#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:
staticvoid
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_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.
#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:
#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_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.