Of itu-t sg16 wp3 and iso/iec jtc1/SC29/WG11

JCTVC-O0298 Non-SCE4: Cross-check of JCTVC-O0195 “Picture and region adaptive gain-offset prediction for color space scalability” [J. Zhao, S. H. Kim (Sharp)] [late]

6.2.6Up-/downsampling process (6 - done)

(Reviewed Sat 26th evening JRO)
JCTVC-O0071 AhG13: Performance analysis of scalable systems with different down-samplers [X.Li, J.Chen, M.Karczewicz (Qualcomm), E.Alshina, A.Alshin (Samsung), J.Dong, Y.Ye (InterDigital), E.Francois (Technicolor)]

This contribution compares performance of scalable systems in which 2 different down-samplers are used. Since base layer is different, BD-rate performance is reported compared to the single layer HEVC. Depending on scalability ratio performance drop of scalable system is 10.5%...14.0% (AI), 16.2%...19.0% (RA), 24.8%...28.5% (LD-B) and 22.8%...26.6% (LD-P) when so-called SHVC down-sampler and 13.5%...15.5% (AI), 17.3%...20.3% (RA), 24.9%...28.7% (LD-B) and 22.8%...26.6% (LD-P) when JSVM down-sampler is used. So the usage of so-called SHVC down-sampler is preferable for all-intra, random access and low-delay-B configurations.

“SHVC downsampler” has slightly less loss against single layer than “JSVM downsampler” (which has lower frequency cutoff) – both operated with zero phase shift. Scalability ratios 2x, 1.75x and 1.5x were used

Bit rate increases against single layer for SHVC:

BD-rate vs HM11.0
Y	U	V	Y	U	V	Y	U	V
AI HEVC 2x			AI HEVC ~1.75x			AI HEVC 1.5x
12,8%	14,9%	14,6%	14,0%	15,3%	14,8%	10,5%	9,8%	9,3%
RA HEVC 2x			RA HEVC ~1.75x			RA HEVC 1.5x
19,0%	33,1%	31,8%	19,5%	33,2%	32,1%	16,2%	28,9%	29,2%
LD-B HEVC 2x			LD-B HEVC ~1.75x			LD-B HEVC 1.5x
28,3%	38,9%	39,7%	28,5%	38,6%	39,9%	24,8%	33,2%	36,0%
LD-P HEVC 2x			LD-P HEVC ~1.75x			LD-P HEVC 1.5x
26,5%	37,9%	38,9%	26,6%	38,0%	39,5%	22,8%	32,8%	35,6%

Decision (SW): Adopt the new version of SHVC downsampler provided in JCTVC-O0071

Note: The downsampler generates reasonable output only in the range of approx. 1.3x ... 2.2x. This should be documented in the SHM description.

JCTVC-O0072 AhG13: On Chroma accurate position alignment during re-sampling [E.Alshina, A.Alshin (Samsung)]

The contribution presents the set performance tests with misaligned Chroma position during re-sampling process. If accurate Chroma position is not taken into account then re-sampling process can be simplified: reference position calculation for both Luma and Chroma, vertical and horizontal interpolation becomes the same. This simplification leads to no performance degradation in terms of Luma BD-rate (0,1% Chroma BD-rate drop is observed).

The results shown in this contribution unveil that the loss by using a different phase in chroma downsampling and upsampling is low (0.1-0.2% depending on phase deviation introduced).

One expert reports about having investigated this with other test material and found larger losses.

Definitely, the same phase position should be used in down and upsampling, and position “b” (half luma sample shift vertical of the chroma) was asserted as default and used in the base layer of test sequences.

By defining the corresponding upsampler with phase position “b” as the only option, it would enforce always using this phase position in downsampling. If the original material was using different phase positions, this might be undesirable for displaying the base layer.

Evidence should be brought that it is important to be able changing and signalling the downsampling phase of chroma.

Further study in AHG.

JCTVC-O0215 On phase alignment of up-sampling process in SHVC [J. Chen, L. Guo, X. Li, S. Fan, M. Karczewicz (Qualcomm)]

This contribution proposes signalling a flag to indicate phase alignment between reference layer picture and enhancement layer picture in down-sampling process and accordingly a matched up-sampling is used in decoding process. With the proposed method, SHVC can take both zero position aligned sequences and central position aligned sequences as input. It is reported that for central position aligned sequences, the proposed method can achieve −8.8%, −6.3% and −5.4% luma bit rate saving, respectively, for AI, RA and LD-B configurations.

Results relate to the case where center position in used for downsampling and zero phase for upsampling

“central position aligned” means that the center of base and enhancement pictures are aligned, regardless of the ratio.

Zero phase has better compression performance than center position

However some existing downsamplers use center position

additional complexity by determining the position (once per line/column, not per sample)

Commercial downsamplers may have other frequency cutoff characteristics, which may also affect compression performance

Likely, other than zero and center position would not be required

If only one position would be supported, center position would be desirable

Down and upsampling phase should be aligned

The additional complexity is minimum, due to the fact that arbitrary upsampling has been adopted per SCE1

The additional flexibility is desirable

Decision: Adopt JCTVC-O0215 (signalling zero or center phase shift of upsampling).
JCTVC-O0274 Cross-check report for JCTVC-O0215 on phase alignment of up-sampling process in SHVC [Jie Dong, Yan Ye (InterDigital)]