mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-30 13:41:48 +00:00
90 lines
3.9 KiB
Text
90 lines
3.9 KiB
Text
A lot of data streams need a few initial buffers to make sense of the
|
|
data stream. With these initial buffers, it can pick up at any point in
|
|
the rest of the data stream.
|
|
|
|
Exampes:
|
|
- Vorbis and Theora have three initial Ogg packets
|
|
- MPEG4 has codec data
|
|
- GDP protocol serializes the initial new_segment event and the initial caps
|
|
|
|
It is important that elements that get connected after the stream starts,
|
|
receive these initial buffers. Also, sink elements should know that these
|
|
initial buffers are necessary for every new "client" of the sink element;
|
|
for example, multifdsink needs to send these initial buffers before the
|
|
normal buffers.
|
|
|
|
Currently, this is done by putting a 'streamheader' value on the caps.
|
|
'streamheader' is a GST_TYPE_ARRAY of GstBuffer, ie. an ordered list of
|
|
buffers.
|
|
|
|
The buffers themselves still get pushed as normal buffers, but with the
|
|
IN_CAPS flag set. This allows elements that do not know about streamheader
|
|
to still function correctly (ie. filesink and others)
|
|
|
|
It is of course important that these streamheader buffers are not sent twice
|
|
(once because they're in the caps, and once because they're received as
|
|
buffers).
|
|
|
|
So, an element that is aware of streamheader in the caps should probably best
|
|
drop all incoming IN_CAPS buffers.
|
|
|
|
Rules for sending streamheaders:
|
|
- all streamheader buffers should have IN_CAPS set
|
|
- from this list of streamheader buffers, a "streamheader" caps field should
|
|
be created as a GST_TYPE_ARRAY of GstBuffer. This should be done by
|
|
copying the buffers.
|
|
(the only important thing about the buffers in this list is the data;
|
|
other attributes of the buffers get ignored)
|
|
- each of these buffers should then have these caps set on them.
|
|
- when all streamheader buffers are collected in the element, pad caps should
|
|
be set, including this streamheader
|
|
- streamheader buffers should be sent consecutively, and before any of the
|
|
data (non-IN_CAPS) buffers they apply to. If necessary, the element
|
|
should internally queue non-IN_CAPS buffers until the streamheaders
|
|
are completely assembled.
|
|
- when new streamheader buffers need to be pushed out, this process is
|
|
repeated. Receiving a new IN_CAPS buffer after a non-IN_CAPS buffer
|
|
signifies resetting streamheader, as does the new set_caps with different
|
|
streamheader right before. (FIXME: it's probably better to explicitly
|
|
have an event/buffer that clears streamheaders; consider the case of
|
|
an element like GDP that has created streamheader from the first newsegment
|
|
and the first caps, and then receives a new tag event that it also wants
|
|
to put on the streamheader - it should be able to invalidate the previous
|
|
ones)
|
|
|
|
Elements that can send streamheader caps:
|
|
- vorbisenc
|
|
- theoraenc
|
|
- gdppay
|
|
|
|
Elements that can receive streamheader caps:
|
|
- multifdsink
|
|
|
|
Elements that can receive and send streamheader caps:
|
|
- oggmux
|
|
|
|
Elements that could/should use this:
|
|
- ffenc_mpeg4 (currently uses/sets codec_data)
|
|
- theoradec, vorbisdec (currently only look at incoming buffers)
|
|
- oggdemux
|
|
|
|
FUTURE PLANS
|
|
------------
|
|
The concept of streamheader is more generally applicable. We may want to
|
|
find a better way of implementing this than having two ways of telling
|
|
downstream elements about them. Currently an element that is
|
|
streamheader-aware needs to look both at caps and incoming buffers.
|
|
|
|
One option would be to only rely on the buffer flow, and make all elements
|
|
aware of a HEADER flag, causing them to keep these buffers around and push
|
|
them for each new pad link. This needs to be done in every element; a
|
|
scenario like
|
|
videotestsrc ! theoraenc
|
|
where later on an oggmux gets connected, should still work without dropping
|
|
these header buffers. But similarly,
|
|
videotestsrc ! theoraenc ! identity
|
|
with a later connection of oggmux to identity should work.
|
|
|
|
This could tie in with an idea to have pads store some initial data (the
|
|
first new_segment event, tag events, ...), which at the moment also get
|
|
dropped on dynamic in-stream links.
|