XviD configuration: GMC vs QPel, fixed avg bitrate, and more

[merlinhd]

Hi,

I'm afraid I'm very new to this and I'm having questions that I can't get answered anywhere else, including searching this forum.

First issue is very simple. What is the meaning of "Avg bitrate (kbps)" in the "XviD Status" window in the first pass? Is it something like "ideal bitrate"? Because it's happened to me a couple of times already that after the first pass this bitrate was some value around 1200 kbps or so, while most other times it's a much higher value.

Since I don't really understand what this value means in the first pass, I'm afraid I might be wasting space by using a higher bitrate in the second. Is there any type of relationship to take into account between the avg bitrate given by XviD in the first pass and that specified in the second?

Another issue is GMC. I read somewhere that it didn't really make sense to use GCM if VHQ wasn't set to 4. Is this true? Is VHQ related to any other configuration parameter? Now, in the forum, I've read that GMC actually performs better with slow motion movies. Is this right? Should I deactivate this option for action movies?

And just to have a better idea about how these things work... How do GMC and QPel compare in terms of compression quality and time? Is one option better than other, maybe for specific cases? Or is one option much slower than the other?

A last question is about 'Quatization type'. I'm not going to get in the whole business of custom tables, so I'd rather choose among what there is available: H.263 and MPEG. It's said everywhere that H.263 should be chosen for 1-CD rips and MPEG for 2-CD rips. Now, what does this mean? What is the average bitrate at which it is worth it to use MPEG? Shouldn't this also depend on the frame size (pixels)? Because something I don't like doing is resizing moving horizontally, I just resize them vertically to correct the aspect ratio. But this means more information to compress per frame, so that I don't have very clear what quatization type is more appropriate (trying with different movies just seems to give pretty much random results to me).

I know this is a long one, sorry. I just hope that answering these questions will not raise even more.

Regards,

Álvaro

[Blue_MiSfit]

please read crusty's xvid faq, and search about mpeg and h263 matricies, and compression ratios specifically the bits/(pixel*frame) ratio

I would suggest you use auto GK or Gordian Knot

[merlinhd]

Thanks for your reply.

I've read the FAQ you mentioned - I've read lots of stuff about XviD (can't understand most of it though) but somehow I had not reached to this one.

It explains more about GMC and QPel, but it doesn't really answer my questions. It doesn't say what the avg bitrate of the first pass means, neither whether there really is a relationship between QMC and VHQ - i.e., whether it's only worth it to use GMC if VHQ = 4.

It, yet again, says that MPEG should be used for higher bitrate encodings... How high does a bitrate need to be in order to be considered "high"?

Regards,

Álvaro

[Sharktooth]

I dont know if average bitrate in 1st pass is accurate unless you check the "Full quality 1st pass" checkbox.
In that case it will be the maximum achievable bitrate (unless you want to waste space doing a Q1 encoding) with your settings, coz first pass is encoded at quant 2 (+bframes if enabled).
GMC is usefull even with VHQ4 coz an "S" frame (a GMCed P frame) does not contain all the motion vectors data (only the GMC data) so the frame size is reduced significantly.

[Ark]

Bitrate on first-pass is the base for compressibility check for second-pass.

A rough compressibility test is done doing a normal first pass and then calculate how much lower than found value target bitrate is.

If i've a first-pass of 1000 average kbps, and my target bitrate is 750 avg kbps, then i've a "compressibility ratio" of 75% (which is not all that bad).

If target bitrate is higher then the codec is saturated for this encode, so it's better to increase resolution, or do less denoising, or use higher birate matrix.

[malkion]

I believe others before me sufficiently answered the vague question on average bitrate. If my memory serves me, it used to be recommended for bitrate < 900 use the H.263 matrix, if > 900 try the MPEG matrix.

XviD GMC is uniquely it's own standard I believe. I dont think decoders (except ffdshow) or stand alone players support it fully. Plus using GMC consumes a lot of CPU. Qpel is rather nice. But again, these "extras" are best verified using your eyes to test. Try encoding a small clip or sample before you encode the entire project. Any extra options you select will make encoding slower, of course.

Not sure if you have more specific questions after this. Good luck.

(edit; added p.s.)

p.s. As far as I know GMC is unrelated to VHQ. If you enjoy GMC, then try it with VHQ4. For that matter, a lot of people have different experience with VHQ 0-4, so in the end it's really up to you.

[Teegedeck]

Hm. Bitrates themselves mean nothing, it is mostly about compressiblity but also about the type of input. 900kbps are very little at full PAL resolution but, I believe, OK for CIF resolution (176x144). High-motion/low-motion, anime/life-footage etc. etc.

For example if you have this really lo-quality TV series with lots of noise you have two options: 1) denoise it heavily and or use either a matrix that filters out high frequencies (low bitrate matrices or h.263) or a general-purpose matrix at a high quantizer. 2) denoise only to some extent and encode it with a high-bitrate matrix at a low quantizer so you keep the noise but also the picture detail hidden in it. This means of course either using a high bitrate or downsizing the picture until considerably.

It really isn't about bitrates but about the 'look' you want to reach.

Anyway, I would not use h.263, because I don't like its look, but rather choose a reasonable resolution and go for MPEG or SixOfNine.

XviD GMC is uniquely it's own standard I believe. I dont think decoders (except ffdshow) or stand alone players support it fully.
No, GMC is not XviD's 'uniquely own standard', it is MPEG-4 ASP standard, and VLC for example fully supports it. It's unfortunate that there's still standalones around that cannot play it back.

[SeeMoreDigital]

Originally posted by Teegedeck
...No, GMC is not XviD's 'uniquely own standard', it is MPEG-4 ASP standard, and VLC for example fully supports it. It's unfortunate that there's still standalones around that cannot play it back. Agreed.

Nero's Recode2 GMC implementation uses 3warp-points. And not the useless 1warp-point DivX has lumbered us all with for use in stand-alone players, due to their "limited" certification process!


Cheers

[malkion]

That is good to hear. I'm waiting for Nero to get stand alone support (wasn't there talk of it already). Do you think this will help clear the way for better stand alone support for XviD as well?

[merlinhd]

Hi,

Thanks for all the replies, I'm starting to see things a little bit clearer now.

It's a pity that it's not possible to give more detailed instructions about the appropriate configuration of XviD to compress movies. According to Crusty's FAQ, there is no way of knowing in advance whether QPel will give good results or not - although people seem to like it -, and it seems to be the other way around with GMC.

Something that could be very helpful in this sense would be a forum in which to add reports of different XviD configurations. Someone could add a report only if he/she has compressed the same movie more than once with different parameters, and it would include:

[list=1]
The name of the movie.
A detailed description of the XviD configurations used.
A short conlusion referring to the quality of the results.
[/list=1]

This way of gathering information would help people, especially those with little experience with XviD - such as me -, to get a better idea of the effects of the different compression mechanisms. And it would keep itself updated, unlike many guides available in the Internet.

Just an idea. Well, thanks again.

Regards,

Álvaro

[SeeMoreDigital]

Originally posted by malkion
That is good to hear. I'm waiting for Nero to get stand alone support (wasn't there talk of it already). Do you think this will help clear the way for better stand alone support for XviD as well? It's already been announced that Nero and Sigma are "working together".

However and most bizarrely, Sigma's brand new chip-set does not presently support 3warp-point GMC. In fact according to their latest FAQ (http://www.sigmadesigns.com/products/em8620Lseries_faq.htm), it does not support GMC at all :eek:

Now it may be wishful thinking on my behalf but hopefully this means Sigma are distancing themselves away from "all things DivX"


Cheers

[SeeMoreDigital]

merlinhd,

I understand your frustration. You want to generate the best looking encodes you can... and there's nothing wrong with this.

Unfortunately "you can't please all of the people all of the time" and when it comes to implementations such as GMC and Qpel, some people like the effects generated, some don't.... And some people can't use them (even if the wanted too) because they might not spin in their stand-alone players.

For me... I like to generate encodes at 1:1 (ie: using 720x480/576 pixels). Encodes with small pixel frame sizes are no good for me because when viewing on a big screen I find it more relaxing to view images comprising of lots of small pixels rather not so many larger pixels, even if the bitrate is lower than suggested during a compressibility test!


Cheers

[Teegedeck]

Originally posted by merlinhd

Something that could be very helpful in this sense would be a forum in which to add reports of different XviD configurations. Someone could add a report only if he/she has compressed the same movie more than once with different parameters, and it would include:

[list=1]
The name of the movie.
A detailed description of the XviD configurations used.
A short conlusion referring to the quality of the results.
[/list=1]

You would get as many different recommendations as there are people who've tried to encode the movie. Experience tells me that.

But seriously, there is not much sense in that. Apart from QPel there is only AQ as a setting people strongly disagree about. Find out whether you generally like the look of QPel, and if you decide you like QPel only deactivate QPel and AQ in those cases where you stumble upon a single source where you strongly feel that QPel or AQ didn't do it any good. Simple as that. No need for a movie-database.

[merlinhd]

I see your point. But the idea of the movie database wouldn't be to have absolute recommendations on what configurations are best for a movie. Rather, to compare specific configurations and decide which one is better; and as there is more information available, typically a pattern emerges. This is a common technique used to gather information on subjects about which it's difficult to give absolute answers.

You have also said something very interesting: that people tend to disagree on Qpel and AQ. Does that mean that there is some common opinion about other options such as GMC, VHQ and Trellis? If that's the case, what options are typically recommended?

Thanks.

[SeeMoreDigital]

Originally posted by merlinhd
...You have also said something very interesting: that people tend to disagree on Qpel and AQ. Does that mean that there is some common opinion about other options such as GMC, VHQ and Trellis? If that's the case, what options are typically recommended?

Thanks. I all honesty you have to try these things for yourself... there are no "holy grail" settings, because we all see different things and have our own expectations!


Cheers

[malkion]

I dont think there is a big advantage to having people post their recommended settings. I would certainly try the recommends if I was new, but ultimately I would arrive at my own conclusions on what I like for settings. I have encoded a short chapter from a DVD with all the various settings and this is how I have arrived on my own preferences. My favorite settings will yield bigger file sizes at very basic settings. Others will enjoy better compression using all the options. There simply is no one way to help people recompress video. The Crusty FAQ is about as honest as they come. If it says there is no way to tell, or if it says to try it for yourself, that's just the bottom line. What can I say?! It will certainly take time as you progress. Your later encodes will look much better to you, and you wonder if you want to go back and recompress your earlier efforts.

And that is just for XviD. Throw in other variables such as using AviSynth and the plethora of video filters available, you may begin to see just how many decisions the encoders has to deal with.