[FFmpeg-devel] [PATCH] libvpx: alt reference frame / lag

James Zern jzern
Tue Jun 15 22:23:27 CEST 2010

On Tue, Jun 15, 2010 at 14:51, Reimar D?ffinger
<Reimar.Doeffinger at gmx.de> wrote:
> On Tue, Jun 15, 2010 at 12:18:29PM -0400, John Koleszar wrote:
>> The encoder doesn't and shouldn't know what the user is going to do
>> with the data. People can and have defined their own transports.
>> Framing aside, I don't think it makes sense to combine the two packets
>> because they can be semantically different. One of the two could be
>> droppable, for example. Many applications want to treat these packets
>> independently (consider streaming, video conferencing). One example
>> might be to apply different error correction to the two packets.
> Could maybe someone explain what exactly this is about?
> You seem to be talking about two packets with the same time stamp that
> a decoder might want to decode differently for each?
> That makes no sense to me, if the have the same time stamp, they are
> supposed to be displayed at the same time, which makes no sense and the
> decoder really should output only one of them...
Essentially with this option the encoder will inject another reference
frame at the same timestamp as the dependent frame (ignoring time_base
changes) [1]. The question is how best to mux the frame as it has to
be decoded prior to the one following it, but won't produce a frame on
its own. The current recommendation [1] has the problems mentioned
here. Other options seem to be framing the two packets or perhaps
putting more dependence on the container, e.g., the invisible bit in
the SimpleBlock header.
I was thinking about starting a new thread with these comments in mind
on webm-discuss, but held back as this one gained history. Should I cc
the list or prefer to keep it here?

[1]: http://www.webmproject.org/code/specs/container/#vp8_alternate_reference_frames

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