Are these B-frames useful?

[el_rico]

Hello,

I'd like to use Xvid to compress movies, but actually relatively short sequences. Then size is not much of a matter for me. Not much, but it is not worth wasting space neither! Also, I am using 1 pass quality=100%, mainly for speed reason.

I have read that, with quality in mind, one should not use B-frames. I do not understand the technical aspect behind this but it seems that B-frames can hurt quality because of higher quantizer (quantizer ratio > 100 and positive treshold). Then are B-frames still useful (ie reduce size) when having a quantizer ratio of 100 and null treshold? (Do not hesitate to laugh here, I am having a strange feeling that this is idiotic, but since I do not very well understand all of this...), without hurting quality?

Thanks.

[mikeson]

@el_rico:

If you want best possible quality and you don't care about size (I guess you don't as you use 1 pass quality 100%), just turn them off.

[el_rico]

That's right,I do not care a lot on size. It was just to know if, keeping 1 pass (for speed) 100% quality (result wanted), it was possible to avoid wasting bytes using B-frames. Since I archive the sequences on CD, I do prefer use the remaining space with parchive recovery files.

[Alxemi]

Just try and take a look, make 1 pass quality 100% with a standar bframes configuration (2 150 100 0 for example) and another one without them, the first will be bigger, maybe you could notice the diference or maybe not.

IMO, the bframes will harm the video more as more dirty is the source, if you have a clean source, you can use them safely without notice any loss of quality (especially if you use quality=100% mode). If you notice just go to a lower configuration, you will always save bits, and the perfect bframes configuration is different for each video so you will have to try and try if you want the best quality/size balance (for your eyes)

[mf]

Why does everyone in this forum have the idea that turning off b-frames is a good idea. It'll just increase filesize. If you don't want quality impairment, use lower quant settings, like B-frames 2/100/100/15. Not to mention that this question has been asked a GAZILLION times before.
http://forum.doom9.org/images/top_search.gif (http://forum.doom9.org/search.php?s=)
http://forum.doom9.org/images/top_search.gif (http://forum.doom9.org/search.php?s=) <-- get to know this thing.
http://forum.doom9.org/images/top_search.gif (http://forum.doom9.org/search.php?s=)
http://forum.doom9.org/images/top_search.gif (http://forum.doom9.org/search.php?s=)
</rant>.

[el_rico]

@mf
Well, I have searched... But generally it is oriented towards maximizing the quality, given a target size. Also, I was not coming with the idea that turning off B-frames is good. Hence my question, that was to know if an extreme B-frame settings like ration=100 and treshold=0 still has a sense.

All,

Same question for VHQ. Is it useful with 1-pass 100% quality? (yes, I have problems regarding speed...)
In general, it appears that 1-pass quality based is not used a lot here. Am I totally on the wrong way. IMHO, quality as objective function rather than size is not so weird.

Thanks!

[mf]

Originally posted by el_rico
@mf
Well, I have searched... But generally it is oriented towards maximizing the quality, given a target size. Also, I was not coming with the idea that turning off B-frames is good. Hence my question, that was to know if an extreme B-frame settings like ration=100 and treshold=0 still has a sense.
Ok, sorry, let me clarify. The first part of my rant was meant at everyone who has replied saying you should turn off b-frames, and the second part at you. Strange that you couldn't find results, because I know there must be more threads out there like this. I know cause I read/replied to them. Anyway, VHQ is very slow for a relatively low quality increase. It's nice if you wanna aim for the max (it does increase quality at fixed size, so even when you've maxed out the codec), but not if you're going for speed.

[el_rico]

The point is that my knowledge is too limited, even to give the right sequence of keywords to the search engine. And since I have tried variations around "xvid B-frames quantizer ratio quality" or so, I let you imagine the number of hits I get....

Anyway, I now know that I can go further in this direction... only have to make a lot of tests!

Thanks!

[Alxemi]

The first part of my rant was meant at everyone who has replied saying you should turn off b-frames
Maybe i didn´t explain myself, that´s no what i tried to say.
I just think that you should do some visual test because sometimes, when the codec get saturated, the only way to increase quality is using less bframes or turning them off (specially in dirty/foggy sources).
But the normal thing is that you can use a normal bframes configuration, have the same good quality and save a good space.

[mf]

Originally posted by Alxemi
Maybe i didn´t explain myself, that´s no what i tried to say.
I just think that you should do some visual test because sometimes, when the codec get saturated, the only way to increase quality is using less bframes or turning them off (specially in dirty/foggy sources).
But the normal thing is that you can use a normal bframes configuration, have the same good quality and save a good space.
It is clear our friend here is trying to achieve high quality at constant quant 2. This means saturation is irrelevant, because he is aiming at 100% saturation. And B-frames do indeed increase quality when their quant ratio is set correctly.