Sink
Object Hierarchy:
Description:
[ GIR ( name = "BaseSink" ) ]
public abstract class Sink : Element
Sink 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. Sink 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 Sink infrastructure should be sufficient.
Sink provides support for exactly one sink pad, which should be named "sink". A sink implementation (subclass of Sink) 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>");
}Sink will handle the prerolling correctly. This means that it will return
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, Sink will synchronise on the clock using the times returned from
GstBaseSinkClass::get_timess. If this function returns
CLOCK_TIME_NONE for the start time, no synchronisation will be done. Synchronisation can be disabled entirely by setting the object
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, Sink will wait for the clock to reach the time indicated
by the stop time of the last GstBaseSinkClass::get_timess 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.
Sink will internally use the 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.
Sink will by default report the current playback position in 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_capss 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_capss
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 Sink.
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 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_timess 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 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 @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.