This base class is for parser elements that process data and splits it into separate audio/video/whatever frames.
It provides for:
* provides one sink pad and one source pad * handles state changes * can operate in pull mode or push mode * handles seeking in both
modes * handles events (SEGMENT/EOS/FLUSH) * handles queries (POSITION/DURATION/SEEKING/FORMAT/CONVERT) * handles flushing
The purpose of this base class is to provide the basic functionality of a parser and share a lot of rather complex code.
typeof (unichar2) Description of the parsing mechanism:
typeof (unichar2)typeof
(unichar2) Set-up phase
* Parse calls GstBaseParseClass::start to inform subclass that data processing is about
to start now.
* Parse class calls GstBaseParseClass::set_sink_capss to inform the subclass about
incoming sinkpad caps. Subclass could already set the srcpad caps accordingly, but this might be delayed until calling
finish_frame with a non-queued frame.
* At least at this point subclass needs to tell the Parse class how big data chunks it wants to
receive (minimum frame size ). It can do this with set_min_frame_size.
* Parse class sets up appropriate data passing mode (pull/push) and starts to process the data.
typeof (unichar2)typeof
(unichar2) Parsing phase
* Parse gathers at least min_frame_size bytes of data either by pulling it from upstream or
collecting buffers in an internal Adapter.
* A buffer of (at least) min_frame_size bytes is passed to subclass with GstBaseParseClass::handle_frame. Subclass checks
the contents and can optionally return OK along with an amount of data to be skipped to find a valid frame
(which will result in a subsequent DISCONT). If, otherwise, the buffer does not hold a complete frame,
GstBaseParseClass::handle_frame can merely return and will be called again when additional data is available. In push mode this
amounts to an additional input buffer (thus minimal additional latency), in pull mode this amounts to some arbitrary reasonable buffer
size increase.
Of course, set_min_frame_size could also be used if a very specific
known amount of additional data is required. If, however, the buffer holds a complete valid frame, it can pass the size of this frame to
finish_frame.
If acting as a converter, it can also merely indicate consumed input data while simultaneously providing custom output data. Note that
baseclass performs some processing (such as tracking overall consumed data rate versus duration) for each finished frame, but other state
is only updated upon each call to GstBaseParseClass::handle_frame (such as tracking upstream input timestamp).
Subclass is also responsible for setting the buffer metadata (e.g. buffer timestamp and duration, or keyframe if applicable). (although
the latter can also be done by Parse if it is appropriately configured, see below). Frame is provided
with timestamp derived from upstream (as much as generally possible), duration obtained from configuration (see below), and offset if
meaningful (in pull mode).
Note that GstBaseParseClass::handle_frame might receive any small amount of input data when leftover data is being drained
(e.g. at EOS).
* As part of finish frame processing, just prior to actually pushing the buffer in question, it is passed to
GstBaseParseClass::pre_push_frame which gives subclass yet one last chance to examine buffer metadata, or to send some custom
(tag) events, or to perform custom (segment) filtering.
* During the parsing process Parse will handle both srcpad and sinkpad events. They will be passed
to subclass if GstBaseParseClass::sink_event or GstBaseParseClass::src_event implementations have been provided.
typeof (unichar2)typeof
(unichar2) Shutdown phase
* Parse class calls GstBaseParseClass::stop to inform the subclass that data parsing
will be stopped.
Subclass is responsible for providing pad template caps for source and sink pads. The pads need to be named "sink" and "src". It also
needs to set the fixed caps on srcpad, when the format is ensured (e.g. when base class calls subclass'
GstBaseParseClass::set_sink_capss function).
This base class uses DEFAULT as a meaning of frames. So, subclass conversion routine needs to know that
conversion from TIME to DEFAULT must return the frame number that can be
found from the given byte position.
Parse uses subclasses conversion methods also for seeking (or otherwise uses its own default one, see
also below).
Subclass start and stop functions will be called to inform the beginning and end of data processing.
Things that subclass need to take care of:
* Provide pad templates * Fixate the source pad caps when appropriate * Inform base class how big data chunks should be retrieved. This
is done with set_min_frame_size function. * Examine data chunks passed
to subclass with GstBaseParseClass::handle_frame and pass proper frame(s) to
finish_frame, and setting src pad caps and timestamps on frame. * Provide
conversion functions * Update the duration information with set_duration *
Optionally passthrough using set_passthrough * Configure various
baseparse parameters using set_average_bitrate,
set_syncable and
set_frame_rate.
* In particular, if subclass is unable to determine a duration, but parsing (or specs) yields a frames per seconds rate, then this can be
provided to Parse to enable it to cater for buffer time metadata (which will be taken from upstream as
much as possible). Internally keeping track of frame durations and respective sizes that have been pushed provides
Parse with an estimated bitrate. A default GstBaseParseClass::convert (used if not
overridden) will then use these rates to perform obvious conversions. These rates are also used to update (estimated) duration at regular
frame intervals.