BaseSink

class BaseSink(**properties: Any)

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 segment

Added 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_latency() → int: Get the currently configured latency.

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. see set_processing_deadline() for more information about the processing deadline.

Added in version 1.16.

get_render_delay() → int: Get the render delay of sink. see set_render_delay() for more information about the render delay.

get_stats() → Structure

Return various BaseSink statistics. This function returns a Structure with name application/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_sync() → bool: Checks if sink is currently configured to synchronize against the clock.

get_throttle_time() → int: Get the time that will be inserted between frames to control the maximum buffers per second.

get_ts_offset() → int: Get the synchronisation offset of sink.

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 be True if sink is configured to synchronize against the clock. upstream_live will be True if an upstream element is live.

If both live and upstream_live are True, the sink will want to compensate for the latency introduced by the upstream elements by setting the min_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 segment

Added 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 to delay. 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. When sync is False, incoming samples will be played as fast as possible. If sync is True, 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 with offset. 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 the GstBaseSinkClass::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 the GstBaseSinkClass::unlock method and cause the GstBaseSinkClass::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 to False, 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.blocksize: int: The amount of bytes to pull when operating in pull mode.

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 to None. 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.max_lateness: int

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.qos: bool

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 a Structure with name application/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.

props.sync: bool

props.throttle_time: int: The time to insert between buffers. This property can be used to control the maximum amount of buffers per second to render. Setting this property to a value bigger than 0 will make the sink create THROTTLE QoS events.

props.ts_offset: int: Controls the final synchronisation, a negative value will render the buffer earlier while a positive value delays playback. This property can be used to fix synchronisation in bad files.

Virtual Methods

class BaseSink

do_activate_pull(active: bool) → bool

Parameters:: active

do_event(event: Event) → bool

Parameters:: event

do_fixate(caps: Caps) → Caps

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

Parameters:: buffer

do_prepare_list(buffer_list: BufferList) → FlowReturn

Parameters:: buffer_list

do_preroll(buffer: Buffer) → FlowReturn

Parameters:: buffer

do_propose_allocation(query: Query) → bool

Parameters:: query

do_query(query: Query) → bool

Parameters:: query

do_render(buffer: Buffer) → FlowReturn

Parameters:: buffer

do_render_list(buffer_list: BufferList) → FlowReturn

Parameters:: buffer_list

do_set_caps(caps: Caps) → bool

Parameters:: caps

do_start() → bool

do_stop() → bool

do_unlock() → bool

do_unlock_stop() → bool

do_wait_event(event: Event) → FlowReturn

Parameters:: event

Fields

class BaseSink

can_activate_pull

can_activate_push

clock_id

element

eos

flushing

have_newsegment

have_preroll

max_lateness

need_preroll

offset

pad_mode

playing_async

preroll_cond

preroll_lock

priv

running

segment

sinkpad

sync