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Appsink is a sink plugin that supports many different methods for making the application get a handle on the GStreamer data in a pipeline. Unlike most GStreamer elements, Appsink provides external API functions.
appsink can be used by linking to the gstappsink.h header file to access the methods or by using the appsink action signals and properties.
The normal way of retrieving samples from appsink is by using the
gst_app_sink_pull_sample() and gst_app_sink_pull_preroll() methods.
These methods block until a sample becomes available in the sink or when the
sink is shut down or reaches EOS.
Appsink will internally use a queue to collect buffers from the streaming thread. If the application is not pulling samples fast enough, this queue will consume a lot of memory over time. The "max-buffers" property can be used to limit the queue size. The "drop" property controls whether the streaming thread blocks or if older buffers are dropped when the maximum queue size is reached. Note that blocking the streaming thread can negatively affect real-time performance and should be avoided.
If a blocking behaviour is not desirable, setting the "emit-signals" property
to TRUE will make appsink emit the "new-sample" and "new-preroll" signals
when a sample can be pulled without blocking.
The "caps" property on appsink can be used to control the formats that appsink can receive. This property can contain non-fixed caps, the format of the pulled samples can be obtained by getting the sample caps.
If one of the pull-preroll or pull-sample methods return NULL, the appsink
is stopped or in the EOS state. You can check for the EOS state with the
"eos" property or with the gst_app_sink_is_eos() method.
The eos signal can also be used to be informed when the EOS state is reached to avoid polling.
void gst_app_sink_set_caps (GstAppSink *appsink,const GstCaps *caps);
Set the capabilities on the appsink element. This function takes
a copy of the caps structure. After calling this method, the sink will only
accept caps that match caps
. If caps
is non-fixed, you must check the caps
on the buffers to get the actual used caps.
GstCaps *
gst_app_sink_get_caps (GstAppSink *appsink);
Get the configured caps on appsink
.
gboolean
gst_app_sink_is_eos (GstAppSink *appsink);
Check if appsink
is EOS, which is when no more samples can be pulled because
an EOS event was received.
This function also returns TRUE when the appsink is not in the PAUSED or
PLAYING state.
void gst_app_sink_set_emit_signals (GstAppSink *appsink,gboolean emit);
Make appsink emit the "new-preroll" and "new-sample" signals. This option is by default disabled because signal emission is expensive and unneeded when the application prefers to operate in pull mode.
gboolean
gst_app_sink_get_emit_signals (GstAppSink *appsink);
Check if appsink will emit the "new-preroll" and "new-sample" signals.
void gst_app_sink_set_max_buffers (GstAppSink *appsink,guint max);
Set the maximum amount of buffers that can be queued in appsink
. After this
amount of buffers are queued in appsink, any more buffers will block upstream
elements until a sample is pulled from appsink
.
guint
gst_app_sink_get_max_buffers (GstAppSink *appsink);
Get the maximum amount of buffers that can be queued in appsink
.
void gst_app_sink_set_drop (GstAppSink *appsink,gboolean drop);
Instruct appsink
to drop old buffers when the maximum amount of queued
buffers is reached.
gboolean
gst_app_sink_get_drop (GstAppSink *appsink);
Check if appsink
will drop old buffers when the maximum amount of queued
buffers is reached.
GstSample *
gst_app_sink_pull_preroll (GstAppSink *appsink);
Get the last preroll sample in appsink
. This was the sample that caused the
appsink to preroll in the PAUSED state. This sample can be pulled many times
and remains available to the application even after EOS.
This function is typically used when dealing with a pipeline in the PAUSED state. Calling this function after doing a seek will give the sample right after the seek position.
Note that the preroll sample will also be returned as the first sample
when calling gst_app_sink_pull_sample().
If an EOS event was received before any buffers, this function returns
NULL. Use gst_app_sink_is_eos() to check for the EOS condition.
This function blocks until a preroll sample or EOS is received or the appsink element is set to the READY/NULL state.
a GstSample or NULL when the appsink is stopped or EOS.
Call gst_sample_unref() after usage.
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GstSample *
gst_app_sink_pull_sample (GstAppSink *appsink);
This function blocks until a sample or EOS becomes available or the appsink element is set to the READY/NULL state.
This function will only return samples when the appsink is in the PLAYING state. All rendered buffers will be put in a queue so that the application can pull samples at its own rate. Note that when the application does not pull samples fast enough, the queued buffers could consume a lot of memory, especially when dealing with raw video frames.
If an EOS event was received before any buffers, this function returns
NULL. Use gst_app_sink_is_eos() to check for the EOS condition.
a GstSample or NULL when the appsink is stopped or EOS.
Call gst_sample_unref() after usage.
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void gst_app_sink_set_callbacks (GstAppSink *appsink,GstAppSinkCallbacks *callbacks,gpointer user_data,GDestroyNotify notify);
Set callbacks which will be executed for each new preroll, new sample and eos. This is an alternative to using the signals, it has lower overhead and is thus less expensive, but also less flexible.
If callbacks are installed, no signals will be emitted for performance reasons.
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typedef struct {
void (*eos) (GstAppSink *appsink, gpointer user_data);
GstFlowReturn (*new_preroll) (GstAppSink *appsink, gpointer user_data);
GstFlowReturn (*new_sample) (GstAppSink *appsink, gpointer user_data);
} GstAppSinkCallbacks;
A set of callbacks that can be installed on the appsink with
gst_app_sink_set_callbacks().
Called when the end-of-stream has been reached. This callback is called from the steaming thread. |
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Called when a new preroll sample is available.
This callback is called from the steaming thread.
The new preroll sample can be retrieved with
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Called when a new sample is available.
This callback is called from the steaming thread.
The new sample can be retrieved with
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