Gets the capabilities this pad can produce or consume. Note that this method doesn't necessarily return the caps set by sending a Event.newCaps - use Pad.getCurrentCaps for that instead. gst_pad_query_caps returns all possible caps a pad can operate with, using the pad's CAPS query function, If the query fails, this function will return filter, if not NULL, otherwise ANY.
When called on sinkpads filter contains the caps that upstream could produce in the order preferred by upstream. When called on srcpads filter contains the caps accepted by downstream in the preferred order. filter might be NULL but if it is not NULL the returned caps will be a subset of filter.