[x264-devel] Clipping min-keyint

[BugMaster]

On Thu, 22 Sep 2016 15:37:36 +0530, Pradeep Ramachandran wrote:
> Hi all,I see in encoder/encoder.c, the following line


h->>param.i_keyint_min = x264_clip3( h->param.i_keyint_min, 1, h->param.i_keyint_max/2+1 );



> Essentially, the keyint_min is clipped to be a max of keyint_max/2
> +1. Now, this results in the problem that if scenecut detects a
> scene transition after keyint_min, it will insert IDR frame and
> break the GOP. So, if I were interested in a fixed GOP length (no,
> not max GOP length, but fixed GOP length for other reasons), I would
> have to disable scenecut detection which would result in a loss of efficiency. 


> Can someone please help me understand why this clipping was done?
> It seems like losing the benefits of scenecut detection if I want
> fixed GOP isn't the best trade-off.


> Thanks,
> Pradeep.

If you want fixed GOP length than you SHOULD disable scenecut
detection otherwise it wouldn't be fixed. --min-keyint was introduced
only to prevent closing GOP due flashes almost immediately after
previous key-frame. If you meant "min-keyint == keyint" use case for
fixed GOP than imho it is incorrect way to force fixed GOP. It
wouldn't result in any quality improvement vs disabling scenecut imho.
Because main quality hit of fixed length GOP is due placing of IDR at
non-scenecut frame which happened not long before (which we already
coded almost as IDR). Coding scenecuts itself as I-frame instead of
P-frame in fixed GOP wouldn't really give you significant quality gain
imho.