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29th June 2010, 12:36 | #1 | Link |
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Still severe bitrate loss with aq-mode 2
Okay, I'm still experiencing severe bitrate loss with aq-mode 2.
Yesterday I re-encoded Blade Runner -- The Final Cut again (not having made the same mistake of throwing away the old encode first). Old Encode: CRF 16, aq=1 = 16.2G New Encode: CRF 14, aq=2 = 11.5G As you can tell from the MediaInfo below, for the New Encode I lowered CRF by 2 (which ought to add ~25% filesize/bitrate). And still, the bitrate under aq=2 drops drastically below that of the Old Encode (by ca. 30%). So, simple question, what's causing this? Is the newer x264 that much more efficient? I just can't explain this huge bitrate loss (overall nearly 6 Mbps). ----- Old Encode: Code:
Video Bit rate : 19.7 Mbps Maximum bit rate : 35.0 Mbps Stream size : 16.2 GiB (71%) Writing library : x264 core 66 r1093M 1df50b9 Encoding settings : cabac=1 / ref=3 / deblock=1:0:0 / analyse=0x3:0x133 / me=tesa / subme=9 / psy_rd=1.0:0.0 / mixed_ref=1 / me_range=16 / chroma_me=1 / trellis=1 / 8x8dct=1 / cqm=0 / deadzone=21,11 / chroma_qp_offset=-2 / threads=3 / nr=0 / decimate=1 / mbaff=0 / bframes=3 / b_pyramid=1 / b_adapt=2 / b_bias=0 / direct=3 / wpredb=1 / keyint=250 / keyint_min=25 / scenecut=40(pre) / rc=crf / crf=16.0 / qcomp=0.60 / qpmin=10 / qpmax=51 / qpstep=4 / vbv_maxrate=35000 / vbv_bufsize=35000 / ip_ratio=1.40 / pb_ratio=1.30 / aq=1:1.00 Code:
Video Bit rate : 14.0 Mbps Maximum bit rate : 40.0 Mbps Stream size : 11.5 GiB (64%) Writing library : x264 core 98 r1649 c54c47d Encoding settings : cabac=1 / ref=4 / deblock=1:0:0 / analyse=0x3:0x133 / me=tesa / subme=10 / psy=1 / psy_rd=1.00:0.00 / mixed_ref=1 / me_range=32 / chroma_me=1 / trellis=2 / 8x8dct=1 / cqm=0 / deadzone=21,11 / fast_pskip=0 / chroma_qp_offset=-2 / threads=6 / sliced_threads=0 / nr=0 / decimate=1 / interlaced=0 / constrained_intra=0 / bframes=8 / b_pyramid=2 / b_adapt=2 / b_bias=0 / direct=3 / weightb=1 / weightp=2 / keyint=250 / keyint_min=25 / scenecut=40 / intra_refresh=0 / rc_lookahead=60 / rc=crf / mbtree=1 / crf=14.0 / qcomp=0.60 / qpmin=10 / qpmax=51 / qpstep=4 / vbv_maxrate=60000 / vbv_bufsize=70000 / crf_max=0.0 / ip_ratio=1.40 / aq=2:1.00 / nal_hrd=non
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29th June 2010, 13:06 | #3 | Link |
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Asarian, as you are using CRF mode, so you cannot make any assumption about the output size, except that a lower CRF value yields better quality (at the cost of bigger file) and a higher CRF value yields a smaller file (at the cost of reduced quality). Moreover you cannot expect that the same CRF value still gives the same file size, after you changed other options, such as AQ mode! CRF isn't an exact "constant quality" mode. Such mode doesn't exist! The same CRF value will give roughly the same quality for different source (of the same nature), as long as you don't change any other options. But that's it! It's also wrong to believe that using "slower" settings will necessarily result in a smaller file at the same CRF value. Actually the opposite might happen.
If you want to know if AQ mode 1 or AQ mode 2 works better for you, you must visually(!) compare them in 2-Pass mode. And if you do a visual comparison, always make sure you compare files of the same size. Then, after you have decided between AQ mode 1 or AQ mode 2, you can start using your preferred AQ mode in CRF mode. The final step is re-adjusting the CRF value to your needs...
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29th June 2010, 13:23 | #4 | Link | |
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I understand what you're saying. I'm just surprised that even lowering CRF by two, for aq-mode 2, still yields a file which is so much smaller. For another movie (Evangelion 1.11) I had tested and calculated that I needed to go from CRF 16 to 14.2, in order to compensate for the bitrate loss under aq=2. Seems for this Blade Runner Blu-Ray I need to lower CRF even significantly further. So, predicting a 'general-compensation factor' for aq=2 proves more difficult than I thought. For the record: I'm not judging aq=2 (yet). At this stage I'm just baffled why it appears to be needing/wanting to use so much less bitrate.
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29th June 2010, 13:40 | #6 | Link | |
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Quote:
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29th June 2010, 14:00 | #7 | Link | |
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I don't really target for a specific filesize/bitrate. The only time when I'm concerned about it, is times like these, when going to aq-mode 2 makes the output file so much smaller, even at lower CRF. If someone were to come out and say: "Don't worry, aq-2 is supposed to use less bitrate!", then I'd be happy too. Could simply be that auto-variance truly is that much more efficient; but I'm not knowledgeable enough in these matters to convince myself of that to the point of not worrying about it. :P
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29th June 2010, 14:11 | #8 | Link |
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In lossy video compression there's no 'number' that works better than your eye. Look closely with your eyes and decide if the bitrate is enough to obtain your standard of 'good quality'. If not, raise the bitrate/change settings/etc until you're satisfied.
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29th June 2010, 14:21 | #9 | Link |
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Unlike the Evangelion 1.11 disc, with (as you know) its very specific gradient/dither issues, here aq=2 actually, visually at least, looks better. ;) Dunno how that's possible, but here aq=2 definitely seems to produce superior visual quality.
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29th June 2010, 14:23 | #10 | Link | ||
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But it's important to understand that CRF mode never was supposed the give the same filesize/bitrate for the same CRF value when other influential options are changed. Or in other words: After you switched form AQ1 to AQ2, your old CRF value became meaningless. Hence you might have to re-adjust your CRF value. Quote:
If you want to draw useful conclusions about compression efficiency, you need either: (a) Two files that have identical quality (hard to produce, because "identical quality" is difficult to find) (b) Two files that have identical size/bitrate (easy to produce, thanks to 2-Pass mode) In case (b) you will either notice that one file has significant better/worse quality than the other one (in this case it's quite clear which setting compressed more/less efficient) or you will notice that you cannot clearly tell which of the two files looks better/worse. In the latter case the conclusion would be that the impact on efficiency of the settings under test is negligible.
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29th June 2010, 15:52 | #12 | Link |
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I didn't notice that there are more than 500 x264 revisions between the encodes. MB-Tree would be a good candidate. Lots of people complained about much smaller filesizes at the same RF when MB-Tree was added.
edit: Anyway, I still agree with everything Lord Mulder said in this thread. |
29th June 2010, 17:43 | #13 | Link |
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Seriously... If you raise all those settings and update x264 by 500 revisions, of course you're going to get a smaller filesize (or rather, better compression). A year and a half of x264 updates is definitely going to make a difference. On top of that, crf has been adjusted at least a few times over that period. Try again using the same x264 revision (and same settings except for AQ-mode) for both encodes and tell us if you still get the same results.
Last edited by Soichiro; 29th June 2010 at 17:47. |
29th June 2010, 18:11 | #14 | Link | |
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And there are also some important differences between the settings you have used: Code:
Old: New: ref=3 ref=4 trellis=1 trellis=2 bframes=3 bframes=8 b_pyramid=1 b_pyramid=2 |
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29th June 2010, 18:34 | #15 | Link |
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Also the newer version has weighted p-frames.
Other settings which were changed that affects a comparisona are (albeit very slightly) p-skip, subme, and the me-range chosen. A real comparison determining the difference between aq-modes can only be done with the same version, same settings except having one set to 2 and the other set to 1 If you want to compare just the versions, feature independent, you'd have to disable the new features (mb-tree, weighted p-frames etc), and use the same settings. Its not the filesize that matters, its what it actually looks like if the new one looks better (even with the higher settings), its irrelevant whether its smaller or not ! Changing ME from TESA to UMH may make encoding a little less slow |
29th June 2010, 19:08 | #16 | Link |
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Okay, I'm convinced. :) Indeed, too much settings have changed since the previous encode. I should do a new re-encode, with the same settings, only with aq=1 this time.
Thank you all for your constructive ideas and input!
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aq-mode, bitrate, crf, x264 |
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