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

JCTVC-O0345 BoG Report on single loop decoding and key pictures in SHVC [K. Ugur]

Discussed Sat. p.m. GJS.

The BoG met on 24 October 2013 at 11:30 to discuss single loop decoding and key picture concepts for SHVC. The proposals were analyzed with respect to the following points:

• 
  Coding efficiency and complexity characteristics.
  
• 
  Profiling considerations for SNR scalability and single loop operation
  
• 
  Burden on multi-loop encoder/decoder that doesn’t operate in single loop fashion
  
  • 
    Access to unfiltered samples (in variant supporting single-loop decoding unless base layer is required to disable deblocking)
    
  • 
    TMVP problem (discovered by cross-checker, asserted minor fixable by proponent)
    

It was remarked that we might need a special profile for a single-loop approach that requires low-level modifications (as in JCTVC-O0145), since our intent has been to have an approach that can be built without low-level changes.

There is also an approach already supported in the standard that supports single-loop operation without low-level changes by forbidding ILP for certain temporal sub-layers. This has a more substantial loss for the enhancement layer decoder – unless the base layer non-key pictures are removed from the bitstream, in which case it actually has gain.

JCTVC-O0162 is more of an AVC-like single-loop design. It requires access to the base layer residual and to the unfiltered base layer samples and full-resolution motion field. It also requires the base layer and enchancement layer to reference the same pictures.

Conclusion: A new phase of work as development of additional profile(s) would need to be justified and approved to adopt such a scheme that requires low level changes. This is not within our current plan of work.

JCTVC-O0227 has a different scheme – not a single-loop decoding approach. It allows base layer pictures to refer to enhancement layer pictures, whereas a base layer decoder that operates only on base layer data would generate somewhat degraded picture quality due to drift (e.g., 4% on average). The enhancement layer decoder would actually produce improved quality (e.g., 3%) – although only in the case where temporal scalability is in use. Relative to operation without temporal scalability, e.g., in low-delay use, this scheme does not provide a benefit.

The text changes are asserted to be relatively small (e.g. less than a page of text changes).

It was remarked that in this scheme the BL decoder needs access to the pictures (all of them) in the EL DPB.

It was commented that the interaction of this with rate control may reveal problems that would not be seen with fixed-QP testing.

Further study is encouraged – e.g. on distribution and perceptibility of drift, encoder drift control, relationship to temporal scalability, interaction with rate control, DPB access feasibility.