[x265] Adaptove GoP sizes - not sure if its working

[Steve Borho]

On 08/22, Roshantha Mendis wrote:
> Thanks Steve and Deepthi, for explaining things and for that link.
> 
> I understand now, that the encoder will always attempt to use P-frames at
> scene-cuts, whenever possible. Is this to improve on compression efficiency
> ?
> 
> However in the docs it says :
> --scenecut <integer>, --no-scenecut
> 
> How aggressively I-frames need to be inserted. *The higher the threshold
> value, the more aggressive the I-frame placement*. --scenecut
> <https://x265.readthedocs.org/en/default/cli.html#cmdoption--scenecut> 0 or
> --no-scenecut
> <https://x265.readthedocs.org/en/default/cli.html#cmdoption--no-scenecut>
> disables adaptive I frame placement. Default 40
> 
> So I thought , increasing this *'--scenecut <int>'* param would cause the
> encoder to insert I-frames instead of P-frames. is there an upper limit to
> the scenecut param ?
> 
> and on the x265 website (https://x265.com/about-x265/) it says : Scenecut:
> Insert I-frames at scene changes
> which is why I thought the encoder would insert I-frames rather than
> P-frames. Initially I was going to use x264 to get GoP size distribution
> results. But when I saw those lines in the docs/website I thought of trying
> out x265 instead.

That is what the scenecut bias does, but increasing --scenecut does not
necessarily make the encoder always use I frames for scene-cuts.  The
keyframe adaption function uses a bias that increases the likelyhood of
an I (vs a P) the closer it gets to the max keyframe interval.  The
--scenecut bias adds to that bias, making early keyframes more likely.

FWIW: x264 has almost the exact same logic

> For, the purpose of presenting results in an academic paper - do you think
> calculating a GoP size as distance between : [ I-SLICE <--> I/PP SLICE] is
> correct ?

GOP size is generally the distance between keyframes.

> Finally - with respect to closed-GOPs (IDR is being used) - Is it right to
> say that in this case, GoP size (I to I distance), will always be fixed at
> the --keyint inverval : no variation, not even slight. Is this because with
> closed-gops it is not possible to have adaptive GoPs ? (i.e. it doesn't
> specify in the standards) or just the x265 encoder doesn't support it ?

no, closed-GOP only means that at the keyframes the decoded picture
buffer is cleared, so pictures decoded after the keyframe cannot use any
references that were coded before the keyframe. It has nothing to do
with keyframe cadence.

fixed keyframe interval is achieved by --no-scenecut

-- 
Steve Borho