[x265] x265-devel Digest, Vol 29, Issue 69

[Hoang Dzung]

What Joshua said is correct. I'd like to add a use case where a P slice might be better than a B slice. Consider a scene change where the forward and backward references are in different scenes. Using a P slice with a reference from the same scene is likely better than using a B slice where the two references are in different scenes. With a B slice, it is possible to observe "bleed-through" artifacts where some content from the other scene is blended in with content from the current scene.

With that said, it is possible to use a B slice and improve upon the P slice if both list 0 and list 1 references were chosen from the same scene as the current picture.

If not already supported, I encourage the x265 development team to optimize the reference list based upon scene change information.

Best regards,
- Dzung Hoang
@Freescale



-----Original Message-----
From: x265-devel [mailto:x265-devel-bounces at videolan.org] On Behalf Of Joshua Bowman
Sent: Monday, October 26, 2015 5:50 AM
To: x265-devel at videolan.org
Subject: Re: [x265] x265-devel Digest, Vol 29, Issue 69

On Mon, 26 Oct 2015 09:34:33 +0100, Mario *LigH* Rohkr?mer <contact at ligh.de> wrote:
> 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.
On Mon, 26 Oct 2015 14:17:13 +0530, Deepthi Nandakumar <deepthi at multicorewareinc.com> wrote:
> Mario - Yes, and P slices use only I-slices or other P-slices as reference.

This is incorrect. P-blocks may reference any one previously encoded frame in the DPB, while B-blocks may reference any two previously encoded frames, that's the only difference between them. (No frame is explicitly marked reference or non-reference, but they fall out of the DPB if they are unreferenced.) A P-frame may only have I- and P-blocks, a B-frame may have I-, P-, and B-blocks. Each frame has two lists of a practically unlimited number of references frames (2^16 short-term, 2^24 long-term) for each block to use, P-frames getting one list and B-frames two, but realistic DPB limits and usefulness mean lists are rarely more than a few frames long. A strict hierarchical encoding will enforce that no P references a B and no B-ref references a throwaway B, but the spec allows much more flexibility than that; strict hierarchical is just a poor-man's temporal scaling, and there's no need to emulate MPEG-1 with HEVC.

Reference list management is easily one of the most complex parts of the spec. That's why simplifications are usually used, but it's absolutely possible to do more.
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