[webrtcbin] Multiple codecs on offer

[Marc Durand]

I'm working a project to stream video from a camera to a web client (basically firefox or chrome).  For the server i'm using gstreamer/webrtcbin and the video feed is encoded with H264/baseline.   Depending on my server capabilities, i could also offer streams encoded with more efficient combinations of codecs (vp9/H265) and settings (high profile ...).  However those codecs are not mandatory for webrtc so,  to get things working,  i need a mechanism to negociate stream encoding and if required, fall back to H264/baseline.    Apparently, this mechanism exists on webrtc but i don't know how to implement it with webrtcbin.
My server is based on https://github.com/centricular/gstwebrtc-demos/tree/master/sendonly   So i just wait for the on-negotiation-needed signal and then i call the create-offer action which generates the sdp offer.   On my understanding, the SDP offer is filled with the negotiated caps, so i have to build and run a full pipeline in order to send the offer. Therefore, i have to choose a codec before the webrtc negotiation starts.   

Is there a way to make webrtcbin advertise a list of encoding formats we could provide before having built the pipeline ?  


Marc,






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