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
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>>
>> iD8DBQFGFDgx/ZEwc0fz/xMRAlqvAJ925c/kKewQhDqDNPERNyCLH4b/YACcDsEY
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>>
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