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Field-to-frame scalability (3)14.1.97.1.1.1.1.1.282JCTVC-P0163 AHG15: Interlaced to progressive scalability for SHVC hybrid codec use case [Y. Ye, Y. He, Y. W. He (InterDigital)] Discussed 01-10 a.m. (JRO). SHVC draft 4 supports hybrid codec scalability in concept, where the base layer is coded using AVC, and the enhancement layers are coded using HEVC. This contribution, however, asserts that, SHVC draft 4 does not provide a complete solution when the AVC coded base layer contains interlaced content. This contribution focuses on interlaced-to-progressive scalability for the hybrid codec use case. It is proposed proposed to apply a field-parity-based resampling process on reconstructed base layer field pictures to generate the inter layer reference pictures, which are then used as additional reference pictures for coding of the progressive content in the enhancement layer. Experiments reportedly show that, compared to simulcast, the proposed method achieves {Y, U, V} BD-rate reduction of {−20.3%, −15.6%, −14.8%} for Random Access. Equivalently, for the HEVC coded progressive content (EL-only), {Y, U, V} BD-rate reduction of {−42.6%, −38.6%, −38.0%} can reportedly be achieved. Combination of 1080i AVC base layer and 1080p enhancement. Signalling of top/bottom field in slice header, upsampling phases are determined from that For the AVC base layer, PAFF and MBAFF was turned off Results with five sequences (not available) that were reportedly captured in 1080p60 and professionally downconverted to 1080i were described. The gain compared to the SHM (without correcting the vertical upsampling phase) was reportedly 1.6% for RA. One expert pointed out that the combination of 1080i base layer and UHD progressive enhancement layer could also be interesting; however, other opinions are that in that case the gain over simulcast could be significantly less attractive due to the larger difference of resolutions. The question was raised whether the signalling in the slice header of the enhancement layer is the appropriate position; another option could be to determine from the base layer bitstream whether frame or field coding is applied, and whether the field is top or bottom (relates to HLS concepts). Signalling in PPS could be another option, however this might be inappropriate under the expectation that the information changes quite frequently. 14.1.97.1.1.1.1.1.283JCTVC-P0165 Interlaced to progressive scalability in SHVC [J. Chen, K. Rapaka, Y.-K. Wang, M. Karczewicz (Qualcomm)] Discussed 01-10 a.m. (JRO). In this contribution, an approach to interlaced-to-progressive scalability was proposed. The proposal uses signaling in a PPS flag whether it is a frame or field picture and in the slice header whether it is a top or bottom field. Results with SVT sequences and HEVC base layer, 1080i/1080p60 were described. Reported gain for AI: 9.1%, RA 0.2%, IbbB coding 4.1% (in the latter case, B toggles between top and bottom field and therefore the gain is higher). Results with AVC were described including MBAFF. The reported gain is 7.3% for AI, 0.8% for RA. No results were provided on IbbB or other configurations. Some more discussion was held about the PPS flag. It seemed unclear what happens in the case of frame structured pictures:
See notes on other related contributions. 14.1.97.1.1.1.1.1.284JCTVC-P0175 On field to frame scalability [K. Minoo, D. Baylon, A. Luthra (ARRIS)] Discussed 01-10 a.m. (JRO). This document discusses field-to-frame scalability, such as in conversion from 1080i to 1080p. If spatial upsampling of a field is performed to generate the “de-interlaced” frame, then it is asserted to be important that particular vertical field offsets be used. Simulation results where such phase offsets are used reportedly show BD-rate changes relative to SHM 4.0 for luma and chroma, respectively of −11.7% and −12.8% for AI, −1.2% and −1.3% for RA, −3.9% and −2.3% for LD-B, and −4.3% and −2.6% for LD-P. The results reportedly show BD-rate changes relative to HM 12.0 simulcast for luma and chroma, respectively of −30.1% and −30.7% for AI, −25.1% and −16.3% for RA, −21.9% and −15.9% for LD-B, and −21.0% and −14.2% for LD-P. Results were reported with current class B sequences, using HEVC base and enhancement layers. The approach is different to that in other contributions, in that instead of signalling top/bottom field the phase offset is signalled in the PPS. This means that two PPSs need to be present for top/bottom field offset differences. It was suggested that such a method could also be applied to signal top/bottom field usage (without explicit phase offset). As a general conclusion, it seems manageable to do field/frame switching at sequence level, where either vertical field upsampling (with filter phase dependent on top/bottom or unchanged with some compression loss) is used, or in the case of field-to-frame merging, temporal scalability would be used. The phase precision computed in the upsampling process is 1/16th sample. Questions:
Next steps were suggested as follows:
Parent-level joint discussion of 01-13 was noted on 01-14 (GJS). A similarity was noted between the phase offset used for rescaling filtering and the phase offset that would be used for field-based operation, and the parent-level guidance was to consider a general approach. It was agreed to establish a phase adjustment BoG P0312 (coordinated by E. Alshina) to consider what should be done along those lines. 14.1.97.1.1.1.1.1.285JCTVC-P0312 BoG report on phase adjustment in SHVC re-sampling process [E. Alshina] Discussed 01-15 (GJS). It was clarified that the enhancement layer is not envisioned to be switching between frame and field referencing to the base layer on a picture-by-picture basis within a CVS. So the scalability resampling ratio is fixed within a CVS. (At least if the referenced "picture" array is supplied by external means, this does not constrain how that array was coded before it was presented to the enhancement layer for referencing.) Only the 2:1 case has been tested. Currently the draft has a cross-layer phase alignment flag at the VPS level to control vertical phase. The BoG was considering (but had not concluded on discussion of) a four-flag scheme
Alternatives discussed included having 4 bits for luma and 4 bits for chroma in the PPS (with some gating flag(s)). For horizontal phase, the same possibilities exist, but there was less interest in having additional flexibility horizontally. For upsampling ratios other than 2:1, the scheme would not necessarily provide optimal phase behaviour. Further BoG discussion was held. BoG report (r2) reviewed. Decision: Add the proposed signalling (3 added bits and the previously drafted bit) and its mechanism for phase adjustment described above in to the SHVC specification draft text and next release of SHM s/w. Do not constrain the use of the flags to particular scalability ratios. It was noted that the vertical alignment of chroma relative to luma built into the scheme corresponds to that used for interlaced fields in the 2:1 case. However, the chroma phase alignment does not seem critical. Further study was requested to determine whether constraints should be imposed or different syntax should be used.
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