low '-keyint' = reduced quality?

[liquidskin76]

Hi,

Just wanted to clear up something up. Would having a lower '-keyint' value, for example '-keyint 24' produce a lower quality encode?

I've read in a few places that that's the case...

http://forum.doom9.org/archive/index.php/t-165197.html

...being one.

Obviously by using a lower '-keyint' value, seek times improve but file size increases.

Just wondering about any possible hit on quality?

Cheers

[detmek]

If you use encode at same bitrate (filesize) then yes, lower keying may lower the quality. But will it dependes of the particular video and used bitrate.

[liquidskin76]

Cheers for the info. What sort of quality hit would you likely see? Visible or nothing really to worry about?

Guess the insane Blu-ray bitrates compensate for the keyint 24 there!

Cheers

[Atak_Snajpera]

frame pulsing every second. very visible in static scenes if you use low bitrate.

[Asmodian]

Obviously by using a lower '-keyint' value, seek times improve but file size increases.

Just wondering about any possible hit on quality?

If the file size increases quality probably went up or stayed the same. It is only when targeting a bit rate (i.e. not using --crf) that quality will take a hit.

[shon3i]

frame pulsing every second. very visible in static scenes if you use low bitrate.
Should use open gop to avoid frame pulsing ? with small gops

[liquidskin76]

If the file size increases quality probably went up or stayed the same. It is only when targeting a bit rate (i.e. not using --crf) that quality will take a hit.

Sorry, should have mentioned... I'm using crf.

So re-encoding Blu-ray using '-keyint 24' and '--crf' is usually ok then?

I'm not a massive fan of seek performance on keyint240. Seeing as I have plenty of space on my NAS, I'd take a file size hit for better seeking, as long as quality doesn't take a hit (obviously quality has taken a hit already by re-encoding!)

Cheers

[Sharc]

--ipratio 1.1 instead of default 1.4 can also help for GOP pulsing reduction, but at the expense of larger file size.
(same applies for -- pbratio, but only effective without --mbtree)
The question is perhaps whether you want to encode for blu-ray compliancy or not. If yes, other x264 settings (not just --keyint) must be selected compliant as well. If no, you can leave --keyint at its default value.

[Asmodian]

In my experience this is only an issue if you use such a low bit rate you notice the I frames anyway; true the effect is more annoying if they are closer together. At the same bit-rate compression is worse so the effect is worse but at the same crf the file is bigger so the pulsing effect is the same as it would have been, just more often.

I always encode at a bit rate where the default 1.4 ipratio does not have visible GOP pulsing.

It is fine to use -keyint 24 and --crf, at least at reasonable --crf settings. However you might want to try --keyint 48 and lowering --crf a small amount, better quality at the same size and seeking is still quite fast. :)

[benwaggoner]

Sorry, should have mentioned... I'm using crf.

So re-encoding Blu-ray using '-keyint 24' and '--crf' is usually ok then?

I'm not a massive fan of seek performance on keyint240. Seeing as I have plenty of space on my NAS, I'd take a file size hit for better seeking, as long as quality doesn't take a hit (obviously quality has taken a hit already by re-encoding!)
Well, there's a HUGE gap between 240 and 24. Maybe something in-between would help? The player doing the seeking can also matter; a good player will snap to IDR frames, so it's only seeking with precision greater than --keyint that is a problem. Also, normally --keyint finds the first frames of shots which are good places to start. In practice, average GOP duration for modern film/TV content would be less than 240 as generally edits come faster than that.

Using Open GOP helps quality with low --keyint some, although can make scrubbing slower depending on how the player does it.

[Warperus]

So re-encoding Blu-ray using '-keyint 24' and '--crf' is usually ok then?General blue-ray already has GoP size of about 0.5s length, that is equivalent of keyint 15 (or even less). So I don't think it will hit quality down to noticable amount.
I used to reencode frapses to save drive space. To make them more NLE-friendly I use key-int in range 15-25. At 18-20 crf it looks to me the same as for default key-int (240? 250?).