Argh, I screwed this up. (So much for "simple rules.")
It should have been:
(1) "Reference frames" means I-frames or P-frames.
(2) If you hit a B-frame, decode it (using BOTH previously-encountered
"reference frames") and discard it.
(3) If you hit an I-frame, decode it (it stands alone) and display the
*previously encountered* "reference frame" if one exists.
(4) If you hit a P-frame, decode it (using the previous "reference frame")
and display the previously encountered "reference frame" if one exists.
-Keith
On Fri, 6 Apr 2007, Keith Winstein wrote:
> Hi Dinkar,
>
> In the example below, the frames would be displayed according to the
> "display order," i.e., #-3 (B), #-2 (B), #-1 (P), #0 (B), #1 (B), #2 (I).
>
> If you get confused, just remember that MPEG-2 video follows these simple
> rules:
>
> (1) Keep two slots -- the "last I-frame" slot and the "last P-frame" slot.
>
> (2) If you hit an I-frame, decode it, put it into the "last I-frame" slot,
> and display the contents of the "last P-frame" slot
>
> (3) If you hit a P-frame, decode it (using the "last I-frame" slot as a
> reference), put it into the "last P-frame" slot, and display the
> contents of the "last I-frame" slot.
>
> (4) If you hit a B-frame, decode it (using both slots as references) and
> display it immediately.
>
> That's it.
>
> Best,
> Keith
>
> On Fri, 6 Apr 2007, Dinkar Bhat wrote:
>
>> Hi Keith,
>>
>> Thanks for this example. In this example, would the first picture that is
>> displayed
>> be the P-picture in the second GOP followed by the I picture? I am getting
>> a little
>> confused about the display order of the all the frames.
>>
>> Thanks
>> Dinkar
>>
>> Keith Winstein <[EMAIL PROTECTED]> wrote:
>> Hi Sebastian,
>>
>> Unfortunately, MPEG-2 isn't quite this simple. The approach you're
>> describing will only work on a *closed* GOP. In normal (non-closed) GOPs,
>> decoding the first two B frames will require access to the previous P
>> frame -- from the previous GOP.
>>
>> Here's an example of what you'll see right around the GOP boundary:
>>
>> P-picture (coded order #-3, display order #-1)
>> [This guy is predicted from the previous I-picture, not shown here.]
>> B-picture (coded order #-2, display order #-3)
>> [Predicted from prev. I-pic AND P-picture in coded order #-3.]
>> B-picture (coded order #-1, display order #-2)
>> [Predicted from prev. I-pic AND P-picture in coded order #-3.]
>> SEQUENCE header
>> GOP header
>> I-picture (coded order #0, display order #2)
>> [Stands alone.]
>> B-picture (coded order #1, display order #0)
>> [Predicted from I-picture in coded order #0 and P-picture #-3.]
>> B-picture (coded order #2, display order #1)
>> [Predicted from I-picture in coded order #0 and P-picture #-3.]
>> P-picture (coded order #3, display order #5)
>> ...
>>
>> So, you see, decoding the first two B-pictures requires access to the last
>> P-picture in the previous GOP. That's not the case in a closed GOP, but it
>> is in a normal GOP.
>>
>> So, what can you do? The easiest thing is probably to pretend that the GOP
>> starts with the I-picture (the one sent first but not displayed first).
>> Skip any B-pictures until you've seen one I-picture and one P-picture.
>> Then decode and display everything *and continue* beyond the end of the
>> GOP, decoding all the frames until you hit a P-picture, and displaying all
>> the frames until you have to display an I-picture (which will be the first
>> one sent in the next GOP.)
>>
>> This method will require you to do a little extra decoding if you want to
>> hit all the frames, but basically you're just offsetting what you consider
>> to be the start of the "GOP".
>>
>> Hope this helps. Happy to help if you have further questions.
>>
>> Best,
>> Keith
>>
>> On Thu, 5 Apr 2007, Sebastian Vetter wrote:
>>
>>> -----BEGIN PGP SIGNED MESSAGE-----
>>> Hash: SHA1
>>>
>>> Hallo everyone,
>>>
>>> I just started working with libmpeg2 to integrate video support into the
>>> application I'm working at.
>>>
>>> What I would like to do is decode all frames within ONE GOP and store
>>> them in memory. The user should have the ability to jump to a random GOP
>>> resulting in decoding that one and make its frames accessible.
>>>
>>> Unfortunately, I had some problems making it work. It seems that even if
>>> the I-frame is decoded correctly all consecutive frames are decoded with
>>> loads of artifacts.
>>>
>>> Could anyone suggest a good way to achieve what I described before?
>>> Especially the way of using skip and seek functionality still is kind of
>>> a red rag to me.
>>>
>>> Any suggestion are hint will be highly appreciated.
>>>
>>> Cheers,
>>> Sebastian
>>> -----BEGIN PGP SIGNATURE-----
>>> Version: GnuPG v1.4.3 (GNU/Linux)
>>> Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org
>>>
>>> iD8DBQFGFDgx/ZEwc0fz/xMRAlqvAJ925c/kKewQhDqDNPERNyCLH4b/YACcDsEY
>>> clKo+Ac+TOAbhJ0H14CT5EQ=
>>> =OWFQ
>>> -----END PGP SIGNATURE-----
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