Basic tutorial 4: gstreamer time management

Abstract	gstreamer time management
Authors	Walter Fan
Status	WIP as draft
Updated	2024-08-21

目标 

本教程将展示如何使用 GStreamer 关于时间的相关设施。特别是：

如何查询管道以获取流位置或持续时间等信息。
如何在媒体流内搜索（跳转）到不同位置（时间）。

简介 

GstQuery 是一种允许向元素或 pad 询问信息的机制。在此示例中，我们询问管道是否允许搜索（某些源，如直播流，不允许搜索）。如果允许，那么一旦电影播放了十秒，我们就会使用搜索跳转到不同位置。

在之前的教程中，一旦我们设置并运行了管道，我们的主要功能就会等待通过总线接收 ERROR 或 EOS。

在这里，我们修改此功能以定期唤醒并查询管道以获取流位置，以便我们可以将其打印在屏幕上。这类似于媒体播放器会定期更新用户界面。

最后，每当流持续时间发生变化时，都会对其进行查询和更新。

代码 

完整代码如下

#include <gst/gst.h>

/* Structure to contain all our information, so we can pass it around */
typedef struct _CustomData {
   GstElement *playbin;  /* Our one and only element */
   gboolean playing;      /* Are we in the PLAYING state? */
   gboolean terminate;    /* Should we terminate execution? */
   gboolean seek_enabled; /* Is seeking enabled for this media? */
   gboolean seek_done;    /* Have we performed the seek already? */
   gint64 duration;       /* How long does this media last, in nanoseconds */
} CustomData;

/* Forward definition of the message processing function */
static void handle_message (CustomData *data, GstMessage *msg);

int main(int argc, char *argv[]) {
   CustomData data;
   GstBus *bus;
   GstMessage *msg;
   GstStateChangeReturn ret;

   data.playing = FALSE;
   data.terminate = FALSE;
   data.seek_enabled = FALSE;
   data.seek_done = FALSE;
   data.duration = GST_CLOCK_TIME_NONE;

   /* Initialize GStreamer */
   gst_init (&argc, &argv);

   /* Create the elements */
   data.playbin = gst_element_factory_make ("playbin", "playbin");

   if (!data.playbin) {
      g_printerr ("Not all elements could be created.\n");
      return -1;
   }

   /* Set the URI to play */
   g_object_set (data.playbin, "uri", "https://gstreamer.freedesktop.org/data/media/sintel_trailer-480p.webm", NULL);

   /* Start playing */
   ret = gst_element_set_state (data.playbin, GST_STATE_PLAYING);
   if (ret == GST_STATE_CHANGE_FAILURE) {
      g_printerr ("Unable to set the pipeline to the playing state.\n");
      gst_object_unref (data.playbin);
      return -1;
   }

   /* Listen to the bus */
   bus = gst_element_get_bus (data.playbin);
   do {
      msg = gst_bus_timed_pop_filtered (bus, 100 * GST_MSECOND,
         GST_MESSAGE_STATE_CHANGED | GST_MESSAGE_ERROR | GST_MESSAGE_EOS | GST_MESSAGE_DURATION);

      /* Parse message */
      if (msg != NULL) {
         handle_message (&data, msg);
      } else {
         /* We got no message, this means the timeout expired */
         if (data.playing) {
         gint64 current = -1;

         /* Query the current position of the stream */
         if (!gst_element_query_position (data.playbin, GST_FORMAT_TIME, &current)) {
            g_printerr ("Could not query current position.\n");
         }

         /* If we didn't know it yet, query the stream duration */
         if (!GST_CLOCK_TIME_IS_VALID (data.duration)) {
            if (!gst_element_query_duration (data.playbin, GST_FORMAT_TIME, &data.duration)) {
               g_printerr ("Could not query current duration.\n");
            }
         }

         /* Print current position and total duration */
         g_print ("Position %" GST_TIME_FORMAT " / %" GST_TIME_FORMAT "\r",
               GST_TIME_ARGS (current), GST_TIME_ARGS (data.duration));

         /* If seeking is enabled, we have not done it yet, and the time is right, seek */
         if (data.seek_enabled && !data.seek_done && current > 10 * GST_SECOND) {
            g_print ("\nReached 10s, performing seek...\n");
            gst_element_seek_simple (data.playbin, GST_FORMAT_TIME,
               GST_SEEK_FLAG_FLUSH | GST_SEEK_FLAG_KEY_UNIT, 30 * GST_SECOND);
            data.seek_done = TRUE;
         }
         }
      }
   } while (!data.terminate);

   /* Free resources */
   gst_object_unref (bus);
   gst_element_set_state (data.playbin, GST_STATE_NULL);
   gst_object_unref (data.playbin);
   return 0;
}

static void handle_message (CustomData *data, GstMessage *msg) {
   GError *err;
   gchar *debug_info;

   switch (GST_MESSAGE_TYPE (msg)) {
      case GST_MESSAGE_ERROR:
         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);
         data->terminate = TRUE;
         break;
      case GST_MESSAGE_EOS:
         g_print ("\nEnd-Of-Stream reached.\n");
         data->terminate = TRUE;
         break;
      case GST_MESSAGE_DURATION:
         /* The duration has changed, mark the current one as invalid */
         data->duration = GST_CLOCK_TIME_NONE;
         break;
      case GST_MESSAGE_STATE_CHANGED: {
         GstState old_state, new_state, pending_state;
         gst_message_parse_state_changed (msg, &old_state, &new_state, &pending_state);
         if (GST_MESSAGE_SRC (msg) == GST_OBJECT (data->playbin)) {
         g_print ("Pipeline state changed from %s to %s:\n",
               gst_element_state_get_name (old_state), gst_element_state_get_name (new_state));

         /* Remember whether we are in the PLAYING state or not */
         data->playing = (new_state == GST_STATE_PLAYING);

         if (data->playing) {
            /* We just moved to PLAYING. Check if seeking is possible */
            GstQuery *query;
            gint64 start, end;
            query = gst_query_new_seeking (GST_FORMAT_TIME);
            if (gst_element_query (data->playbin, query)) {
               gst_query_parse_seeking (query, NULL, &data->seek_enabled, &start, &end);
               if (data->seek_enabled) {
               g_print ("Seeking is ENABLED from %" GST_TIME_FORMAT " to %" GST_TIME_FORMAT "\n",
                     GST_TIME_ARGS (start), GST_TIME_ARGS (end));
               } else {
               g_print ("Seeking is DISABLED for this stream.\n");
               }
            }
            else {
               g_printerr ("Seeking query failed.");
            }
            gst_query_unref (query);
         }
         }
      } break;
      default:
         /* We should not reach here */
         g_printerr ("Unexpected message received.\n");
         break;
   }
   gst_message_unref (msg);
}

Basic tutorial 4: gstreamer time management

目标

简介

代码

目标 

简介 

代码 