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

[Mario *LigH* Rohkrämer]

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