GStreamer基础教程07

摘要

　　在常见的媒体播放器中，通常可以看到快进，快退，慢放等功能，这部分功能被称为“特技模式（Trick Mode）”，这些模式有个共同点：都通过修改播放的速率来达到相应的目的。 本文将介绍如何通过GStreamer去实现快进，快退，慢放以及单帧播放。

GStreamer Seek与Step事件

　　快进（Fast-Forward），快退（Fast-Rewind）和慢放（Slow-Motion）都是通过修改播放的速率来达到相应的目的。在GStreamer中，将1倍速作为正常的播放速率，将大于1倍速的2倍，4倍，8倍等倍速称为快进，慢放则是播放速率的绝对值小于1倍速，当播放速率小于0时，则进行倒放。
在GStreamer中，我们通过seek与step事件来控制Element的播放速率及区域。Step事件允许跳过指定的区域并设置后续的播放速率（此速率必须大于0）。Seek事件允许跳转到播放文件中的的任何位置，并且播放速率可以大于0或小于0.
　　在播放时间控制中，我们使用gst_element_seek_simple 来快速的跳转到指定的位置，此函数是对seek事件的封装。实际使用时，我们首先需要构造一个seek event，设置seek的绝对起始位置和停止位置，停止位置可以设置为0，这样会执行seek的播放速率直到结束。同时可以支持按buffer的方式进行seek，以及设置不同的标志指定seek的行为。
　　Step事件相较于Seek事件需要更少的参数，更易用于修改播放速率，但是不够灵活。Step事件只会作用于最终的sink，Seek事件则可以作用于Pipeline中所有的Element。Step操作的效率高于Seek。
　　在GStreamer中，单帧播放(Frame Stepping）与快进相同，也是通过事件实现。单帧播放通常在暂停的状态下，构造并发送step event每次播放一帧。
　　需要注意的是，seek event需要直接作用于sink element（eg： audio sink或video sink），如果直接将seek event作用于Pipeline，Pipeline会自动将事件转发给所有的sink，如果有多个sink，就会造成多次seek。通常是先获取Pipeline中的video-sink或audio-sink，然后发送seek event到指定的sink，完成seek的操作。 Seek时间的构造及发送示例如下：

   GstEvent *event;
   gboolean result;
   ...
   // construct a seek event to play the media from second 2 to 5, flush
   // the pipeline to decrease latency.
   event = gst_event_new_seek (1.0,
      GST_FORMAT_TIME,
      GST_SEEK_FLAG_FLUSH,
      GST_SEEK_TYPE_SET, 2 * GST_SECOND,
      GST_SEEK_TYPE_SET, 5 * GST_SECOND);
   ...
   result = gst_element_send_event (video_sink, event);
   if (!result)
     g_warning ("seek failed");
   ...

示例代码

下面通过一个完整的示例，来查看GStreamer是如何通过seek和step达到相应的播放速度。

#include <string.h>
#include <stdio.h>
#include <gst/gst.h>

typedef struct _CustomData
{
  GstElement *pipeline;
  GstElement *video_sink;
  GMainLoop *loop;

  gboolean playing;             /* Playing or Paused */
  gdouble rate;                 /* Current playback rate (can be negative) */
} CustomData;

/* Send seek event to change rate */
static void
send_seek_event (CustomData * data)
{
  gint64 position;
  GstEvent *seek_event;

  /* Obtain the current position, needed for the seek event */
  if (!gst_element_query_position (data->pipeline, GST_FORMAT_TIME, &position)) {
    g_printerr ("Unable to retrieve current position.
");
    return;
  }

  /* Create the seek event */
  if (data->rate > 0) {
    seek_event =
        gst_event_new_seek (data->rate, GST_FORMAT_TIME,
        GST_SEEK_FLAG_FLUSH | GST_SEEK_FLAG_ACCURATE, GST_SEEK_TYPE_SET,
        position, GST_SEEK_TYPE_END, 0);
  } else {
    seek_event =
        gst_event_new_seek (data->rate, GST_FORMAT_TIME,
        GST_SEEK_FLAG_FLUSH | GST_SEEK_FLAG_ACCURATE, GST_SEEK_TYPE_SET, 0,
        GST_SEEK_TYPE_SET, position);
  }

  if (data->video_sink == NULL) {
    /* If we have not done so, obtain the sink through which we will send the seek events */
    g_object_get (data->pipeline, "video-sink", &data->video_sink, NULL);
  }

  /* Send the event */
  gst_element_send_event (data->video_sink, seek_event);

  g_print ("Current rate: %g
", data->rate);
}

/* Process keyboard input */
static gboolean
handle_keyboard (GIOChannel * source, GIOCondition cond, CustomData * data)
{
  gchar *str = NULL;

  if (g_io_channel_read_line (source, &str, NULL, NULL,
          NULL) != G_IO_STATUS_NORMAL) {
    return TRUE;
  }

  switch (g_ascii_tolower (str[0])) {
    case 'p':
      data->playing = !data->playing;
      gst_element_set_state (data->pipeline,
          data->playing ? GST_STATE_PLAYING : GST_STATE_PAUSED);
      g_print ("Setting state to %s
", data->playing ? "PLAYING" : "PAUSE");
      break;
    case 's':
      if (g_ascii_isupper (str[0])) {
        data->rate *= 2.0;
      } else {
        data->rate /= 2.0;
      }
      send_seek_event (data);
      break;
    case 'd':
      data->rate *= -1.0;
      send_seek_event (data);
      break;
    case 'n':
      if (data->video_sink == NULL) {
        /* If we have not done so, obtain the sink through which we will send the step events */
        g_object_get (data->pipeline, "video-sink", &data->video_sink, NULL);
      }

      gst_element_send_event (data->video_sink,
          gst_event_new_step (GST_FORMAT_BUFFERS, 1, ABS (data->rate), TRUE,
              FALSE));
      g_print ("Stepping one frame
");
      break;
    case 'q':
      g_main_loop_quit (data->loop);
      break;
    default:
      break;
  }

  g_free (str);

  return TRUE;
}

int
main (int argc, char *argv[])
{
  CustomData data;
  GstStateChangeReturn ret;
  GIOChannel *io_stdin;

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

  /* Initialize our data structure */
  memset (&data, 0, sizeof (data));

  /* Print usage map */
  g_print ("USAGE: Choose one of the following options, then press enter:
"
      " 'P' to toggle between PAUSE and PLAY
"
      " 'S' to increase playback speed, 's' to decrease playback speed
"
      " 'D' to toggle playback direction
"
      " 'N' to move to next frame (in the current direction, better in PAUSE)
"
      " 'Q' to quit
");

  /* Build the pipeline */
  data.pipeline =
      gst_parse_launch
      ("playbin uri=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm",
      NULL);

  /* Add a keyboard watch so we get notified of keystrokes */
#ifdef G_OS_WIN32
  io_stdin = g_io_channel_win32_new_fd (fileno (stdin));
#else
  io_stdin = g_io_channel_unix_new (fileno (stdin));
#endif
  g_io_add_watch (io_stdin, G_IO_IN, (GIOFunc) handle_keyboard, &data);

  /* Start playing */
  ret = gst_element_set_state (data.pipeline, GST_STATE_PLAYING);
  if (ret == GST_STATE_CHANGE_FAILURE) {
    g_printerr ("Unable to set the pipeline to the playing state.
");
    gst_object_unref (data.pipeline);
    return -1;
  }
  data.playing = TRUE;
  data.rate = 1.0;

  /* Create a GLib Main Loop and set it to run */
  data.loop = g_main_loop_new (NULL, FALSE);
  g_main_loop_run (data.loop);

  /* Free resources */
  g_main_loop_unref (data.loop);
  g_io_channel_unref (io_stdin);
  gst_element_set_state (data.pipeline, GST_STATE_NULL);
  if (data.video_sink != NULL)
    gst_object_unref (data.video_sink);
  gst_object_unref (data.pipeline);
  return 0;
}

　　通过下面的命令编译即可得到可执行文件，在终端输入相应指令可修改播放速率。

gcc basic-tutorial-7.c -o basic-tutorial-7 `pkg-config --cflags --libs gstreamer-1.0`

源码分析

　　本例中，Pipeline的创建与其他示例相同，通过playbin播放文件，采用GLib的I/O接口来处理键盘输入。

/* Process keyboard input */
static gboolean handle_keyboard (GIOChannel *source, GIOCondition cond, CustomData *data) {
  gchar *str = NULL;

  if (g_io_channel_read_line (source, &str, NULL, NULL, NULL) != G_IO_STATUS_NORMAL) {
    return TRUE;
  }

  switch (g_ascii_tolower (str[0])) {
  case 'p':
    data->playing = !data->playing;
    gst_element_set_state (data->pipeline, data->playing ? GST_STATE_PLAYING : GST_STATE_PAUSED);
    g_print ("Setting state to %s
", data->playing ? "PLAYING" : "PAUSE");
    break;

　　在终端输入P时，使用gst_element_set_state ()设置播放状态。

case 's':
  if (g_ascii_isupper (str[0])) {
    data->rate *= 2.0;
  } else {
    data->rate /= 2.0;
  }
  send_seek_event (data);
  break;
case 'd':
  data->rate *= -1.0;
  send_seek_event (data);
  break;

　　通过S和s增加和降低播放速度，d用于改变播放方向（倒放），这里在修改rate后，调用send_seek_event实现真正的处理。

/* Send seek event to change rate */
static void send_seek_event (CustomData *data) {
  gint64 position;
  GstEvent *seek_event;

  /* Obtain the current position, needed for the seek event */
  if (!gst_element_query_position (data->pipeline, GST_FORMAT_TIME, &position)) {
    g_printerr ("Unable to retrieve current position.
");
    return;
  }

　　这个函数会构造一个SeekEvent发送到Pipeline以调节速率。因为Seek Event会跳转到指定的位置，但我们在此例汇总只想改变速率，不跳转到其他位置，所以首先通过gst_element_query_position ()获取当前的播放位置。

/* Create the seek event */
if (data->rate > 0) {
  seek_event = gst_event_new_seek (data->rate, GST_FORMAT_TIME, GST_SEEK_FLAG_FLUSH | GST_SEEK_FLAG_ACCURATE,
      GST_SEEK_TYPE_SET, position, GST_SEEK_TYPE_END, 0);
} else {
  seek_event = gst_event_new_seek (data->rate, GST_FORMAT_TIME, GST_SEEK_FLAG_FLUSH | GST_SEEK_FLAG_ACCURATE,
      GST_SEEK_TYPE_SET, 0, GST_SEEK_TYPE_SET, position);
}

　　通过gst_event_new_seek()创建SeekEvent，设置新的rate，flag，起始位置，结束位置。需要注意的是，起始位置需要小于结束位置。

if (data->video_sink == NULL) {
  /* If we have not done so, obtain the sink through which we will send the seek events */
  g_object_get (data->pipeline, "video-sink", &data->video_sink, NULL);
}
/* Send the event */
gst_element_send_event (data->video_sink, seek_event);

　　正如上文提到的，为了避免Pipeline执行多次的seek，我们在此处获取video-sink，并向其发送SeekEvent。我们直到执行Seek时才获取video-sink是因为实际的sink有可能会根据不同的媒体类型，在PLAYING状态时才创建。

　　以上部分内容就是速率的修改，关于单帧播放的情况，实现方式更加简单：

case 'n':
  if (data->video_sink == NULL) {
    /* If we have not done so, obtain the sink through which we will send the step events */
    g_object_get (data->pipeline, "video-sink", &data->video_sink, NULL);
  }

  gst_element_send_event (data->video_sink,
      gst_event_new_step (GST_FORMAT_BUFFERS, 1, ABS (data->rate), TRUE, FALSE));
  g_print ("Stepping one frame
");
  break;

　　我们通过gst_event_new_step()创建了StepEvent，并指定只跳转一帧，且不改变当前速率。单帧播放通常都是先暂停，然后再进行单帧播放。

　　

　　以上就是通过GStreamer实现播放速率的控制，实际中，有些Element对倒放支持不是很好，不能达到理想的效果。

总结

通过本文我们掌握了：

• 如何通过gst_event_new_seek()构造SeekEvent，通过gst_element_send_event()发送到sink改变速率。
• 如何通过gst_event_new_step()实现单帧播放。

引用

https://gstreamer.freedesktop.org/documentation/tutorials/basic/playback-speed.html?gi-language=c
https://gstreamer.freedesktop.org/documentation/additional/design/seeking.html?gi-language=c
https://gstreamer.freedesktop.org/documentation/additional/design/framestep.html?gi-language=c

作者：John.Leng

出处：http://www.cnblogs.com/xleng/

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