Qpel and Multipass

[PapaBobDole]

This is a relatively straightforward question, but I was prompted to ask after reading in another thread that settings should be duplicated across all passes. I am encoding using divx 5.1.1, 3 pass, first 2 at standard and the last at slowest. All three passes use B frames, GMC, PSY, etc., however Qpel is only enabled for the first two as you cannot enable it under the slowest configuration.

My question is will this adversely affect the quality of my encodes having it enabled for the first two and not the last? Should I disable it for all passes? Please keep in mind that my goal in this process is strictly quality, I do not care about speed. (This system was built for this purpose and is quite fast) The reason I use three passes is that it is moderately faster than two at slowest and the results are identicle as far as my eyes can tell. Also, I'm using Gknot for this.

[CiNcH]

Leave Quarter Pixel disabled for all three passes as it would only screw up the encoder's rate-control strategy (and therefore hurt image quality) if you enabled it for the first two "Standard" passes. Most parameters should be kept consistent between passes in order to be able to benefit from a multipass encoding.

[bond]

afaik some options can be disabled sometimes to save time, without hurting quality. i dunno if qpel is among them. you will have to use search to find out the details

btw for me qpel is a must, if you compare clips encoded with and without qpel you will find out that it brings _far_ more sharpness without hurting compressibility much

[CiNcH]

afaik some options can be disabled sometimes to save time, without hurting quality. i dunno if qpel is among them.

It is nonsense to use QPel in the first two passes here since the final output is being encoded with HPel accurate ME anyway. So there's no quality gain. In this case you can save time by disabling QPel.

Actually it should hurt quality. Enabling QPel for the first two passes only and suddenly switching it off screws up rate-control strategy because QPel accurate ME requires more bits on average. So the recommended bitrate distribution by the multipass log generated in the first two passes gets screwd up a little bit.

[Cyberman]

I would think that the algorithm at slow(est) already took over the work QPel would do - why else isn´t it useable?

In any case, I don´t think it´s good to change any settings during passes, except in the last pass(es), I usually change the bitrate to see how low it can get without (noticeably) losing quality.

[bond]

Originally posted by CiNcH
It is nonsense to use QPel in the first two passes here since the final output is being encoded with HPel accurate ME anyway. So there's no quality gainyep, if you want to use qpel (and i would use it) you have to turn it on in the last pass in anyway...

Originally posted by Cyberman
I would think that the algorithm at slow(est) already took over the work QPel would do - why else isn´t it useable?could be possible, but i doubt it
my guess is more that divx5 is simply not a good codec, for example with xvid you can enable qpel also with the slowest me settings, so...

In any case, I don´t think it´s good to change any settings during passes, except in the last pass(es)ok i now searched for the "Speed Tip" the guys from dxn gave (which means you should be absolutely save to do it this way):
Do all but your last pass in Standard mode with the MV file enabled. Then, disable the MV file (this is imporant) and switch to Slow or Slowest mode for your final pass. If you use Psycho-vis then you can also disable it until your second-last pass as long as you disable the MV file at this point (e.g. if you're doing 4 passes, turn on PV and turn off MV in pass 3)which also means, if you want to use qpel, that you will have to use it in all passes

[CiNcH]

I would think that the algorithm at slow(est) already took over the work QPel would do - why else isn´t it useable?

No, in Slow / Slowest Performance/Quality modes half pixel accurate ME is being performed. Hopefully DXN will implement a "QPel-aware" RDA in DivX Pro 5.2.

[DigitAl56K]

Hey guys :)

You should leave QPel disabled for all passes if you don't want to use it in the final pass.

Here's why.

If you look in the multipass log file ("c:\divx.log" by default) you'll find that for each frame we track various statistics, one of which is motion complexity.

We use these statistics in order to refine our rate control strategy for each successive pass - for example where the motion and texture complexities increase we know that we're probably going to need more bits for that frame.

The motion complexity statistic is based around the proportion of bits that go into representing the motion for each frame. If you change from half-pel to quarter-pel motion accuracy, you can significantly change the number of bits required to store the motion vectors for each frame. Not only this, but the best matching block during a half-pel search may be in a completely different location than the best matching block using a quarter-pel search.

Because searching at quarter-pel resolution takes longer than half-pel searches, you will also increase your encoding time if you use qpel for any pass.

CiNcH is correct in that we do not currently support QPel motion estimation under the rate-distortion enabled modes (Slow and Slowest under 5.1 & 5.1.1).

Hope this helps,
-Al

[PapaBobDole]

Thanks guys, that helped out.

[SeeMoreDigital]

Message to DXN staff/forum members!

Once again, this thread provides another clear example why your GUI needs changing!

It's a total waste of everybody's time allowing the GUI create settings that don't work....

For example if Qpel is not enabled under slow and slowest modes, then disable it in the GUI!

Cheers

[DigitAl56K]

It is disabled in the GUI, try turning on QPel then selecting Slow or Slowest mode ;)

[TheSeeker]

Also, as I understand it, what qpel essentially does is make the stuff in the background of the frame sharper. Which in some cases is not representative of how the film maker wanted his film to look. a lot of the time the stuff in the fore of the frame is what the director wants to stand out and the background of the frame should be slightly blurred.

[Soulhunter]

Originally posted by TheSeeker
...not representative of how the film maker wanted his film to look.He also wants to have it on celluloid, and not compressed to CD-R's... :D

Seriously, in this case "sharper" means only preserving more details !!!


Bye

[TheSeeker]

Soulhunter. You know your right. But for me, the very minimal gain in quality that you get with qpel enabled is most definately not worth the extra 4-6 hours of encoding time. I can do an encode without qpel in about 3 hours. with it it takes about 8 hours.