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GStreamer SDK documentation : Playback tutorial 3: Short-cutting the pipeline
This page last changed on Jun 26, 2012 by xartigas.
Goal
Basic tutorial 8: Short-cutting the
pipeline showed
how an application can manually extract or inject data into a pipeline
by using two special elements called appsrc
and appsink
.
playbin2
allows using these elements too, but the method to connect
them is different. To connect an appsink
to playbin2
see Playback
tutorial 7: Custom playbin2
sinks.
This tutorial shows:
- How to connect
appsrc
withplaybin2
- How to configure the
appsrc
A playbin2 waveform generator
Copy this code into a text file named playback-tutorial-3.c
.
![]() |
This tutorial is included in the SDK since release 2012.7. If you cannot find it in the downloaded code, please install the latest release of the GStreamer SDK. |
playback-tutorial-3.c
#include <gst/gst.h>
#include <string.h>
#define CHUNK_SIZE 1024 /* Amount of bytes we are sending in each buffer */
#define SAMPLE_RATE 44100 /* Samples per second we are sending */
#define AUDIO_CAPS "audio/x-raw-int,channels=1,rate=%d,signed=(boolean)true,width=16,depth=16,endianness=BYTE_ORDER"
/* Structure to contain all our information, so we can pass it to callbacks */
typedef struct _CustomData {
GstElement *pipeline;
GstElement *app_source;
guint64 num_samples; /* Number of samples generated so far (for timestamp generation) */
gfloat a, b, c, d; /* For waveform generation */
guint sourceid; /* To control the GSource */
GMainLoop *main_loop; /* GLib's Main Loop */
} CustomData;
/* This method is called by the idle GSource in the mainloop, to feed CHUNK_SIZE bytes into appsrc.
* The ide handler is added to the mainloop when appsrc requests us to start sending data (need-data signal)
* and is removed when appsrc has enough data (enough-data signal).
*/
static gboolean push_data (CustomData *data) {
GstBuffer *buffer;
GstFlowReturn ret;
int i;
gint16 *raw;
gint num_samples = CHUNK_SIZE / 2; /* Because each sample is 16 bits */
gfloat freq;
/* Create a new empty buffer */
buffer = gst_buffer_new_and_alloc (CHUNK_SIZE);
/* Set its timestamp and duration */
GST_BUFFER_TIMESTAMP (buffer) = gst_util_uint64_scale (data->num_samples, GST_SECOND, SAMPLE_RATE);
GST_BUFFER_DURATION (buffer) = gst_util_uint64_scale (CHUNK_SIZE, GST_SECOND, SAMPLE_RATE);
/* Generate some psychodelic waveforms */
raw = (gint16 *)GST_BUFFER_DATA (buffer);
data->c += data->d;
data->d -= data->c / 1000;
freq = 1100 + 1000 * data->d;
for (i = 0; i < num_samples; i++) {
data->a += data->b;
data->b -= data->a / freq;
raw[i] = (gint16)(500 * data->a);
}
data->num_samples += num_samples;
/* Push the buffer into the appsrc */
g_signal_emit_by_name (data->app_source, "push-buffer", buffer, &ret);
/* Free the buffer now that we are done with it */
gst_buffer_unref (buffer);
if (ret != GST_FLOW_OK) {
/* We got some error, stop sending data */
return FALSE;
}
return TRUE;
}
/* This signal callback triggers when appsrc needs data. Here, we add an idle handler
* to the mainloop to start pushing data into the appsrc */
static void start_feed (GstElement *source, guint size, CustomData *data) {
if (data->sourceid == 0) {
g_print ("Start feeding\n");
data->sourceid = g_idle_add ((GSourceFunc) push_data, data);
}
}
/* This callback triggers when appsrc has enough data and we can stop sending.
* We remove the idle handler from the mainloop */
static void stop_feed (GstElement *source, CustomData *data) {
if (data->sourceid != 0) {
g_print ("Stop feeding\n");
g_source_remove (data->sourceid);
data->sourceid = 0;
}
}
/* This function is called when an error message is posted on the bus */
static void error_cb (GstBus *bus, GstMessage *msg, CustomData *data) {
GError *err;
gchar *debug_info;
/* Print error details on the screen */
gst_message_parse_error (msg, &err, &debug_info);
g_printerr ("Error received from element %s: %s\n", GST_OBJECT_NAME (msg->src), err->message);
g_printerr ("Debugging information: %s\n", debug_info ? debug_info : "none");
g_clear_error (&err);
g_free (debug_info);
g_main_loop_quit (data->main_loop);
}
/* This function is called when playbin2 has created the appsrc element, so we have
* a chance to configure it. */
static void source_setup (GstElement *pipeline, GstElement *source, CustomData *data) {
gchar *audio_caps_text;
GstCaps *audio_caps;
g_print ("Source has been created. Configuring.\n");
data->app_source = source;
/* Configure appsrc */
audio_caps_text = g_strdup_printf (AUDIO_CAPS, SAMPLE_RATE);
audio_caps = gst_caps_from_string (audio_caps_text);
g_object_set (source, "caps", audio_caps, NULL);
g_signal_connect (source, "need-data", G_CALLBACK (start_feed), data);
g_signal_connect (source, "enough-data", G_CALLBACK (stop_feed), data);
gst_caps_unref (audio_caps);
g_free (audio_caps_text);
}
int main(int argc, char *argv[]) {
CustomData data;
GstBus *bus;
/* Initialize cumstom data structure */
memset (&data, 0, sizeof (data));
data.b = 1; /* For waveform generation */
data.d = 1;
/* Initialize GStreamer */
gst_init (&argc, &argv);
/* Create the playbin2 element */
data.pipeline = gst_parse_launch ("playbin2 uri=appsrc://", NULL);
g_signal_connect (data.pipeline, "source-setup", G_CALLBACK (source_setup), &data);
/* Instruct the bus to emit signals for each received message, and connect to the interesting signals */
bus = gst_element_get_bus (data.pipeline);
gst_bus_add_signal_watch (bus);
g_signal_connect (G_OBJECT (bus), "message::error", (GCallback)error_cb, &data);
gst_object_unref (bus);
/* Start playing the pipeline */
gst_element_set_state (data.pipeline, GST_STATE_PLAYING);
/* Create a GLib Main Loop and set it to run */
data.main_loop = g_main_loop_new (NULL, FALSE);
g_main_loop_run (data.main_loop);
/* Free resources */
gst_element_set_state (data.pipeline, GST_STATE_NULL);
gst_object_unref (data.pipeline);
return 0;
}
To use an appsrc
as the source for the pipeline, simply instantiate a
playbin2
and set its URI to appsrc://
/* Create the playbin2 element */
data.pipeline = gst_parse_launch ("playbin2 uri=appsrc://", NULL);
playbin2
will create an internal appsrc
element and fire the
source-setup
signal to allow the application to configure
it:
g_signal_connect (data.pipeline, "source-setup", G_CALLBACK (source_setup), &data);
In particular, it is important to set the caps property of appsrc
,
since, once the signal handler returns, playbin2
will instantiate the
next element in the pipeline according to these
caps:
/* This function is called when playbin2 has created the appsrc element, so we have
* a chance to configure it. */
static void source_setup (GstElement *pipeline, GstElement *source, CustomData *data) {
gchar *audio_caps_text;
GstCaps *audio_caps;
g_print ("Source has been created. Configuring.\n");
data->app_source = source;
/* Configure appsrc */
audio_caps_text = g_strdup_printf (AUDIO_CAPS, SAMPLE_RATE);
audio_caps = gst_caps_from_string (audio_caps_text);
g_object_set (source, "caps", audio_caps, NULL);
g_signal_connect (source, "need-data", G_CALLBACK (start_feed), data);
g_signal_connect (source, "enough-data", G_CALLBACK (stop_feed), data);
gst_caps_unref (audio_caps);
g_free (audio_caps_text);
}
The configuration of the appsrc
is exactly the same as in Basic
tutorial 8: Short-cutting the
pipeline:
the caps are set to audio/x-raw-int
, and two callbacks are registered,
so the element can tell the application when it needs to start and stop
pushing data. See Basic tutorial 8: Short-cutting the
pipeline
for more details.
From this point onwards, playbin2
takes care of the rest of the
pipeline, and the application only needs to worry about generating more
data when told so.
To learn how data can be extracted from playbin2
using the
appsink
element, see Playback tutorial 7: Custom playbin2
sinks.
Conclusion
This tutorial applies the concepts shown in Basic tutorial 8:
Short-cutting the
pipeline to
playbin2
. In particular, it has shown:
- How to connect
appsrc
withplaybin2
using the special URIappsrc://
- How to configure the
appsrc
using thesource-setup
signal
It has been a pleasure having you here, and see you soon!
Attachments:
playback-tutorial-3.c (text/plain)
vs2010.zip (application/zip)
playback-tutorial-3.c (text/plain)
Document generated by Confluence on Oct 08, 2015 10:27