gstbase.base_sink

#GstBaseSink is the base class for sink elements in GStreamer, such as xvimagesink or filesink. It is a layer on top of #GstElement that provides a simplified interface to plugin writers. #GstBaseSink 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 #GstElement or to set functions on pads, because the #GstBaseSink infrastructure should be sufficient.

#GstBaseSink provides support for exactly one sink pad, which should be named "sink". A sink implementation (subclass of #GstBaseSink) 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 #GstStaticPadTemplate 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>");
}

#GstBaseSink will handle the prerolling correctly. This means that it will return [gst.types.StateChangeReturn.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, #GstBaseSink will synchronise on the clock using the times returned from #GstBaseSinkClass::get_times. If this function returns [gst.types.CLOCK_TIME_NONE] for the start time, no synchronisation will be done. Synchronisation can be disabled entirely by setting the object #GstBaseSink: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 [gstbase.base_sink.BaseSink.waitPreroll] to perform the blocking wait.

Upon receiving the EOS event in the PLAYING state, #GstBaseSink 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.

#GstBaseSink will internally use the [gst.types.EventType.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.

#GstBaseSink will by default report the current playback position in [gst.types.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 #GstBaseSink. 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 #GstBaseSink: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 [gstbase.base_sink.BaseSink.setMaxLateness] to configure the max-lateness value.

The #GstBaseSink: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 #GstBaseSink: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.

Fluent builder for [gstbase.base_sink.BaseSink]