BaseSink
Superclasses: Element
, Object
, InitiallyUnowned
, Object
BaseSink
is the base class for sink elements in GStreamer, such as
xvimagesink or filesink. It is a layer on top of Element
that provides a
simplified interface to plugin writers. BaseSink
handles many details
for you, for example: preroll, clock synchronization, state changes,
activation in push or pull mode, and queries.
In most cases, when writing sink elements, there is no need to implement
class methods from Element
or to set functions on pads, because the
BaseSink
infrastructure should be sufficient.
BaseSink
provides support for exactly one sink pad, which should be
named “sink”. A sink implementation (subclass of BaseSink
) should
install a pad template in its class_init function, like so:
static void
my_element_class_init (GstMyElementClass *klass)
{
GstElementClass *gstelement_class = GST_ELEMENT_CLASS (klass);
// sinktemplate should be a :obj:`~gi.repository.Gst.StaticPadTemplate` with direction
// ``%GST_PAD_SINK`` and name "sink"
gst_element_class_add_static_pad_template (gstelement_class, &sinktemplate);
gst_element_class_set_static_metadata (gstelement_class,
"Sink name",
"Sink",
"My Sink element",
"The author <my.sink``my``.email>");
}
BaseSink
will handle the prerolling correctly. This means that it will
return %GST_STATE_CHANGE_ASYNC
from a state change to PAUSED until the first
buffer arrives in this element. The base class will call the
GstBaseSinkClass
::preroll vmethod with this preroll buffer and will then
commit the state change to the next asynchronously pending state.
When the element is set to PLAYING, BaseSink
will synchronise on the
clock using the times returned from GstBaseSinkClass
::get_times. If this
function returns %GST_CLOCK_TIME_NONE
for the start time, no synchronisation
will be done. Synchronisation can be disabled entirely by setting the object
BaseSink
:sync property to False
.
After synchronisation the virtual method GstBaseSinkClass
::render will be
called. Subclasses should minimally implement this method.
Subclasses that synchronise on the clock in the GstBaseSinkClass
::render
method are supported as well. These classes typically receive a buffer in
the render method and can then potentially block on the clock while
rendering. A typical example is an audiosink.
These subclasses can use wait_preroll()
to perform the
blocking wait.
Upon receiving the EOS event in the PLAYING state, BaseSink
will wait
for the clock to reach the time indicated by the stop time of the last
GstBaseSinkClass
::get_times call before posting an EOS message. When the
element receives EOS in PAUSED, preroll completes, the event is queued and an
EOS message is posted when going to PLAYING.
BaseSink
will internally use the %GST_EVENT_SEGMENT
events to schedule
synchronisation and clipping of buffers. Buffers that fall completely outside
of the current segment are dropped. Buffers that fall partially in the
segment are rendered (and prerolled). Subclasses should do any subbuffer
clipping themselves when needed.
BaseSink
will by default report the current playback position in
%GST_FORMAT_TIME
based on the current clock time and segment information.
If no clock has been set on the element, the query will be forwarded
upstream.
The GstBaseSinkClass
::set_caps function will be called when the subclass
should configure itself to process a specific media type.
The GstBaseSinkClass
::start and GstBaseSinkClass
::stop virtual methods
will be called when resources should be allocated. Any
GstBaseSinkClass
::preroll, GstBaseSinkClass
::render and
GstBaseSinkClass
::set_caps function will be called between the
GstBaseSinkClass
::start and GstBaseSinkClass
::stop calls.
The GstBaseSinkClass
::event virtual method will be called when an event is
received by BaseSink
. Normally this method should only be overridden by
very specific elements (such as file sinks) which need to handle the
newsegment event specially.
The GstBaseSinkClass
::unlock method is called when the elements should
unblock any blocking operations they perform in the
GstBaseSinkClass
::render method. This is mostly useful when the
GstBaseSinkClass
::render method performs a blocking write on a file
descriptor, for example.
The BaseSink
:max-lateness property affects how the sink deals with
buffers that arrive too late in the sink. A buffer arrives too late in the
sink when the presentation time (as a combination of the last segment, buffer
timestamp and element base_time) plus the duration is before the current
time of the clock.
If the frame is later than max-lateness, the sink will drop the buffer
without calling the render method.
This feature is disabled if sync is disabled, the
GstBaseSinkClass
::get_times method does not return a valid start time or
max-lateness is set to -1 (the default).
Subclasses can use set_max_lateness()
to configure the
max-lateness value.
The BaseSink
:qos property will enable the quality-of-service features of
the basesink which gather statistics about the real-time performance of the
clock synchronisation. For each buffer received in the sink, statistics are
gathered and a QOS event is sent upstream with these numbers. This
information can then be used by upstream elements to reduce their processing
rate, for example.
The BaseSink
:async property can be used to instruct the sink to never
perform an ASYNC state change. This feature is mostly usable when dealing
with non-synchronized streams or sparse streams.
Methods
- class BaseSink
- get_blocksize() int
Get the number of bytes that the sink will pull when it is operating in pull mode.
- get_drop_out_of_segment() bool
Checks if
sink
is currently configured to drop buffers which are outside the current segmentAdded in version 1.12.
- get_last_sample() Sample | None
Get the last sample that arrived in the sink and was used for preroll or for rendering. This property can be used to generate thumbnails.
The
Caps
on the sample can be used to determine the type of the buffer.Free-function: gst_sample_unref
- get_max_bitrate() int
Get the maximum amount of bits per second that the sink will render.
Added in version 1.2.
- get_max_lateness() int
Gets the max lateness value. See
set_max_lateness()
for more details.
- get_processing_deadline() int
Get the processing deadline of
sink
. seeset_processing_deadline()
for more information about the processing deadline.Added in version 1.16.
- get_render_delay() int
Get the render delay of
sink
. seeset_render_delay()
for more information about the render delay.
- get_stats() Structure
Return various
BaseSink
statistics. This function returns aStructure
with nameapplication/x-gst-base-sink-stats
with the following fields:“average-rate” G_TYPE_DOUBLE average frame rate
“dropped” G_TYPE_UINT64 Number of dropped frames
“rendered” G_TYPE_UINT64 Number of rendered frames
Added in version 1.18.
- get_throttle_time() int
Get the time that will be inserted between frames to control the maximum buffers per second.
- is_async_enabled() bool
Checks if
sink
is currently configured to perform asynchronous state changes to PAUSED.
- is_last_sample_enabled() bool
Checks if
sink
is currently configured to store the last received sample in the last-sample property.
- is_qos_enabled() bool
Checks if
sink
is currently configured to send Quality-of-Service events upstream.
- query_latency() tuple[bool, bool, bool, int, int]
Query the sink for the latency parameters. The latency will be queried from the upstream elements.
live
will beTrue
ifsink
is configured to synchronize against the clock.upstream_live
will beTrue
if an upstream element is live.If both
live
andupstream_live
areTrue
, the sink will want to compensate for the latency introduced by the upstream elements by setting themin_latency
to a strictly positive value.This function is mostly used by subclasses.
- set_async_enabled(enabled: bool) None
Configures
sink
to perform all state changes asynchronously. When async is disabled, the sink will immediately go to PAUSED instead of waiting for a preroll buffer. This feature is useful if the sink does not synchronize against the clock or when it is dealing with sparse streams.- Parameters:
enabled – the new async value.
- set_blocksize(blocksize: int) None
Set the number of bytes that the sink will pull when it is operating in pull mode.
- Parameters:
blocksize – the blocksize in bytes
- set_drop_out_of_segment(drop_out_of_segment: bool) None
Configure
sink
to drop buffers which are outside the current segmentAdded in version 1.12.
- Parameters:
drop_out_of_segment – drop buffers outside the segment
- set_last_sample_enabled(enabled: bool) None
Configures
sink
to store the last received sample in the last-sample property.- Parameters:
enabled – the new enable-last-sample value.
- set_max_bitrate(max_bitrate: int) None
Set the maximum amount of bits per second that the sink will render.
Added in version 1.2.
- Parameters:
max_bitrate – the max_bitrate in bits per second
- set_max_lateness(max_lateness: int) None
Sets the new max lateness value to
max_lateness
. This value is used to decide if a buffer should be dropped or not based on the buffer timestamp and the current clock time. A value of -1 means an unlimited time.- Parameters:
max_lateness – the new max lateness value.
- set_processing_deadline(processing_deadline: int) None
Maximum amount of time (in nanoseconds) that the pipeline can take for processing the buffer. This is added to the latency of live pipelines.
This function is usually called by subclasses.
Added in version 1.16.
- Parameters:
processing_deadline – the new processing deadline in nanoseconds.
- set_qos_enabled(enabled: bool) None
Configures
sink
to send Quality-of-Service events upstream.- Parameters:
enabled – the new qos value.
- set_render_delay(delay: int) None
Set the render delay in
sink
todelay
. The render delay is the time between actual rendering of a buffer and its synchronisation time. Some devices might delay media rendering which can be compensated for with this function.After calling this function, this sink will report additional latency and other sinks will adjust their latency to delay the rendering of their media.
This function is usually called by subclasses.
- Parameters:
delay – the new delay
- set_sync(sync: bool) None
Configures
sink
to synchronize on the clock or not. Whensync
isFalse
, incoming samples will be played as fast as possible. Ifsync
isTrue
, the timestamps of the incoming buffers will be used to schedule the exact render time of its contents.- Parameters:
sync – the new sync value.
- set_throttle_time(throttle: int) None
Set the time that will be inserted between rendered buffers. This can be used to control the maximum buffers per second that the sink will render.
- Parameters:
throttle – the throttle time in nanoseconds
- set_ts_offset(offset: int) None
Adjust the synchronisation of
sink
withoffset
. A negative value will render buffers earlier than their timestamp. A positive value will delay rendering. This function can be used to fix playback of badly timestamped buffers.- Parameters:
offset – the new offset
- wait(time: int) tuple[FlowReturn, int]
This function will wait for preroll to complete and will then block until
time
is reached. It is usually called by subclasses that use their own internal synchronisation but want to let some synchronization (like EOS) be handled by the base class.This function should only be called with the PREROLL_LOCK held (like when receiving an EOS event in the ::event vmethod or when handling buffers in ::render).
The
time
argument should be the running_time of when the timeout should happen and will be adjusted with any latency and offset configured in the sink.- Parameters:
time – the running_time to be reached
- wait_clock(time: int) tuple[ClockReturn, int]
This function will block until
time
is reached. It is usually called by subclasses that use their own internal synchronisation.If
time
is not valid, no synchronisation is done and%GST_CLOCK_BADTIME
is returned. Likewise, if synchronisation is disabled in the element or there is no clock, no synchronisation is done and%GST_CLOCK_BADTIME
is returned.This function should only be called with the PREROLL_LOCK held, like when receiving an EOS event in the
GstBaseSinkClass
::event vmethod or when receiving a buffer in theGstBaseSinkClass
::render vmethod.The
time
argument should be the running_time of when this method should return and is not adjusted with any latency or offset configured in the sink.- Parameters:
time – the running_time to be reached
- wait_preroll() FlowReturn
If the
GstBaseSinkClass
::render method performs its own synchronisation against the clock it must unblock when going from PLAYING to the PAUSED state and call this method before continuing to render the remaining data.If the
GstBaseSinkClass
::render method can block on something else than the clock, it must also be ready to unblock immediately on theGstBaseSinkClass
::unlock method and cause theGstBaseSinkClass
::render method to immediately call this function. In this case, the subclass must be prepared to continue rendering where it left off if this function returns%GST_FLOW_OK
.This function will block until a state change to PLAYING happens (in which case this function returns
%GST_FLOW_OK
) or the processing must be stopped due to a state change to READY or a FLUSH event (in which case this function returns%GST_FLOW_FLUSHING
).This function should only be called with the PREROLL_LOCK held, like in the render function.
Properties
- class BaseSink
- props.async: bool
If set to
True
, the basesink will perform asynchronous state changes. When set toFalse
, the sink will not signal the parent when it prerolls. Use this option when dealing with sparse streams or when synchronisation is not required.
- props.enable_last_sample: bool
Enable the last-sample property. If
False
, basesink doesn’t keep a reference to the last buffer arrived and the last-sample property is always set toNone
. This can be useful if you need buffers to be released as soon as possible, eg. if you’re using a buffer pool.
- props.last_sample: Sample
The last buffer that arrived in the sink and was used for preroll or for rendering. This property can be used to generate thumbnails. This property can be
None
when the sink has not yet received a buffer.
- props.max_bitrate: int
Control the maximum amount of bits that will be rendered per second. Setting this property to a value bigger than 0 will make the sink delay rendering of the buffers when it would exceed to max-bitrate.
Added in version 1.2.
- props.processing_deadline: int
Maximum amount of time (in nanoseconds) that the pipeline can take for processing the buffer. This is added to the latency of live pipelines.
Added in version 1.16.
- props.render_delay: int
The additional delay between synchronisation and actual rendering of the media. This property will add additional latency to the device in order to make other sinks compensate for the delay.
- props.stats: Structure
Various
BaseSink
statistics. This property returns aStructure
with nameapplication/x-gst-base-sink-stats
with the following fields:“average-rate” G_TYPE_DOUBLE average frame rate
“dropped” G_TYPE_UINT64 Number of dropped frames
“rendered” G_TYPE_UINT64 Number of rendered frames
Added in version 1.18.
Virtual Methods
- class BaseSink
- do_activate_pull(active: bool) bool
Subclasses should override this when they can provide an alternate method of spawning a thread to drive the pipeline in pull mode. Should start or stop the pulling thread, depending on the value of the “active” argument. Called after actually activating the sink pad in pull mode. The default implementation starts a task on the sink pad.
- Parameters:
active
- do_event(event: Event) bool
Override this to handle events arriving on the sink pad
- Parameters:
event
- do_fixate(caps: Caps) Caps
Only useful in pull mode. Implement if you have ideas about what should be the default values for the caps you support.
- Parameters:
caps
- do_get_caps(filter: Caps | None = None) Caps
Called to get sink pad caps from the subclass.
- Parameters:
filter
- do_get_times(buffer: Buffer) tuple[int, int]
Get the start and end times for syncing on this buffer.
- Parameters:
buffer
- do_prepare(buffer: Buffer) FlowReturn
Called to prepare the buffer for
render
andpreroll
. This function is called before synchronisation is performed.- Parameters:
buffer
- do_prepare_list(buffer_list: BufferList) FlowReturn
Called to prepare the buffer list for
render_list
. This function is called before synchronisation is performed.- Parameters:
buffer_list
- do_preroll(buffer: Buffer) FlowReturn
Called to present the preroll buffer if desired.
- Parameters:
buffer
- do_render(buffer: Buffer) FlowReturn
Called when a buffer should be presented or output, at the correct moment if the
BaseSink
has been set to sync to the clock.- Parameters:
buffer
- do_render_list(buffer_list: BufferList) FlowReturn
Same as
render
but used with buffer lists instead of buffers.- Parameters:
buffer_list
- do_unlock() bool
Unlock any pending access to the resource. Subclasses should unblock any blocked function ASAP and call
wait_preroll()
- do_unlock_stop() bool
Clear the previous unlock request. Subclasses should clear any state they set during
GstBaseSinkClass
::unlock, and be ready to continue where they left off afterwait_preroll()
,wait()
or gst_wait_sink_wait_clock() return orGstBaseSinkClass
::render is called again.
- do_wait_event(event: Event) FlowReturn
Override this to implement custom logic to wait for the event time (for events like EOS and GAP). Subclasses should always first chain up to the default implementation.
- Parameters:
event