H.264 Profiles and Levels discussion

[LoRd_MuldeR]

Originally Posted by fvisagie

TDue to the subtlety you allude to in the first quote, --level is needed to ensure --ref is set correctly. If the command line omits --level, --ref defaults to 16. I just retested that to confirm.

Nope, it defaults to three reference frames:


You probably have been using "very slow" or "placebo" Preset, which crank up the number of reference frames

[fvisagie]

Originally Posted by sneaker_ger

Wrong. Two-pass encoding without --vbv-XXX is variable, not constant bit rate.

In the strict sense of the word what you say is correct, of course. However, for the purposes of that suggestion (achieving a bitrate predictable enough for level estimation) I believe two-pass encoding to be sufficient. I did use the word "roughly", BTW . Certainly, in my experience, I have not seen any bitrate swings causing problems to constrained players with correctly level-constrained two-pass encodes.

[fvisagie]

Originally Posted by LoRd_MuldeR

You probably have been using "very slow" or "placebo" Preset, which crank up the number of reference frames

Well spotted, I neglected to mention that, thanks.

This brings us to the more general point that --preset preselects --ref values independently of encoding level:

• - ultrafast: --ref 1
• - superfast: --ref 1
• - veryfast: --ref 1
• - faster: --ref 2
• - fast: --ref 2
• - medium: Default settings apply.
• - slow: --ref 5
• - slower: --ref 8
• - veryslow: --ref 16
• - placebo: --ref 16

- unless they are explicitly managed via --level.

[detmek]

Yep. Here what presets, tunings and profiles change compared to defaults:

--profile <string> Force the limits of an H.264 profile
Overrides all settings.
- baseline:
--no-8x8dct --bframes 0 --no-cabac
--cqm flat --weightp 0
No interlaced.
No lossless.
- main:
--no-8x8dct --cqm flat
No lossless.
- high:
No lossless.
- high10:
No lossless.
Support for bit depth 8-10.
- high422:
No lossless.
Support for bit depth 8-10.
Support for 4:2:0/4:2:2 chroma subsampling.
- high444:
Support for bit depth 8-10.
Support for 4:2:0/4:2:2/4:4:4 chroma subsampling.
--preset <string> Use a preset to select encoding settings [medium]
Overridden by user settings.
- ultrafast:
--no-8x8dct --aq-mode 0 --b-adapt 0
--bframes 0 --no-cabac --no-deblock
--no-mbtree --me dia --no-mixed-refs
--partitions none --rc-lookahead 0 --ref 1
--scenecut 0 --subme 0 --trellis 0
--no-weightb --weightp 0
- superfast:
--no-mbtree --me dia --no-mixed-refs
--partitions i8x8,i4x4 --rc-lookahead 0
--ref 1 --subme 1 --trellis 0 --weightp 1
- veryfast:
--no-mixed-refs --rc-lookahead 10
--ref 1 --subme 2 --trellis 0 --weightp 1
- faster:
--no-mixed-refs --rc-lookahead 20
--ref 2 --subme 4 --weightp 1
- fast:
--rc-lookahead 30 --ref 2 --subme 6
--weightp 1
- medium:
Default settings apply.
- slow:
--b-adapt 2 --direct auto --me umh
--rc-lookahead 50 --ref 5 --subme 8
- slower:
--b-adapt 2 --direct auto --me umh
--partitions all --rc-lookahead 60
--ref 8 --subme 9 --trellis 2
- veryslow:
--b-adapt 2 --bframes 8 --direct auto
--me umh --merange 24 --partitions all
--ref 16 --subme 10 --trellis 2
--rc-lookahead 60
- placebo:
--bframes 16 --b-adapt 2 --direct auto
--slow-firstpass --no-fast-pskip
--me tesa --merange 24 --partitions all
--rc-lookahead 60 --ref 16 --subme 11
--trellis 2
--tune <string> Tune the settings for a particular type of source
or situation
Overridden by user settings.
Multiple tunings are separated by commas.
Only one psy tuning can be used at a time.
- film (psy tuning):
--deblock -1:-1 --psy-rd <unset>:0.15
- animation (psy tuning):
--bframes {+2} --deblock 1:1
--psy-rd 0.4:<unset> --aq-strength 0.6
--ref {Double if >1 else 1}
- grain (psy tuning):
--aq-strength 0.5 --no-dct-decimate
--deadzone-inter 6 --deadzone-intra 6
--deblock -2:-2 --ipratio 1.1
--pbratio 1.1 --psy-rd <unset>:0.25
--qcomp 0.8
- stillimage (psy tuning):
--aq-strength 1.2 --deblock -3:-3
--psy-rd 2.0:0.7
- psnr (psy tuning):
--aq-mode 0 --no-psy
- ssim (psy tuning):
--aq-mode 2 --no-psy
- fastdecode:
--no-cabac --no-deblock --no-weightb
--weightp 0
- zerolatency:
--bframes 0 --force-cfr --no-mbtree
--sync-lookahead 0 --sliced-threads
--rc-lookahead 0

[r0lZ]

Someone knows if x264 supports level 5.2 ? (x264 doesn't complain if I type --level 5.2 in the command line, but I'm not sure it's an evidence that it has been really implemented.)

And if it's the case, what values should be used for the --vbv-bufsize and --vbv-maxrate arguments to force x264 to be compatible with level 5.2 ? (The table given by sneaker_ger in post #25 stops at level 5.1 and I have been unable to find the information on the internet.)

[fvisagie]

http://www.itu.int/rec/T-REC-H.264/en (and the direct link to the latest publicly available version).

[detmek]

Originally Posted by r0lZ

Someone knows if x264 supports level 5.2 ? (x264 doesn't complain if I type --level 5.2 in the command line, but I'm not sure it's an evidence that it has been really implemented.)

And if it's the case, what values should be used for the --vbv-bufsize and --vbv-maxrate arguments to force x264 to be compatible with level 5.2 ? (The table given by sneaker_ger in post #25 stops at level 5.1 and I have been unable to find the information on the internet.)

Yes, x264 does support level 5.2. Here are specs. Your 1080p video must be over 120fps to be level 5.2.
You can use --vbv-maxrate 300000. --vbv-bufsize is your choice.
P.S. I already gave you a link to that Wikipedia page (DPB). Did you read that page?

[sneaker_ger]

Originally Posted by detmek

You can use --vbv-maxrate 300000. --vbv-bufsize is your choice.

Maximum --vbv-bufsize is 300000 as well for level 5.2. (See tables on pages 292 and 295 in the document linked by fvisagie)

[r0lZ]

Thanks!

Originally Posted by detmek

Your 1080p video must be over 120fps to be level 5.2.

Why? The table on that page shows the maximum property values, but there are no minimum values.

Originally Posted by detmek

Did you read that page?

Yes, but it lists the max bitrate and frame size, not the max buffer size. It is somewhat difficult to compute it with the formula that uses several concepts that I don't understand well. (For example, it seems that the macroblocks are 16x16 pixels, but I'm not sure it's always the case. And searching for each bit of information in the jungle of the h264 doc is not easy, when you are a noob like me! Each concept refers to another concept, and it seems that it's an endless quest.)

I'll use 300000, at least if it makes sense to use level 5.2 with 1080p@23.976fps. Can you confirm that? And can you confirm it also when the resolution is the double of 1080p (for Full-SBS or Full-T&B) ?

[detmek]

Originally Posted by sneaker_ger

Maximum --vbv-bufsize is 300000 as well for level 5.2. (See tables on pages 292 and 295 in the document linked by fvisagie)

Thanks. I'll try to read that info.

Originally Posted by r0lZ

Thanks!

Why? The table on that page shows the maximum property values, but there are no minimum values.

Yes, but it lists the max bitrate and frame size, not the max buffer size. It is somewhat difficult to compute it with the formula that uses several concepts that I don't understand well. (For example, it seems that the macroblocks are 16x16 pixels, but I'm not sure it's always the case. And searching for each bit of information in the jungle of the h264 doc is not easy, when you are a noob like me! Each concept refers to another concept, and it seems that it's an endless quest.)

I'll use 300000, at least if it makes sense to use level 5.2 with 1080p@23.976fps. Can you confirm that? And can you confirm it also when the resolution is the double of 1080p (for Full-SBS or Full-T&B) ?

x264 will try to flag your video with lowest possible level it can. If your 1080p video has 16 reference frames and less then 120,5fps it fits more to Level 5.1 so, x264 will flag it as Level 5.1.
Usually it does not make sense to use 16 reference frames with any video resolution because it take way too much time to compress and it offers almost none quality improvement but makes video incopatible with a lot of hardware players. Gain of using so much reference frames is less then 1% most of the time.
My advice, do some tests with different settings at the same bitrate and use SSIM to measure difference. You will be surprised how low gain is when you use settings from placebo or veryslow preset compared to slow or slower.

[r0lZ]

I see. That means that using level 5.2 is not recommended for 1080p@23.976fps, but theoretically not impossible, if you are crazy enough to push the limits to their maximums. Right?

BTW, what do you use for SSIM measurement?

[detmek]

x264 itself. Just add --tune ssim --ssim into command line and you will get ssim values in x264 log after encoding is done. Then compare values from two different encodes and you will get % of changes. Formula for comparing SSIM is:

((1-old ssim)/(1-new ssim)-1)*100

Or use this Excel file.

NB! Always use 2-pass and same bitrate.

[r0lZ]

OK, thanks.

[foxyshadis]

Originally Posted by r0lZ

I see. That means that using level 5.2 is not recommended for 1080p@23.976fps, but theoretically not impossible, if you are crazy enough to push the limits to their maximums. Right?

5.2 is exactly the same in terms of bitrate and frame-size as 5.1, it only exists to support full-speed 3K and 4K video (or 1080p at more than 120fps). 5.1 supports 4K @ 30, 5.2 supports 4K @ 60. Specifying 5.2 at 1080p is pointless unless you want all hardware players to reject your encodes.

[r0lZ]

I understand. Thanks. BTW, I did a placebo encode of a Full-SBS movie (3840x1080, 23.976 fps), without forcing any other option, and x264 has printed level 5.2. I suppose it's because the placebo preset uses a large number of ref frames. But that means that level 5.2 must be supported by my program, although I will not recommend to use it.

[sneaker_ger]

5.1 and 5.2 have the same DPB (for references) size. Technically your encode is neither 5.1 nor 5.2 because it is exceeded.

[r0lZ]

Originally Posted by sneaker_ger

Technically your encode is neither 5.1 nor 5.2 because it is exceeded.

Why? Afaik, level 5.0 is sufficient for the image size and frame rate of Full-SBS or Full-T&B (3840x1080 or 1920x2160, at 23.976 fps). Right?

[sneaker_ger]

DPB size is exceeded, you can't use 16 references with 3840x1080.