	U	V
YCbCr 4:4:4	-5.4%	-0.2%	-1.3%
	Random Access HE Main-tier
	Y	U	V
YCbCr 4:4:4	-4.4%	-3.8%	-2.3%
	Low delay B HE Main-tier
	Y	U	V
YCbCr 4:4:4	-6.3%	-0.6%	-0.2%

Tests:

1. 
   Adaptive inter-color-component residual prediction in JCTVC-N0266 (at TU level).
   

• 
  Chroma residual signal is predicted from the reconstructed luma residual signal scaled by alpha
  
• 
  Alpha is selected among {-8/8, -4/8, -2/8, -1/8, 0, 1/8, 2/8, 4/8, 8/8}
  
• 
  Value of alpha is not predicted, but adaptively context coded, using 4 contexts for each chroma component
  
• 
  Alpha is signaled for each TU if luma cbf is not zero
  
• 
  In case of intra coding, it is applied only for DM mode (i.e. when chroma uses the same prediction mode as luma)
  

2. 
   Use of non-reconstructed luma residue (encode-only trick).
   
3. 
   PU based rather than TU based signalling for inter coding (but not for intra coding).
   
4. 
   Combination of 2 and 3.
   

Summary of results

• 
  Coding efficiency improvement
  
  • 
    Large gain in RGB coding
    
    • 
      17/13/11 % in AI/RA/LB
      
  • 
    Large gain in chroma in YCbCr 4:4:4 coding
    
    • 
      7/7/6 % in AI/RA/LB, which was reported to be more than 5% luma gain when compensated for as described above.
      
  • 
    More gain in screen content coding
    
    • 
      21/26/19 % in AI/RA/LB in RGB
      
    • 
      4/2/2 % luma and 9/8/7 % chroma in AI/RA/LB in YCbCr 4:4:4
      
• 
  High color fidelity
  
• 
  Similar PSNR to RGB coding
  
• 
  Better PSNR than YCbCr 4:4:4 coding
  
• 
  Method 2 almost as good as Method 1.
  
• 
  Method 3 does not seem so promising.
  

It was asked how much gain would be obtained if the technique is applied to non-4:4:4 cases. For that, there would still be some gain, but less.

Some prior objections to the technique were related to the encoder side. The technique #2 somewhat alleviates that concern. Some prior similar techniques required the decoder to calculate the prediction scale factor, which is not the case in this scheme. The need for cross-component process is, however, undesirable.

The complexity concerns seemed substantially alleviated, compared to some previous proposals.

(somewhat overlapping notes below)

• 
  Experiments 1 & /2: Adaptive prediction (coefficient alpha for predicting the chroma component from luma is adapted locally). Adaptation performed per TU. Experiment 2 uses the original luma sample for prediction when determining the alpha value.
  
  • 
    factors are -1, -0.5, -0.25, -0.125, 0, 0.125, ... , 1
    
  • 
    CABAC coded with 4 contexts per alpha coefficient value (i.e. 4 new contexts)
    
• 
  Experiment 3: Adaptation at PU level instead of TU
  
• 
  Experiment 4: Combination of Exp. 2 & 3
  

Large gain in RGB coding

For RExt video test set in main tier conditions: Gain in YCbCr: 1.4%/0.5%/0.2% in Y AI/RA/LD; approx. 7/7/6 for the chroma components.

Higher gain for screen content.

The proponents claim that this could turn into 5% overall gain and that results exist where rate was shifted from chroma to luma, but this is not included in the contribution.

It was discussed whether the signalling at PU level (exp. 3) would give benefit. It is more appropriate to do signalling at TU level, as the prediction is conditional on CBF (non-zero luma).

Non concern iswas expressed about the complexity;, several experts expressed support for the method, which has significant compression benefit.

Text in JCTVC-O0202 was later reviewed in BoG.
Decision: Adopt (per O0202, enable flag at PPS level), with text later reviewed in BoG, experiment 2 method to be used in software.

5.1.2RCE1 primary contributions

JCTVC-O0202 RCE1: Descriptions and Results for Experiments 1, 2, 3, and 4 [W. Pu, W.-S. Kim, J. Chen, J. Sole, M. Karczewicz (Qualcomm)]

5.1.3RCE1 cross checks

JCTVC-O0086 RCE1: Cross-check on Experiment 1 [A. Minezawa, S. Sekiguchi (Mitsubishi)]
JCTVC-O0131 RCE1: Cross Check Results for Experiment 3 and 4 [T. Nguyen (Fraunhofer HHI)]
JCTVC-O0264 RCE1: cross-check results of test 2 in inter color component residual prediction (JCTVC-O0202) [K. Kawamura, S. Naito (KDDI)]