JVET-D0123 Enhanced Motion Vector Difference Coding [J. Chen, W.-J. Chien, N. Hu, V. Seregin, M. Karczewicz, X. Li (Qualcomm)]

Discussed Sunday 1700 (GJS & JRO)

This contribution describes modifications to Motion Vector Difference signalling. The proposed method includes two aspects: a) 4-pel accuracy for MVD signalling (in addition to ¼ pel and integer-pel MVD accuracy); b) switchable binarization and enhanced context model selection. The document also investigates the impact of large search range since the current search range (64 luma samples) are too small for 4K resolution with a GOP size equal to 16. In addition, an encoder-only change, with more accurate motion information bit estimation during ME stage, was proposed. Simulation results reportedly show that, for the Random Access configuration, the enlarged search range and encoder-only more accurate motion information bit estimation bring around 1.01% luma BD rate saving. Together with the modified MVD coding, around 1.55% luma BD rate saving is reportedly obtained.

The gain is mainly in class A2 (4%) and there mainly for 2 sequences with large motion.

The better bit rate estimation gives approx. 0.1–0.2% gain without an increased encoder runtime.

An encoder runtime increase of approx. 5% was reported with an encoder-only change (increased search range).

It was reported that the gain by increasing the search range with HM provides less gain, and has more increase in runtime.

HM and JEM encoder settings should be harmonized.

Decision (SW): Adopt encoder-only changes.

Decision (CTC): Use a maximum motion range 256 for the RA configuration.

Further normative changes are proposed: Switching MV precision at CU level ¼, full and four-pel; and modifying binarization and context modelling for the different precisions. All this together reportedly gives another 0.5% bit rate reduction and increases encoder runtime by another 10%.

It was agreed to investigate normative aspects in EE, in particular answering how much the different elements are giving.

General comments on HM comparison:

• It was commented that the search range issue could be considered for the HM
• HM lambda/QP handling is a bit different between JCT-VC CTC and JVET CTC.

It was suggested to try to harmonize these.

Agreed: Use 256 search for RA only (all picture resolutions).

JVET-D0123 Enhanced Motion Vector Difference Coding [J. Chen, W.-J. Chien, N. Hu, V. Seregin, M. Karczewicz, X. Li (Qualcomm)]

Decision (SW): Adopt encoder-only changes.

Decision (CTC): Max motion range 256 for RA configuration.