GstBase.BaseParse¶
- Subclasses:
None
Methods¶
- Inherited:
- Structs:
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Virtual Methods¶
- Inherited:
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Properties¶
- Inherited:
Name |
Type |
Flags |
Short Description |
|---|---|---|---|
r/w |
Force processing (disables passthrough) |
Signals¶
- Inherited:
Fields¶
- Inherited:
Name |
Type |
Access |
Description |
|---|---|---|---|
element |
r |
the parent element. |
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flags |
r |
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segment |
r |
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sinkpad |
r |
||
srcpad |
r |
Class Details¶
- class GstBase.BaseParse(**kwargs)¶
- Bases:
- Abstract:
Yes
- Structure:
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.
- Description of the parsing mechanism:
- Set-up phase
GstBase.BaseParsecalls #GstBaseParseClass::start to inform subclass that data processing is about to start now.GstBase.BaseParseclass calls #GstBaseParseClass::set_sink_caps to inform the subclass about incoming sinkpad caps. Subclass could already set the srcpad caps accordingly, but this might be delayed until callingGstBase.BaseParse.finish_frame() with a non-queued frame.At least at this point subclass needs to tell the
GstBase.BaseParseclass how big data chunks it wants to receive (minimum frame size ). It can do this withGstBase.BaseParse.set_min_frame_size().GstBase.BaseParseclass sets up appropriate data passing mode (pull/push) and starts to process the data.
- Parsing phase
GstBase.BaseParsegathers at least min_frame_size bytes of data either by pulling it from upstream or collecting buffers in an internalGstBase.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
Gst.FlowReturn.OKalong 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,
GstBase.BaseParse.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 toGstBase.BaseParse.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
GstBase.BaseParseif 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
GstBase.BaseParseClasswill handle both srcpad and sinkpad events. They will be passed to subclass if #GstBaseParseClass::sink_event or #GstBaseParseClass::src_event implementations have been provided.
- Shutdown phase
GstBase.BaseParseclass 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_caps function).
This base class uses
Gst.Format.DEFAULTas a meaning of frames. So, subclass conversion routine needs to know that conversion fromGst.Format.TIMEtoGst.Format.DEFAULTmust return the frame number that can be found from the given byte position.GstBase.BaseParseuses 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
GstBase.BaseParse.set_min_frame_size() function.Examine data chunks passed to subclass with #GstBaseParseClass::handle_frame and pass proper frame(s) to
GstBase.BaseParse.finish_frame(), and setting src pad caps and timestamps on frame.Provide conversion functions
Update the duration information with
GstBase.BaseParse.set_duration()Optionally passthrough using
GstBase.BaseParse.set_passthrough()Configure various baseparse parameters using
GstBase.BaseParse.set_average_bitrate(),GstBase.BaseParse.set_syncable() andGstBase.BaseParse.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
GstBase.BaseParseto 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 providesGstBase.BaseParsewith 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.
- add_index_entry(offset, ts, key, force)[source]¶
- Parameters:
- Returns:
boolindicating whether entry was added- Return type:
Adds an entry to the index associating offset to ts. It is recommended to only add keyframe entries. force allows to bypass checks, such as whether the stream is (upstream) seekable, another entry is already “close” to the new entry, etc.
- convert_default(src_format, src_value, dest_format)[source]¶
- Parameters:
src_format (
Gst.Format) –Gst.Formatdescribing the source format.src_value (
int) – Source value to be converted.dest_format (
Gst.Format) –Gst.Formatdefining the converted format.
- Returns:
Trueif conversion was successful.- dest_value:
Pointer where the conversion result will be put.
- Return type:
Default implementation of #GstBaseParseClass::convert.
- drain()[source]¶
Drains the adapter until it is empty. It decreases the min_frame_size to match the current adapter size and calls chain method until the adapter is emptied or chain returns with error.
New in version 1.12.
- finish_frame(frame, size)[source]¶
- Parameters:
frame (
GstBase.BaseParseFrame) – aGstBase.BaseParseFramesize (
int) – consumed input data represented by frame
- Returns:
a
Gst.FlowReturnthat should be escalated to caller (of caller)- Return type:
Collects parsed data and pushes it downstream. Source pad caps must be set when this is called.
If frame's out_buffer is set, that will be used as subsequent frame data, and size amount will be flushed from the input data. The output_buffer size can differ from the consumed size indicated by size.
Otherwise, size samples will be taken from the input and used for output, and the output’s metadata (timestamps etc) will be taken as (optionally) set by the subclass on frame's (input) buffer (which is otherwise ignored for any but the above purpose/information).
Note that the latter buffer is invalidated by this call, whereas the caller retains ownership of frame.
- merge_tags(tags, mode)[source]¶
- Parameters:
tags (
Gst.TagListorNone) – aGst.TagListto merge, orNoneto unset previously-set tagsmode (
Gst.TagMergeMode) – theGst.TagMergeModeto use, usuallyGst.TagMergeMode.REPLACE
Sets the parser subclass’s tags and how they should be merged with any upstream stream tags. This will override any tags previously-set with
GstBase.BaseParse.merge_tags().Note that this is provided for convenience, and the subclass is not required to use this and can still do tag handling on its own.
New in version 1.6.
- push_frame(frame)[source]¶
- Parameters:
frame (
GstBase.BaseParseFrame) – aGstBase.BaseParseFrame- Returns:
- Return type:
Pushes the frame’s buffer downstream, sends any pending events and does some timestamp and segment handling. Takes ownership of frame’s buffer, though caller retains ownership of frame.
This must be called with sinkpad STREAM_LOCK held.
- set_average_bitrate(bitrate)[source]¶
- Parameters:
bitrate (
int) – average bitrate in bits/second
Optionally sets the average bitrate detected in media (if non-zero), e.g. based on metadata, as it will be posted to the application.
By default, announced average bitrate is estimated. The average bitrate is used to estimate the total duration of the stream and to estimate a seek position, if there’s no index and the format is syncable (see
GstBase.BaseParse.set_syncable()).
- set_duration(fmt, duration, interval)[source]¶
- Parameters:
fmt (
Gst.Format) –Gst.Format.duration (
int) – duration value.interval (
int) – how often to update the duration estimate based on bitrate, or 0.
Sets the duration of the currently playing media. Subclass can use this when it is able to determine duration and/or notices a change in the media duration. Alternatively, if interval is non-zero (default), then stream duration is determined based on estimated bitrate, and updated every interval frames.
- set_frame_rate(fps_num, fps_den, lead_in, lead_out)[source]¶
- Parameters:
If frames per second is configured, parser can take care of buffer duration and timestamping. When performing segment clipping, or seeking to a specific location, a corresponding decoder might need an initial lead_in and a following lead_out number of frames to ensure the desired segment is entirely filled upon decoding.
- set_has_timing_info(has_timing)[source]¶
- Parameters:
has_timing (
bool) – whether frames carry timing information
Set if frames carry timing information which the subclass can (generally) parse and provide. In particular, intrinsic (rather than estimated) time can be obtained following a seek.
- set_infer_ts(infer_ts)[source]¶
-
By default, the base class might try to infer PTS from DTS and vice versa. While this is generally correct for audio data, it may not be otherwise. Sub-classes implementing such formats should disable timestamp inferring.
- set_latency(min_latency, max_latency)[source]¶
-
Sets the minimum and maximum (which may likely be equal) latency introduced by the parsing process. If there is such a latency, which depends on the particular parsing of the format, it typically corresponds to 1 frame duration.
If the provided values changed from previously provided ones, this will also post a LATENCY message on the bus so the pipeline can reconfigure its global latency.
- set_min_frame_size(min_size)[source]¶
- Parameters:
min_size (
int) – Minimum size in bytes of the data that this base class should give to subclass.
Subclass can use this function to tell the base class that it needs to be given buffers of at least min_size bytes.
- set_passthrough(passthrough)[source]¶
-
Set if the nature of the format or configuration does not allow (much) parsing, and the parser should operate in passthrough mode (which only applies when operating in push mode). That is, incoming buffers are pushed through unmodified, i.e. no #GstBaseParseClass::handle_frame will be invoked, but #GstBaseParseClass::pre_push_frame will still be invoked, so subclass can perform as much or as little is appropriate for passthrough semantics in #GstBaseParseClass::pre_push_frame.
- set_pts_interpolation(pts_interpolate)[source]¶
-
By default, the base class will guess PTS timestamps using a simple interpolation (previous timestamp + duration), which is incorrect for data streams with reordering, where PTS can go backward. Sub-classes implementing such formats should disable PTS interpolation.
- set_syncable(syncable)[source]¶
- Parameters:
syncable (
bool) – set if frame starts can be identified
Set if frame starts can be identified. This is set by default and determines whether seeking based on bitrate averages is possible for a format/stream.
- set_ts_at_offset(offset)[source]¶
- Parameters:
offset (
int) – offset into current buffer
This function should only be called from a handle_frame implementation.
GstBase.BaseParsecreates initial timestamps for frames by using the last timestamp seen in the stream before the frame starts. In certain cases, the correct timestamps will occur in the stream after the start of the frame, but before the start of the actual picture data. This function can be used to set the timestamps based on the offset into the frame data that the picture starts.New in version 1.2.
- do_convert(src_format, src_value, dest_format, dest_value) virtual¶
- Parameters:
src_format (
Gst.Format) –src_value (
int) –dest_format (
Gst.Format) –dest_value (
int) –
- Return type:
Optional. Convert between formats.
- do_detect(buffer) virtual¶
- Parameters:
buffer (
Gst.Buffer) –- Return type:
Optional. Called until it doesn’t return
Gst.FlowReturn.OKanymore for the first buffers. Can be used by the subclass to detect the stream format.
- do_get_sink_caps(filter) virtual¶
-
Optional. Allows the subclass to do its own sink get caps if needed.
- do_handle_frame(frame) virtual¶
- Parameters:
frame (
GstBase.BaseParseFrame) –- Return type:
(
Gst.FlowReturn, skipsize:int)
Parses the input data into valid frames as defined by subclass which should be passed to
GstBase.BaseParse.finish_frame(). The frame’s input buffer is guaranteed writable, whereas the input frame ownership is held by caller (so subclass should make a copy if it needs to hang on). Input buffer (data) is provided by baseclass with as much metadata set as possible by baseclass according to upstream information and/or subclass settings, though subclass may still set buffer timestamp and duration if desired.
- do_pre_push_frame(frame) virtual¶
- Parameters:
frame (
GstBase.BaseParseFrame) –- Return type:
Optional. Called just prior to pushing a frame (after any pending events have been sent) to give subclass a chance to perform additional actions at this time (e.g. tag sending) or to decide whether this buffer should be dropped or not (e.g. custom segment clipping).
- do_set_sink_caps(caps) virtual¶
-
Optional. Allows the subclass to be notified of the actual caps set.
- do_sink_event(event) virtual¶
-
Optional. Event handler on the sink pad. This function should chain up to the parent implementation to let the default handler run.
- do_sink_query(query) virtual¶
-
Optional. Query handler on the sink pad. This function should chain up to the parent implementation to let the default handler run
New in version 1.2.
- do_src_event(event) virtual¶
-
Optional. Event handler on the source pad. Should chain up to the parent to let the default handler run.
- do_src_query(query) virtual¶
-
Optional. Query handler on the source pad. Should chain up to the parent to let the default handler run
New in version 1.2.
- do_start() virtual¶
- Return type:
Optional. Called when the element starts processing. Allows opening external resources.
Property Details¶
- GstBase.BaseParse.props.disable_passthrough¶
-
If set to
True, baseparse will unconditionally force parsing of the incoming data. This can be required in the rare cases where the incoming side-data (caps, pts, dts, …) is not trusted by the user and wants to force validation and parsing of the incoming data. If set toFalse, decision of whether to parse the data or not is up to the implementation (standard behaviour).