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    Old 21st February 2019, 23:16   #1481  |  Link
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    Quote:
    Originally Posted by benwaggoner View Post
    I worry that AV1's interframe CABAC dependencies will impair random access enough to make the practical maximum GOP duration a lot smaller. Some of that could probably be addressed via encoder tweaks, at the loss of a little efficiency.
    You don't need to worry - AV1 probability dependencies can only come from one of the reference frames, so it doesn't place any additional impairment on seekability.

    (also note that CABAC is a misnomer, as it's not binary. The spec doesn't give it an acronym, but dav1d uses MSAC).
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    Old 22nd February 2019, 03:01   #1482  |  Link
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    You don't need to worry - AV1 probability dependencies can only come from one of the reference frames, so it doesn't place any additional impairment on seekability.
    I think his concern that a decoder (or a stupid player, which is 99.9% of them) don't know this. A good container format (like mp4) can represent the reference structure in its atoms, and then a good decoder + good container + good encoder can do the right thing. But if any one of them fails, you'll have a worse seeking experience if you want to do frame-exact user experience. It's up to all devs to make sure that doesn't happen, and like I said, this is multi-factorial so it's easy to forget and screw up.
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    Old 22nd February 2019, 21:41   #1483  |  Link
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    Quote:
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    I think his concern that a decoder (or a stupid player, which is 99.9% of them) don't know this. A good container format (like mp4) can represent the reference structure in its atoms, and then a good decoder + good container + good encoder can do the right thing. But if any one of them fails, you'll have a worse seeking experience if you want to do frame-exact user experience. It's up to all devs to make sure that doesn't happen, and like I said, this is multi-factorial so it's easy to forget and screw up.
    Yeah, the goal is for the decoder to determine the minimum sequence of frames required to decode a particular frame. With a IbbbBbbbPbbbBbbbPbbbbBbbbI kind of structure, decoding the last "b" frame in an Open GOP should require just six frames (IPPBIb) in decode order typically. But that requires the reference list because sometimes a b could reference two B frames back and things like that. With multiple reference frames it's impossible to reliably know the hierarchy without knowing what each frame references.

    And there are patterns that can be spec-legal but that existing encoders might not do. And then better encoders add those to improve quality.
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    Old 22nd February 2019, 22:37   #1484  |  Link
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    Quote:
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    Yeah, the goal is for the decoder to determine the minimum sequence of frames required to decode a particular frame. With a IbbbBbbbPbbbBbbbPbbbbBbbbI kind of structure, decoding the last "b" frame in an Open GOP should require just six frames (IPPBIb) in decode order typically. But that requires the reference list because sometimes a b could reference two B frames back and things like that. With multiple reference frames it's impossible to reliably know the hierarchy without knowing what each frame references.
    Yeah, if you want to do that you'll need a reference list parser. But that's been true for a very long time - even x264 produces streams that require you to do this.

    But regardless, whatever structure you pick, the CDFs always follow that same structure, so they are "free" from a seekability point of view.

    Last edited by TD-Linux; 22nd February 2019 at 22:42.
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    Old Yesterday, 00:31   #1485  |  Link
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    Yeah, if you want to do that you'll need a reference list parser. But that's been true for a very long time - even x264 produces streams that require you to do this.

    But regardless, whatever structure you pick, the CDFs always follow that same structure, so they are "free" from a seekability point of view.
    That's good news. I had heard suggestions otherwise.
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  • “创新从来都是九死一生”(人民论坛) 2019-02-14
  • 端午假期广州铁路运客640.5万人次 创历史新高 2019-02-14
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