Upstream caps (re)negotiation

Upstream negotiation's primary use is to renegotiate (part of) an already-negotiated pipeline to a new format. Some practical examples include to select a different video size because the size of the video window changed, and the video output itself is not capable of rescaling, or because the audio channel configuration changed.

Upstream caps renegotiation is requested by sending a GST_EVENT_RECONFIGURE event upstream. The idea is that it will instruct the upstream element to reconfigure its caps by doing a new query for the allowed caps and then choosing a new caps. The element that sends out the RECONFIGURE event would influence the selection of the new caps by returning the new preferred caps from its GST_QUERY_CAPS query function. The RECONFIGURE event will set the GST_PAD_FLAG_NEED_RECONFIGURE on all pads that it travels over.

It is important to note here that different elements actually have different responsibilities here: