[x264-devel] Global optimization of keyframes

[Jason Garrett-Glaser]

On Sun, Dec 26, 2010 at 5:38 AM, Nicolas George
<nicolas.george at normalesup.org> wrote:
> Hi.
>
> I am wondering if you considered implementing a global optimization of
> keyframes using two-pass encoding, even in CRF mode.
>
> The way I see it, allocating keyframes with regard to the GOP size is a lot
> like allocating line breaks in a paragraph with regard to the page width.
> And we know since TeX that this can be optimized globally in an efficient
> way and greatly increase the quality. Keyframes have even more freedom than
> line breaks, the benefit could be even bigger¹.
>
> I tried to encode something to have figures to illustrate this mail;
> unfortunately, it seems that forcing the frame type, even to what it would
> have been anyway, greatly decreases the quality of the encoding.
>
> Anyway, I believe this idea may be worth some thought.
>
> Regards,

This idea has a few problems.

x264 currently makes all frametype decisions in the first pass.  It
also makes various decisions *based on* these frametype decisions,
such as MB-tree.  Changing frametypes in the second pass would require
either ignoring the inaccuracy of the other decisions, or redoing them
all.

Secondly, I don't really get your example.  Why would you want
keyframes at all in that case?  Keyframes are for scenecuts, and your
described video has no scenecuts.

Jason