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12th February 2004, 15:22 | #1 | Link |
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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. |
12th February 2004, 16:06 | #2 | Link |
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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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13th February 2004, 16:54 | #6 | Link |
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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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13th February 2004, 20:57 | #7 | Link |
Ahhh....Nuts!!
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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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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... |
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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14th February 2004, 08:26 | #10 | Link | |
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14th February 2004, 08:54 | #11 | Link | |
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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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14th February 2004, 09:02 | #12 | Link | |
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14th February 2004, 12:54 | #17 | Link | |
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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! Regards Koepi
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