AutoGK - What does quality mean ?

[dbl]

Ok, this is my first post, but I swear to god I did :search: and I did :readfaq: and I certainly did :readguid: .

However, I've not found anything approaching an answer to my particular question: What does the quality setting in AutoGK mean?

I realise that overall it controls the quality and therefore the resulting filesize of an encode. But I wondered exactly what encoding paramaters are used for a particular setting.

Presumably, quantizers change, and maybe profiles change, and maybe resolutions change??

Is there a rough guide to what the parameters that change are and there (approximate values)?

Hope that's clear, and I really hope this hasn't been asked before.

Thanks,
<dbl>

[jggimi]

Hello, and welcome to the forum!

"Quality-based" encoding is a fixed-quantizer, variable bitrate encoding, done in a single pass. Size cannot be predetermined, but the output has a constant quality frame by frame.

For much more on encoding options such as this -- even if you're using XviD -- see the original DivX 5 user guide -- the clarity of the explanations is superb. Since they've moved to DivX 6, the link to this old guide is no longer obvious. Try this link:

http://support.divx.com/cgi-bin/divx.cfg/php/enduser/std_adp.php?p_faqid=444&p_created=1081444519&p_sid=nL1sZqZh&p_lva=&p_sp=cF9zcmNoPTEmcF9zb3J0X2J5PSZwX2dyaWRzb3J0PSZwX3Jvd19jbnQ9Mjk0JnBfcHJvZHM9MCZwX2NhdHM9JnBfcHY9JnBfY3Y9JnBfc2VhcmNoX3R5cGU9YW5zd2Vycy5zZWFyY2hfbmwmcF9wYWdlPTEmcF9zZWFyY2hfdGV4dD1kaXZ4IHVzZXIgZ3VpZGU*&p_li=&p_topview=1

[dbl]

Hmm, that doesn't quite answer my question, but I dare say that's because I didn't phrase it very well in the first place :o .

I'll try it a different way:
Let's say I do two xvid encodes with autogk, one at 75% quality and one at 50% quality. What parameters change in vdub/xvid accordingly, and how are these parameters derived?
From what you say it sounds like the quantizer would be different in a 75% encode than a 50% enode? But is that all that changes?



(Oh, that divx guide is fantastic btw, thanks).

[jggimi]

But is that all that changes?
Yes. The scale reaches from 100%, Q1 to 0%, Q31. So at 50% its using Q16, and at 75%, Q8.

[dbl]

Ahh! Who'd have thought it would be so beautifully simple :-).

Thanks jggimi.

Does lead me to another question though, if that's all that changes, what's the compressibility test for? Is that just to choose the target resolution perhaps?

[BigDid]

...what's the compressibility test for? Is that just to choose the target resolution perhaps?
Resolution and matrix and resizer, retreave the infos in the logs; I don't use SAP compatibility except MTK in rare occasions, so for AGK-XVID encoding 2 passes:

-Best case-
Resolution around 640 width, with quality around or above 70%
Sharp matrix
Lanczos resizer (very sharp, good at keeping details)
I do get very good final quality in this case but I often have to raise the size to stick with these parameters (exemple:go from 900Mb to 1100Mb size for a movie longer than 2 hours)

When not enough bitrate (or size, both are related), AGK will:
- lower the resolution
- change the resizer (softer)
- under 608 width (for 16/9) will change to soft matrix then
- change to soft resizer again (not sure about the step of that one)
- lower the resolution again and again

Only Len0x can confirm the exacts stepping, this is from my memory :)

Did

Edited: Related to AGK-Xvid encoding, 2 passes only ( jggimi's answer below is best suited for general explanation, mine is just recalling my experiences and opinions :D )

[jggimi]

BigDid replied as I was typing this. My reply covered both "simplicity" of quality based encoding, as well as compressibility testing.

----
Its not quite so beautifully simple. In DivX, for example, you can choose a percentage that gives you a non-integer Q. If I recall correctly -- and I may not -- the codec will average frames using the two neighboring quantizers, but ... it's still considered a constant quantizer encoding.

----

A compressibility test is only used for multipass encoding, when a fixed filesize is expected. Its automatic with AGK for filesize based encodings, and optional -- but highly recommended -- with GK.

The test takes a sampling of frame sequences from the entire content, and does a first pass -- this determines the maximum bitrate possible at that particular resolution. AGK will both pick a resolution and make adjustments to codec settings in order to reach a compression level it considers acceptable.

[nixo]

----
Its not quite so beautifully simple. In DivX, for example, you can choose a percentage that gives you a non-integer Q. If I recall correctly -- and I may not -- the codec will average frames using the two neighboring quantizers, but ... it's still considered a constant quantizer encoding.
----


Isn't that the same in XviD? I did a test a while ago with AutoGK to find the corresponding quantizer for each percentage, and it came out like this:

75% = Quant 2.67
74% = Quant 2.70
73% = Quant 2.74
72% = Quant 2.78
71% = Quant 2.82
70% = Quant 2.86
69% = Quant 2.90
68% = Quant 2.94
67% = Quant 2.99
66% = Quant 3.03
65% = Quant 3.08
64% = Quant 3.12
63% = Quant 3.17
62% = Quant 3.23
61% = Quant 3.28
60% = Quant 3.33