[Asterisk-video] Re: [asterisk-dev] Video packetization proposal

[Peter Grayson]

On 6/4/07, Peter Grayson <jpgrayson at gmail.com> wrote:
> On 6/2/07, Sergio Garcia Murillo <sergio.garcia at fontventa.com> wrote:
> > > > I,P,B framet indication is not needed as it should the decoder the
> > > > one who is in charge of that.
> > > Some application benefit from knowing the type of frame without
> > > actually decoding it.  One example would be video conferencing
> > > applications, and more specifically app_conference.  When you want to
> > > switch the video stream from one person to another, you want to do it
> > > on a key frame, otherwise you get garbage.
> >
> > Yes, It will make life easier for app_conference, but again it will make
> > very difficult the rtp>IAX transaltion as you'll have to dig into the encoded
> > bitstream to get that info.
>
> I'm no RTP expert, but doesn't the 7-bit payload type (PT) field in
> the RTP header serve the same purpose as the 8-bit payload type that
> Mihai proposes? RFC 3551 describes some well-known mappings for the
> A/V profile.
>
> You seem to be proposing that instead of identifying the video format
> in this header field, the receiver should pick apart the data payload
> to figure out what kind of data is present. This seems wrought with
> peril for two reasons.
>
> First, I don't know of any guarantee that various video codecs will
> have identifiable payloads. Without the payload type header, we would
> be demanding that the data payload have sufficient information for
> both identifying the codec _and_ determining codec parameters.
>
> Secondly, even if there was a way for receivers to identify the
> payload type, they would then have the burden of knowing the magic
> algorithms for determining the payload types for all the
> codecs/formats it might run into. This seems like an undue burden for
> clients.

Upon further review, I see that I misread what Sergio was saying. For
some reason I thought we were talking about the payload type field,
but this was really about the flags field. My above arguments are then
totally bogus. Sorry about the noise.

Pete