8.2.1. Dissecting capabilities
A pads capabilities are described in a GstCaps object. Internally, a GstCaps will contain one or more GstStructure that will describe one media type. A negotiated pad will have capabilities set that contain exactly one structure. Also, this structure will contain only fixed values. These constraints are not true for unnegotiated pads or pad templates.
这段话很精妙，要看特别定义的 GstStructure 有哪些
究竟是谁发起协商的？
http://gstreamer.freedesktop.org/data/doc/gstreamer/head/manual/html/section-bus-message-types.html
Tags: emitted when metadata was found in the stream. This can be
emitted multiple times for a pipeline (e.g. once for descriptive
metadata such as artist name or song title, and another one for
stream-information, such as samplerate and bitrate). Applications
should cache metadata internally. gst_message_parse_tag should be used to parse the taglist, which should
()
be gst_tag_list_free ()'ed when no longer
needed.
这个.. 没想到。这个东西不能作为 stream，本来很奇怪是怎么 chaining 和 mux/demux 的。话说回来，究竟是怎么 mux 呢，难道必须在 encode chain 里面等这个信号？怎么可能同步吗。
