webrtcbin and caps on sink pads

Olivier Crête olivier.crete at collabora.com
Fri Mar 19 19:09:51 UTC 2021

On Fri, 2021-03-19 at 12:20 -0500, Trey Hutcheson wrote:
> Ok, so I know that before generating an offer/answer, that we need to
> wait on all sink pads to get caps from upstream. But we are
> encountering a problem: sometimes caps are never sent.

You can also skip all of that by just setting the exact codec you will
receive as the codec preferences on the transceiver itself.

> Example:
> * webrtcbin1 is receiving audio and video from its remote peer. Each
> incoming media stream is terminated by a tee element. 
> * I create a new webrtcbin instance, and branch off the tee's hanging
> off webrtcbin1; each branch contains a queue and then depayloads and
> repayloads the rtp, and connected via new sink pads on webrtcbin2. 
> Repeat that process for each new webrtc peer/webrtcbin instance that
> needs to participate in the media session.
> Problem #1: the first time a branch is created from the source tee,
> caps are sent downstream. However, new branches never receive caps.
> To get around that, we explicitly create a caps event and send it
> down the branch.

> Problem #2: sometimes that caps event takes several seconds (2
> seconds, 5 seconds, it's indeterminate) to actually get to the new
> sink pad. And I have no idea why. This delay creates all kinds of
> problems for us internally. 

Possibly blocking on a keyframe in the parser ?

> Why is the tee element not sending caps down to new elements after
> they are linked? Is it supposed to? If it's not, is there any built
> in element that will function like a tee, cache the last known caps,
> and send them downstream when new src pads are linked?

Tee should do exactly that already.

Olivier Crête
olivier.crete at collabora.com

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