GStreamer基礎教程07 – 播放速率控制

  • 2019 年 10 月 3 日
  • 筆記

摘要

  在常見的媒體播放器中,通常可以看到快進,快退,慢放等功能,這部分功能被稱為“特技模式(Trick Mode)”,這些模式有個共同點:都通過修改播放的速率來達到相應的目的。 本文將介紹如何通過GStreamer去實現快進,快退,慢放以及單幀播放。

 

GStreamer Seek與Step事件

  快進(Fast-Forward),快退(Fast-Rewind)和慢放(Slow-Motion)都是通過修改播放的速率來達到相應的目的。在GStreamer中,將1倍速作為正常的播放速率,將大於1倍速的2倍,4倍,8倍等倍速稱為快進,慢放則是播放速率的絕對值小於1倍速,當播放速率小於0時,則進行倒放。
在GStreamer中,我們通過seek與step事件來控制Element的播放速率及區域。Step事件允許跳過指定的區域並設置後續的播放速率(此速率必須大於0)。Seek事件允許跳轉到播放文件中的的任何位置,並且播放速率可以大於0或小於0.
  在播放時間控制中,我們使用gst_element_seek_simple 來快速的跳轉到指定的位置,此函數是對seek事件的封裝。實際使用時,我們首先需要構造一個seek event,設置seek的絕對起始位置和停止位置,停止位置可以設置為0,這樣會執行seek的播放速率直到結束。同時可以支援按buffer的方式進行seek,以及設置不同的標誌指定seek的行為。
  Step事件相較於Seek事件需要更少的參數,更易用於修改播放速率,但是不夠靈活。Step事件只會作用於最終的sink,Seek事件則可以作用於Pipeline中所有的Element。Step操作的效率高於Seek。
  在GStreamer中,單幀播放(Frame Stepping)與快進相同,也是通過事件實現。單幀播放通常在暫停的狀態下,構造並發送step event每次播放一幀。
  需要注意的是,seek event需要直接作用於sink element(eg: audio sink或video sink),如果直接將seek event作用於Pipeline,Pipeline會自動將事件轉發給所有的sink,如果有多個sink,就會造成多次seek。通常是先獲取Pipeline中的video-sink或audio-sink,然後發送seek event到指定的sink,完成seek的操作。 Seek時間的構造及發送示例如下:

   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");     ...

示例程式碼

下面通過一個完整的示例,來查看GStreamer是如何通過seek和step達到相應的播放速度。

#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.n");      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: %gn", 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 %sn", 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 framen");        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:n"        " 'P' to toggle between PAUSE and PLAYn"        " 'S' to increase playback speed, 's' to decrease playback speedn"        " 'D' to toggle playback directionn"        " 'N' to move to next frame (in the current direction, better in PAUSE)n"        " 'Q' to quitn");      /* 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.n");      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;  }

  通過下面的命令編譯即可得到可執行文件,在終端輸入相應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`

源碼分析

  本例中,Pipeline的創建與其他示例相同,通過playbin播放文件,採用GLib的I/O介面來處理鍵盤輸入。

/* 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 %sn", data->playing ? "PLAYING" : "PAUSE");      break;

  在終端輸入P時,使用gst_element_set_state ()設置播放狀態。

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;

  通過S和s增加和降低播放速度,d用於改變播放方向(倒放),這裡在修改rate後,調用send_seek_event實現真正的處理。

/* 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.n");      return;    }

  這個函數會構造一個SeekEvent發送到Pipeline以調節速率。因為Seek Event會跳轉到指定的位置,但我們在此例匯總只想改變速率,不跳轉到其他位置,所以首先通過gst_element_query_position ()獲取當前的播放位置。

/* 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);  }

  通過gst_event_new_seek()創建SeekEvent,設置新的rate,flag,起始位置,結束位置。需要注意的是,起始位置需要小於結束位置。

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,為了避免Pipeline執行多次的seek,我們在此處獲取video-sink,並向其發送SeekEvent。我們直到執行Seek時才獲取video-sink是因為實際的sink有可能會根據不同的媒體類型,在PLAYING狀態時才創建。

  以上部分內容就是速率的修改,關於單幀播放的情況,實現方式更加簡單:

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 framen");    break;

  我們通過gst_event_new_step()創建了StepEvent,並指定只跳轉一幀,且不改變當前速率。單幀播放通常都是先暫停,然後再進行單幀播放。

  

  以上就是通過GStreamer實現播放速率的控制,實際中,有些Element對倒放支援不是很好,不能達到理想的效果。

 

總結

通過本文我們掌握了:

  • 如何通過gst_event_new_seek()構造SeekEvent,通過gst_element_send_event()發送到sink改變速率。
  • 如何通過gst_event_new_step()實現單幀播放。

 

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
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