[x265] P-frames referencing B-frames, Generalised B-frames in HEVC

[Deepthi Nandakumar]

Mario - Yes, and P slices use only I-slices or other P-slices as reference.

On Mon, Oct 26, 2015 at 2:04 PM, Mario *LigH* Rohkrämer <contact at ligh.de>
wrote:

> The question was about P slices.
>
> I would be surprised if P slices were allowed to have B slices as
> reference, which are below in hierarchy. From what I know (which may be
> partially limited to AVC), usually only more dependent types can reference
> less dependent types, to avoid error accumulation (quality reduction and
> even possible data errors with decoding artefacts in slices used as a
> reference will remain in all dependent slices):
>
> I(ntra) slices are independent, have no references (prediction generation
> 0).
>
> P(redicted) slices can have I slices as reference (prediction generation
> 1), possibly also previous P slices (not sure about this).
>
> B(idirectionally predicted) slices can have I and P slices as reference
> (prediction generation 2), and in a pyramid hierarchy even B slices which
> have no other B slices as possible reference.
>
> The more dependent a slice is, the more loss of quality may accumulate
> with each further prediction generation, therefore it should only be
> visible for a short time and less likely be used as reference for even more
> dependent slices and even more accumulation of quality loss. Because P
> slices could potentially be a reference of many dependent slices, it
> wouldn't make sense to allow them to have references in B slices which are
> already predicted in second or even third generation, compared to an
> original I(DR) slice of the current GOP.
>
> Disclaimer: I may not yet know about exceptions in HEVC. The statement
> above is based on knowledge from MPEG-1/2 to AVC.
>
>
>
> Am 26.10.2015, 04:31 Uhr, schrieb Aarthi Priya Thirumalai <
> aarthi at multicorewareinc.com>:
>
> Hi,
>>
>> When b-pyramid is on, then b slices can refer to both B , P and I slices.
>> We are creating a hierarchy among B frames, that is some B frames can be
>> used as reference for other b frames.
>>
>> Regards
>> Aarthi
>> On Oct 26, 2015 3:23 AM, "Roshantha Mendis" <hrm506 at york.ac.uk> wrote:
>>
>> Hi,
>>>
>>> When --b-pyramid  is set to true, can P-SLICES refer to B-SLICES ?
>>>
>>> I haven't come across a video sequence that this happens, but just want
>>> to
>>> know if this can happen.
>>>
>>> Also if this situation can occur, then is it valid according to the HEVC
>>> specs ?
>>>
>>> Many thanks
>>>
>>> --
>>> Rosh Mendis
>>> Research Student - EngD in LSCITS
>>> Dept. of Computer Science
>>> University of York
>>>
>>> Disclaimer: http://www.york.ac.uk/docs/disclaimer/email.htm
>>>
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>>>
>>>
>
> --
>
> Fun and success!
> Mario *LigH* Rohkrämer
> mailto:contact at ligh.de
>
>
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>



-- 
Deepthi Nandakumar
Engineering Manager, x265
Multicoreware, Inc
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