Higher number of ReFrames = higher quality?

[ankutsa]

I was wondering if there's any correlation between the two. I see that MediaInfo and others mention this number, so I guess it's for a reason

[KarMa]

I was wondering if there's any correlation between the two. I see that MediaInfo and others mention this number, so I guess it's for a reason

It gives better compression efficiency, which will give smaller file sizes or better quality for the same file size. Depending on if you use two pass or CRF.

[locotus]

I was wondering if there's any correlation between the two. I see that MediaInfo and others mention this number, so I guess it's for a reason

You should read this thread in doom9 forum:

http://forum.doom9.org/showthread.php?t=165490

[KarMa]

Also should not go above 9 ref frames, as that is the limit for High@L4.1 is probably the most compatible profile.

Update: Yeah I forgot, 9 ref frames max for 720p High@L4.1, and 4 ref frames max for 1080p High@L4.1.

[ankutsa]

Is there a trade-off? If higher ref frame numbers lead to smaller file sizes and better quality, there must be a disadvantage, otherwise nobody would use low numbers

[sneaker]

slower to encode, decoding needs more memory (means hardware players may not be able decode the stream if you go too high, many players can only use 4 refs for 1080p for example)

[ankutsa]

What if I, theoretically, go with CRF and 100 ref frames, and say it takes a month to encode 2 hours of video, how much would the file size decrease compared to 4 refs? 10, 50%, 90%?

[sneaker]

Depends on some variables like content type, but think 1%-5% (diminishing returns). It's not a magic weapon. Just use --preset, --tune and --level and let x264 choose optimal value.

[Cornucopia]

Ever hear of the law of diminishing returns? Hyperbolic curve? Asymptotic functions?
It applies here.

That's why there is the often referred to encoding "sweet spot".

Scott

<edit>Sneaker beat me to it.

[ankutsa]

ok thanks!

[johns0]

If you're encoding for bd authoring don't go over 5 ref frames since most newer blu-ray players will show a lot of smeared blocking.

[sneaker]

1080p BluRay (and H.264 level 4.1 in general) is limited to 4 refs, so, yes, that's a bad idea to begin with. For "professional" authoring we have clear specs, no need for guesswork.

[_Al_]

those specs are here, if someone wanted to look at it, it is all interconnected frame rate, ref frames, resolution, with specific equation there

[sophisticles]

What if I, theoretically, go with CRF and 100 ref frames, and say it takes a month to encode 2 hours of video, how much would the file size decrease compared to 4 refs? 10, 50%, 90%?

No encoder supports more than 16 ReFrames so your question is a moot one.