Organisation internationale de normalisation

IC (3)

(Chaired by JRO, Sat evening)

13.0.1.1.1.1.1.1.20JCT3V-J0034 Complexity reduction on illumination compensation for 3D-HEVC [X. Zhang, K. Zhang, J. An, H. Huang, J.-L. Lin, S. Lei (MediaTek)]

In the current 3D-HEVC, for each block coded with illumination compensation (IC), the LLS(linear least square) method is utilized to calculate α and β parameters for each color component. To reduce the usage of LLS which requires numbers of additional multiplication operations and memory accessing, this contribution proposes to disable bi-prediction for IC and set α as 1 for chroma components. Experimental results reportedly show the proposed simplifications only introduces 0.01% BD-rate increase whereas saves the operation number and bandwidth in worst case.

Method 1: Disallow bi-prediction with IC

Method 2: Simplify IC for chroma by setting gain =1

13.0.1.1.1.1.1.1.21JCT3V-J0081 Cross-check on Complexity reduction on illumination compensation for 3D-HEVC (JCT3V-J0034) [T.Ikai (Sharp)] [late]

13.0.1.1.1.1.1.1.22JCT3V-J0099 Crosscheck on complexity reduction on illumination compensation for 3D-HEVC (JCT3V-J0034) [X. Zheng, Y. Lin (Hisilicon)] [late]
13.0.1.1.1.1.1.1.23JCT3V-J0050 Simplification of chroma IC [J. Nam, S. Yea (LGE)]

In the current 3D-HEVC, to compensate illumination change between inter-view texture pictures, IC which designed by based on linear model including scaling factor a close to 1 and an offset b applies both luma and chroma components. Basically, chroma is not an illumination component but a color component. Therefore, it is required to use IC process for chroma different from that for luma. In this contribution, simplification of chroma IC is proposed. The first option is to remove chroma IC completely. IC process only applies luma component. The other option is to use offset model instead of linear model. Experimental results show 0.1% loss for removing chroma IC and no impact for using offset model.

Two methods are proposed:

Method 1: Completely disable IC for chroma – 0.1% rate increase

Method 2: Simplify IC for chroma by setting gain =1 – identical with method 2 of J0034, negligible loss
Both J0034 and J0050 give an interesting tradeoff between complexity reduction and only small change in compression performance. From the discussion, it is desirable to get better understanding about various implications, e.g. the impact on non-CTC condition (IBP coding) for disabling the bi-prediction with illumination compensation. For the chroma simplification part, it is also suggested to investigate whether the saving in complexity could be re-invested to enable IC for 4x4 chroma blocks (which is currently prohibited)

It is also not clear whether the illumination compensation is critical in terms of worst case complexity.

Further information on these aspects would be necessary. If not possible during the meeting, establish CE.

Several experts expressed support for the part of modifying chroma IC (offset only, i.e. method 2 of both proposals). However, as the results indicate that this has at least a little compression loss, it would be desirable to confirm that IC establishes a worst case in terms of complexity

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  Cross-checkers and proponents should investigate the worst case issue and report back
  
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  Proponents of J0034 and J0050 should present a unified version of the draft text concerning their methods 2
  

Based on this, the solution of WD text contained in JCT3V-J0050 v3 was adopted. Decision: Adopt the common solution of J0034 and J0050.

Further study (CE) appears necessary about disabling bi-prediction

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  The consequences of this restriction may require more careful investigation beyond CTC
  
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  The restriction may not be necessary if it is not the worst case
  

13.0.1.1.1.1.1.1.24JCT3V-J0097 Cross-check on Simplification of chroma IC(JCT3V-J0050) [J. Zheng(HiSilicon)] [late]

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