Universal rc-lookahead

[BDHDLIGHT]

Hello

Is there a universal rc-lookahead ?

By universal, I mean that we can put on all films without thinking that our rc-lookahead is not high enough or unnecessarily too large.

I know that the maximum of the rc-lookahead is 250.

I see several different rc lookahead encodings on them.

I saw rc-lookahead lower than 60 when the encoders use presets lower than the one I usually use, which is veryslow.

For example, I saw rc-lookahead at 64, 80, 120, 200, and many other different rc-lookahead when looking at several encodings.

I also noticed that some encoders themselves use different rc-lookahead during their encoding, hence my question about the fact of a variation depending on the movies.

I donÂ’t see why the same encoders would sometimes use a number and sometimes another number during their encoding, so use the same one if it has no impact.

ShouldnÂ’t the rc-lookahead be the same for all movies?

Are there any particular cases of films where the rc-lookahead must be higher?

I will probably be told about the default rc-lookahead for x264, which is 60 in veryslow and placebo.

The default value for x264 merange is 24 in very slow and placebo, and I know that if I change it to 64, I gain quality even though I know it will be minimal during my encoding.
Of course, my encoding will be longer, but I gain quality and security by expanding the search area up to 64 pixels.

All this to say that I canÂ’t rely on the default setting of x264 if IÂ’m looking for possible gains, even minimal ones.

Good weekend.

[Emulgator]

I would use the longest rc-lookahead.
Defaults were tailored to win encoder comparisons, and since most of the time audience was interested in speed over tolerable quality I can not suggest to leave any encoder at defaults.
We can have better quality if we allow encoders to invest more time leading to better decisions.

[Z2697]

Use your lucky number

[Selur]

side note:
I know that the maximum of the rc-lookahead is 250.
max rc-lookahead should be: min(ceil(gop_size / 2.0), 250)
=> you need to take the gop size into account if it's below 250.

[GeoffreyA]

Use the biggest but no need to go over maximum keyframe interval. There is probably diminishing returns past 100, but I just use 240 on all encodings.

[benwaggoner]

rec-lookahead==keyint works well. While there are diminishing returns, they are non-zero as far as I've tested, which is up to 300 IIRC. The benefit is also somewhat content dependent; the more variable the complexity of the content, the more value there is in varying bitrate to keep quality constant. And the more VBV available, the more potential benefit as well, as bits can be redistributed more.