interpolation replacement (from Samsung). [E. Alshina, A. Alshin, J. Chen, J.H. Park (Samsung)]
High memory access during motion compensation is the main drawback for both bi-prediction and long-taps interpolation filters. In this contribution motion compensation process in bi-prediction is modified to reduce memory access: a motion vector which requires high memory access is replaced by nearest one with acceptable level for memory access.
4.3.3 Discussion and conclusions
Discussion: Would it be desirable to define additional testing conditions where mode decision is made without result of motion compensation (as this may likely not be used in practice)? There is agreement that we should stick with the current anchor settings, as it is beyond our purpose to consider any possible optimization or restriction in the implementation of the standard.
4.4 CE4: Quantization 4.4.1 Summary
JCTVC-G034 CE4: Summary report of Core Experiment on Quantization [K. Sato, M.
Budagavi, M. Coban, H. Aoki, X. Li (CE coordinators)]
This document reports activity related to Core Experiment on Quantization (CE4).
This CE contained 3 subtests as follows:
Subtest 1: QP Coding
Subtest 2: De-quantization process modification
Subtest 3: Quantization matrices
All mandatory results had been verified by cross-checkers.
Review of subtest 1:
G721 proposes QP control below the CU level (at TU level for TU larger than a particular size) – emphasis on visual quality (BD rate impact was not an improvement in this CE test, relative to the anchor). No action – for further study.
G773 proposes finer quant step size granularity (seems similar to proposal G850 in subtest 2 below).
Next idea tested is for prediction of QP value: HM uses left prediction unless not available; otherwise last in decoding order.
Anchor is "Step 3 of TM 5" (region characteristic-based QP control for visual quality – no buffer fullness feedback).
Proposals include taking into account things like modes, motion vectors, and intra prediction directions.
It was suggested to focus on 1.3.c (G067) and 1.4 as the best possibilities (0.4% benefit), but higher complexity than current method. But it was pointed out that 1.4 has temporal QP prediction, so it would have loss resilience issues.
It was remarked that this is emphasizing QP control for visual characteristics, while QP control for buffer fullness purposes is a different kind of usage – in that usage, predicting from the last QP in decoding order might be better.
It was suggested to make the qp prediction modal – having one mode that is more friendly to buffer control (changing QP and having that "stick" permanently).
Straw man: two modes (SPS level selection):
Always using the preceding CU QP in coding order (+0.2%, measured previously).
other mode is "1.3.c" (−0.4%) or HM 4 (0.0%), which one TBD.
After discussion, the group wanted to simplify this to just use the preceding QP in scan order. There was then a submission of G1028, and after further discussion, it was agreed to keep the WD and software stable for this meeting and to test two schemes in CE work:
Always use the preceding CU QP in scan order
HM 4 with reset to preceding CU QP in scan order at LCU boundary
G1028
Review of subtest 2: Three proposals in this subtest:
AQO (JCTVC-G278), one offset per colour component per frequency (offset is scaled by quantization weighting matrix entry and by step size as a function of QP)
ARL (JCTVC-G382)
Fine-QP (JCTVC-G843) – encoder can customize the scaling difference for a QP increment.
These proposals, along with a non-CE proposal, a modified Fine-QP scheme (JCTVC-G850), were tested in four ways.
Test results were not provided for some of the cases that were more difficult to test.
Comments in cross-check document JCTVC-G403:
Most of the gain was from non-normative modifications.
Did not seem to work with low QP. It was remarked that G278 did not exhibit this phenomenon.
In the other two cases, the proponents had worked to address this issue with an encoder-only modification for G382, and with the modified fine QP proposal G850.
Effects on luma and chroma somewhat different. Chroma gain is larger than luma gain.
It was suggested to emphasize consideration of JCTVC-G382 and JCTVC-G850.
Gain from normative aspect was estimated at 0.4% for luma and 4% for chroma.
Complexity impact (with quant matrix)? (Any dynamic range impact?)
A BoG (chaired by M. Budagavi) was asked to study the complexity impact, and the subject was discussed again. Revisit to consider which proposal and complexity impact and revisit (BoG M.
Budagavi). Also look atsee notes for G773.
The BoG report commented that an additional Intra/Inter parameter is needed for G382. G382 also uses sgn() and abs() in the dequant equation. No consensus was reached in the BoG on the complexity impact of these. This was further discussed and effect of this was not considered to be significant.
Comment: QP scaling is being done at a finer level. Will this be required if quantization matrices are used? Flat quantization matrices can achieve QP scaling at a finer level.
Comment: Modified version of JCTVC-G382 was cross-checked in JCTVC-G1045 (RDO-Q On BD-Rate matches, code was studied). Modified version of JCTVC-G850 was cross-checked in JCTVC-G1040 (BD-Rate match was observed, code was studied).
Comments: JCTVC-G403 cross-checker commented updated version of their document asserts that all of the gain reported in JCTVC-G382 could be achieved by non-normative modifications only.
Comments: First test: Second bit-allocation in cross-check document JCTVC-G403 asserted to be not constrained within 2%. CE submissions were reported to be within 2%.Second test: Encoder only test reports more gain with two pass algorithm.
Comment: Concerns expressed with regards to complexity at slice level. In worst case bitstreams,there could be many slices in bitstreams.
Suggestions: Study RDOQ off case. What is the complexity impact? On decoder side and on encoder side. RDO-On case:
For further study in CE.
Comments: Interesting to study period of 8 and period of 16 for quantizer.
JCTVC-G773:
Comments: Asserted that dQP rate could increase when QP starts changing.
Comments: Introduces functionality that allows for QP to change at a finer scale.
Comments: Asserted G773 could be implemented using G850.
Comments: G773 was tested with perceptual quantization and not bit-rate control.
Comments: Provides new functionality but functionality is not proven.
Comments: In some implementations, this could lead to doubling of quant matrix tables.
For further study in CE.
Subtest 3: Quantization matrices Three methods evaluated:
Same as AVC
JCTVC-G083
JCTVC-G434
It was recommended to adopt (slightly simplified) AVC method (SPS and PPS) as described in G434 as a starting point (no text! – T. Suzuki was asked to provide – then revised version of G1016 -v4 provided this). For non-square cases, use the even numbered column entries of the next larger square matrix.
Decision: Agreed.
Should we do something about big block sizes? For further study.
4.4.2 Contributions
Subtest 1 (delta QP)