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Old 12th February 2004, 15:22   #1  |  Link
atreya2011
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Quantizers

Another novice (or newbie) question from me. I have this simple question and, thanks in advance, to the kind people who would answer it. Here goes:

1. What's a quantiser?
2. How does that number actually determine the compression of I frame,
P-frame, B-frame?
I mean what kind of mathematical equation is used to determine the filesize of the I frame , P frame, B frame?
(lower quantiser=good quality, higher quantiser=poor quality?)

Sorry if this is a repetition, but time constraints hamper me from searching this forum and using its full potential. If anyone can help me with references to other threads, thanks in advance, for that too.


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Old 12th February 2004, 16:06   #2  |  Link
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http://www.cmlab.csie.ntu.edu.tw/cml...eg/encoder.htm
http://www.ece.purdue.edu/~ace/jpeg-tut/jpgquan1.html
http://www.mpeg.org/MPEG/MSSG/tm5/Ch7/Ch7.html
http://www.acm.org/crossroads/xrds6-...imgcoding.html

...and so on
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Old 13th February 2004, 16:06   #3  |  Link
atreya2011
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Cool Thanx a lot, will look into those...
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Old 13th February 2004, 16:34   #4  |  Link
atreya2011
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I seem to get it, I think, although I am more confused than before now. A more simpler explanation would help... Thanks to anyone who replies
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Old 13th February 2004, 16:45   #5  |  Link
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*VERY* simple explanation: the image is converted to detail signals (DCT), and then those signals are divided (coefficient cutting) to make it smaller. The more signal you cut off, the more detail you lose and more artifacts (blocks, ringing aka mosquitoes) you get. The quantizer is the detail removal factor (DivX3/Nandub thus calls quantizers DRF). The higher the quant, the more detail gets cut off, the lower quality.
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Old 13th February 2004, 16:54   #6  |  Link
atreya2011
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That makes sense now. So DRF in DivX 3 is Quantizer in XviD. Thanks for the info. By the way, mfToon is amazing, all the animes I have ripped so far look damn good. Although the encoding process is going at 4 FPS!!! damn slow man...
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Old 13th February 2004, 20:57   #7  |  Link
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http://www.cmlab.csie.ntu.edu.tw/cm...peg/encoder.htm
http://www.ece.purdue.edu/~ace/jpeg-tut/jpgquan1.html
http://www.mpeg.org/MPEG/MSSG/tm5/Ch7/Ch7.html
http://www.acm.org/crossroads/xrds6...aimgcoding.html

Damn, I think I'll add those to my FAQ. They're good.

Added c3a to my faq : what is a Quantization Matrix?
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Old 14th February 2004, 07:45   #8  |  Link
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In Advanced Options....
Min I frame Quantizer=1
Max I frame Quantizer=31
Min P frame Quantizer=1
Max P frame Quantizer=31

31 gives the worst quality possible, so why let it be 31 when the codec is gonna choose only max quantiser=6 anyways?

Thats what I have noticed when I am ripping...
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Old 14th February 2004, 08:26   #9  |  Link
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It's quite simple really. On slow or static screens the video can be compressed with a quantizer of 31 for some frames with very little real quality loss. But only on relatively long sceens with almost no difference between frames. Like perhaps watching paint dry.
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Old 14th February 2004, 08:26   #10  |  Link
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Quote:
Originally posted by atreya2011
In Advanced Options....
Min I frame Quantizer=1
Max I frame Quantizer=31
Min P frame Quantizer=1
Max P frame Quantizer=31

31 gives the worst quality possible, so why let it be 31 when the codec is gonna choose only max quantiser=6 anyways?

Thats what I have noticed when I am ripping...
Well for one thing, imagine if you have a few seconds of solid black in your video. If it uses quant 31, you lose nothing but gain a whole lot of space. I've seen the quant go all the way up to 15 and never noticed a single scene in that video where it looked like it was quantized too high. It's best just to leave the max quantizer alone.
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Old 14th February 2004, 08:54   #11  |  Link
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Quote:
Originally posted by Prettz
Well for one thing, imagine if you have a few seconds of solid black in your video. If it uses quant 31, you lose nothing but gain a whole lot of space. I've seen the quant go all the way up to 15 and never noticed a single scene in that video where it looked like it was quantized too high. It's best just to leave the max quantizer alone.
Nah, you wouldn't gain anything - black is black, not-coded blocks are not-coded.

The quants are not restricted *because* two-pass will use them wisely. Yes, they can be restricted and it wouldn't change much (for exaple, 2-10 restriction) but it doesn't mean we should restrict them.
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Old 14th February 2004, 09:02   #12  |  Link
atreya2011
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Quote:
Originally posted by sysKin
Nah, you wouldn't gain anything - black is black, not-coded blocks are not-coded.

The quants are not restricted *because* two-pass will use them wisely. Yes, they can be restricted and it wouldn't change much (for exaple, 2-10 restriction) but it doesn't mean we should restrict them.
Since it would'nt change much, why should'nt I restrict them. What do you mean by "two pass will choose them wisely"...
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Old 14th February 2004, 10:48   #13  |  Link
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If a frame is low complexity it will have a small size even at quant 2. Also, with watching paint dry it might be recommendable to watch it dry at quant 2 because of the above reason .
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Old 14th February 2004, 12:00   #14  |  Link
atreya2011
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So its ok to restrict them??? I can use

Min I frame quantizer=2
Max I frame quantizer=5
Min P frame quantizer=2
Max P frame quantizer=5

without any dire consequences I hope...

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Old 14th February 2004, 12:06   #15  |  Link
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The question is, WHY do you want it. It won't have "dire" consequences (though limiting to quant 5 may give you quite an oversize) but rate control performs better without limitations. Why cripple it?
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Old 14th February 2004, 12:27   #16  |  Link
atreya2011
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So... by limiting the range to 2-5, I am actually "choking" the rate control?? I have to check if I get an oversized file if I employ it
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Old 14th February 2004, 12:54   #17  |  Link
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Quote:
Originally posted by atreya2011
So... by limiting the range to 2-5, I am actually "choking" the rate control?? I have to check if I get an oversized file if I employ it
Yeah. There is only one reason why you would want to restrict quants, and that's when RC goes crazy, but that hasn't happened since dev-api-3, so you're better off leaving it unrestricted.
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Old 14th February 2004, 13:00   #18  |  Link
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But there is no harm in experimenting right?
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Old 14th February 2004, 13:06   #19  |  Link
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One more thing I forgot to ask. How does the limiting factor actually choke the rate control function?

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Old 14th February 2004, 13:21   #20  |  Link
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atreya:

such things aren't meant to be asked (well, it sounds worse as I mean it now) - try for yourself and you'll very fast understand what happens (especially when you use the status window and look a bit into the internals and look at the quant distribution - and then at virtualdub's estimation in it's status window).

XviD is an educational project. So do what this implies - educate yourself with it!



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