Conversion Table (H264->XviD)

[SacredCultivator]

Dear Readers,

I was debating where this Thread would apply and thought here would work... If not please move it that deems fit.

As a person that does Dual Encodes in both H264/XviD, I have a question when it comes to setting the Quantizer...

For the longest time I've been following this Site (http://fansubbers.org/index.php/KB/Quanttable) for the values.

And for me, in H264 I generally use either 19 (For PV/Perfs) or 21 (For DVD's/Shows) and it never deemed upon me about the Bitrate output.

For example, for one file, this is according to the Chart I linked to, for H264 I used 19 / XviD 2.24
Bitrates of the Files:
H264: 2431
XviD: 6345

And to me at least, between the two files, I can't really tell much of a difference.

So I was wondering if perhaps that chart is too out of date that the values have changed.

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Additional Information:
For H264 I use meGUI
For XviD I use VirtualDubMod
Both using Single Pass...

If you need me to upload my Profile for them, I'll be more than happy to. Thank you

Again I wasn't too sure where this thread was appropriate to post as it is in regards to BOTH XviD/H264.

[Brother John]

It’s true that you can *mathematically* convert between both quantizer scales. But the usefulness of such conversion tables is severly limited at best. In general I’d say they’re only useful when switching from one format to another to give you a first very rough feeling of how the new quantizer numbers compare to the old ones.

Don’t use conversion tables to draw any conclusions about file size or quality. MPEG-4 ASP and AVC are just too different for that. And of course the codec config details play an important role as well.

[LoRd_MuldeR]

Also note that if we talk about quantizers for Xvid, we usually refer to the "Constant Quantizer" (CQ) mode. But when we talk about quantizers for x264, we usually refer to the "Constant Rate Factor" (CRF) mode. The CRF mode works very different from CQ, resulting in an average quantizer instead of a truly constant one (in fact the specified quantizer will be converted into a "rate factor" internally). CRF is much advanced and gets much closer to the idea of a "Constant Quality" mode than CQ ever could - it's still not "perfect" constant quality though. There is no pendant to CRF in Xvid. So the quantizers you set for x264's CRF mode are not comparable to the quantizers for Xvid's CQ mode at all. And there's no conversion table for that...

[SacredCultivator]

Thanks for the explanations on those, but given so... If I were doing dual encodes, and I want them to be 'similar' even though you mention it's hard to compare... is there a CQ fro XviD that is fairly equal to H264's 19 / 21 (22-25)?

[LoRd_MuldeR]

Thanks for the explanations on those, but given so... If I were doing dual encodes, and I want them to be 'similar' even though you mention it's hard to compare... is there a CQ fro XviD that is fairly equal to H264's 19 / 21 (22-25)?

Not in general. Maybe for a certain type of source. And even then only as long as you don't change any other settings.
And note that the meaning of x264's CRF values change when you change other settings. So you must do it like that:

1. Do an Xvid encode, let's say at CQ=4
2. Find the CRF value for x264 that gives similar subjective quality (using your preferred x264 settings)

This of course requires several encodes and a visual comparison.
Also x264 and Xvid produce different kinds of artifacts, which doesn't make the comparison easy.

The good thing: Usually once you have found the CRF value that makes you happy, you can stick with it!

[SacredCultivator]

Alrights, yeah I see what you mean, shall have to do a lot of encoding ot compare then, and again, thanks.

[Brother John]

Do some tests and find out for yourself. Your eyes are the deciding factor.