GStreamer播放教程03——pipeline的快捷访问

目的

《GStreamer08——pipeline的快捷访问》展示了一个应用如何用appsrc和appsink这两个特殊的element在pipeline中手动输入/提取数据。playbin2也允许使用这两个element，但连接它们的方法有所不同。连接appsink到playbin2的方法在后面还会提到。这里我们主要讲述：

如何把appsrc连接到playbin2

如何配置appsrc

一个playbin2波形发生器

#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,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 feedingn");
    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 feedingn");
    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: %sn",GST_OBJECT_NAME (msg->src),err->message);
  g_printerr ("Debugging information: %sn",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,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;
}

把appsrc用作pipeline的source，仅仅把playbin2的UIR设置成appsrc://即可。

  /* Create the playbin2 element */
  data.pipeline = gst_parse_launch ("playbin2 uri=appsrc://",NULL);

playbin2创建一个内部的appsrc element并且发送source-setup信号来通知应用进行设置。

  g_signal_connect (data.pipeline,&data);

特别地，设置appsrc的caps属性是很重要的，因为一旦这个信号的处理返回，playbin2就会根据返回值来初始化下一个element。

/* This function is called when playbin2 has created the appsrc element,data);
  gst_caps_unref (audio_caps);
  g_free (audio_caps_text);
}

appsrc的配置和《GStreamer08——pipeline的快捷访问》里面一样：caps设置成audio/x-raw-int，注册两个回调，这样element可以在需要/停止给它推送数据时通知应用。具体细节请参考《GStreamer08——pipeline的快捷访问》。

在这个点之后，playbin2接管处理了剩下的pipeline，应用仅仅需要生成数据即可。

至于使用appsink来从从playbin2里面提取数据，在后面的教程里面再讲述。

（编辑：李大同）

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