GStreamer播放教程02——字幕管理

原文：https://gstreamer.freedesktop.org/documentation/tutorials/playback/index.html

译文原文：http://blog.youkuaiyun.com/sakulafly/article/details/22324275

原码：git clone git://anongit.freedesktop.org/gstreamer/gst-docs

编译：gcc playback-tutorial-2.c -o playback-tutorial-2 `pkg-config --cflags --libs gstreamer-1.0`

Need help?

If you need help to compile this code, refer to the Building the tutorials section for your platform: [Linux], [Mac OS X] or [Windows], or use this specific command on Linux:

gcc playback-tutorial-2.c -o playback-tutorial-2 `pkg-config --cflags --libs gstreamer-1.0`

If you need help to run this code, refer to the Running the tutorials section for your platform: [Linux][1], [Mac OS X][2] or [Windows][3].

This tutorial opens a window and displays a movie, with accompanying audio. The media is fetched from the Internet, so the window might take a few seconds to appear, depending on your connection speed. The number of subtitle streams is shown in the terminal, and the user can switch from one to another by entering a number and pressing enter. A small delay is to be expected. Please read the note at the bottom of this page. Bear in mind that there is no latency management (buffering), so on slow connections, the movie might stop after a few seconds. See how Basic tutorial 12: Streaming solves this issue.

Required libraries: gstreamer-1.0

目标

      这篇教程和上一篇非常相似，但不是切换音频流，而是字幕了。这次我们会展示：

      如何选择选择字幕流

      如何引入外部的字幕

      如何客制化字幕使用的字体

介绍

      我们都知道一个文件可以有多个音视频流并且可以使用playerbin2的current-audio和current-video属性很方便的进行切换。切换字幕也是一样的方便。

      就和音视频一样，playbin2会选择解码好的字幕，而且GStreamer的插件设计也很容易支持一种新的文件结构。

      但字幕还是有自己的特殊之处，除了可以嵌入文件里面，playbin2还支持使用外界的URI来提供字幕。

      本教程会打开一个包含5个字幕流的文件，并从外界在导入一个字幕（希腊语）。

多语言字幕的播放器

#include <gst/gst.h>
  
/* Structure to contain all our information, so we can pass it around */
typedef struct _CustomData {
  GstElement *playbin2;  /* Our one and only element */
  
  gint n_video;          /* Number of embedded video streams */
  gint n_audio;          /* Number of embedded audio streams */
  gint n_text;           /* Number of embedded subtitle streams */
  
  gint current_video;    /* Currently playing video stream */
  gint current_audio;    /* Currently playing audio stream */
  gint current_text;     /* Currently playing subtitle stream */
  
  GMainLoop *main_loop;  /* GLib's Main Loop */
} CustomData;
  
/* playbin2 flags */
typedef enum {
  GST_PLAY_FLAG_VIDEO         = (1 << 0), /* We want video output */
  GST_PLAY_FLAG_AUDIO         = (1 << 1), /* We want audio output */
  GST_PLAY_FLAG_TEXT          = (1 << 2)  /* We want subtitle output */
} GstPlayFlags;
  
/* Forward definition for the message and keyboard processing functions */
static gboolean handle_message (GstBus *bus, GstMessage *msg, CustomData *data);
static gboolean handle_keyboard (GIOChannel *source, GIOCondition cond, CustomData *data);
  
int main(int argc, char *argv[]) {
  CustomData data;
  GstBus *bus;
  GstStateChangeReturn ret;
  gint flags;
  GIOChannel *io_stdin;
  
  /* Initialize GStreamer */
  gst_init (&argc, &argv);
   
  /* Create the elements */
  data.playbin2 = gst_element_factory_make ("playbin2", "playbin2");
  
  if (!data.playbin2) {
    g_printerr ("Not all elements could be created.\n");
    return -1;
  }
  
  /* Set the URI to play */
  g_object_set (data.playbin2, "uri", "http://docs.gstreamer.com/media/sintel_trailer-480p.ogv", NULL);
  
  /* Set the subtitle URI to play and some font description */
  g_object_set (data.playbin2, "suburi", "http://docs.gstreamer.com/media/sintel_trailer_gr.srt", NULL);
  g_object_set (data.playbin2, "subtitle-font-desc", "Sans, 18", NULL);
  
  /* Set flags to show Audio, Video and Subtitles */
  g_object_get (data.playbin2, "flags", &flags, NULL);
  flags |= GST_PLAY_FLAG_VIDEO | GST_PLAY_FLAG_AUDIO | GST_PLAY_FLAG_TEXT;
  g_object_set (data.playbin2, "flags", flags, NULL);
  
  /* Add a bus watch, so we get notified when a message arrives */
  bus = gst_element_get_bus (data.playbin2);
  gst_bus_add_watch (bus, (GstBusFunc)handle_message, &data);
  
  /* Add a keyboard watch so we get notified of keystrokes */
#ifdef _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.playbin2, 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.playbin2);
    return -1;
  }
  
  /* 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 */
  g_main_loop_unref (data.main_loop);
  g_io_channel_unref (io_stdin);
  gst_object_unref (bus);
  gst_element_set_state (data.playbin2, GST_STATE_NULL);
  gst_object_unref (data.playbin2);
  return 0;
}
  
/* Extract some metadata from the streams and print it on the screen */
static void analyze_streams (CustomData *data) {
  gint i;
  GstTagList *tags;
  gchar *str;
  guint rate;
  
  /* Read some properties */
  g_object_get (data->playbin2, "n-video", &data->n_video, NULL);
  g_object_get (data->playbin2, "n-audio", &data->n_audio, NULL);
  g_object_get (data->playbin2, "n-text", &data->n_text, NULL);
  
  g_print ("%d video stream(s), %d audio stream(s), %d text stream(s)\n",
    data->n_video, data->n_audio, data->n_text);
  
  g_print ("\n");
  for (i = 0; i < data->n_video; i++) {
    tags = NULL;
    /* Retrieve the stream's video tags */
    g_signal_emit_by_name (data->playbin2, "get-video-tags", i, &tags);
    if (tags) {
      g_print ("video stream %d:\n", i);
      gst_tag_list_get_string (tags, GST_TAG_VIDEO_CODEC, &str);
      g_print ("  codec: %s\n", str ? str : "unknown");
      g_free (str);
      gst_tag_list_free (tags);
    }
  }
  
  g_print ("\n");
  for (i = 0; i < data->n_audio; i++) {
    tags = NULL;
    /* Retrieve the stream's audio tags */
    g_signal_emit_by_name (data->playbin2, "get-audio-tags", i, &tags);
    if (tags) {
      g_print ("audio stream %d:\n", i);
      if (gst_tag_list_get_string (tags, GST_TAG_AUDIO_CODEC, &str)) {
        g_print ("  codec: %s\n", str);
        g_free (str);
      }
      if (gst_tag_list_get_string (tags, GST_TAG_LANGUAGE_CODE, &str)) {
        g_print ("  language: %s\n", str);
        g_free (str);
      }
      if (gst_tag_list_get_uint (tags, GST_TAG_BITRATE, &rate)) {
        g_print ("  bitrate: %d\n", rate);
      }
      gst_tag_list_free (tags);
    }
  }
  
  g_print ("\n");
  for (i = 0; i < data->n_text; i++) {
    tags = NULL;
    /* Retrieve the stream's subtitle tags */
    g_print ("subtitle stream %d:\n", i);
    g_signal_emit_by_name (data->playbin2, "get-text-tags", i, &tags);
    if (tags) {
      if (gst_tag_list_get_string (tags, GST_TAG_LANGUAGE_CODE, &str)) {
        g_print ("  language: %s\n", str);
        g_free (str);
      }
      gst_tag_list_free (tags);
    } else {
      g_print ("  no tags found\n");
    }
  }
  
  g_object_get (data->playbin2, "current-video", &data->current_video, NULL);
  g_object_get (data->playbin2, "current-audio", &data->current_audio, NULL);
  g_object_get (data->playbin2, "current-text", &data->current_text, NULL);
  
  g_print ("\n");
  g_print ("Currently playing video stream %d, audio stream %d and subtitle stream %d\n",
      data->current_video, data->current_audio, data->current_text);
  g_print ("Type any number and hit ENTER to select a different subtitle stream\n");
}
  
/* Process messages from GStreamer */
static gboolean handle_message (GstBus *bus, GstMessage *msg, CustomData *data) {
  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);
      g_main_loop_quit (data->main_loop);
      break;
    case GST_MESSAGE_EOS:
      g_print ("End-Of-Stream reached.\n");
      g_main_loop_quit (data->main_loop);
      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->playbin2)) {
        if (new_state == GST_STATE_PLAYING) {
          /* Once we are in the playing state, analyze the streams */
          analyze_streams (data);
        }
      }
    } break;
    default:
      break;
  }
  
  /* We want to keep receiving messages */
  return TRUE;
}
  
/* 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) {
    int index = atoi (str);
    if (index < 0 || index >= data->n_text) {
      g_printerr ("Index out of bounds\n");
    } else {
      /* If the input was a valid subtitle stream index, set the current subtitle stream */
      g_print ("Setting current subtitle stream to %d\n", index);
      g_object_set (data->playbin2, "current-text", index, NULL);
    }
  }
  g_free (str);
  return TRUE;
}

工作流程

      这篇教程和上篇教程的例子只有很小的差别，让我们就看看这些不同的地方吧。

  /* Set the subtitle URI to play and some font description */
  g_object_set (data.playbin2, "suburi", "http://docs.gstreamer.com/media/sintel_trailer_gr.srt", NULL);
  g_object_set (data.playbin2, "subtitle-font-desc", "Sans, 18", NULL);

      在设置媒体URI之后，我们设置了suburi属性，这就让playbin2获得了字幕流的地址。在这个例子里面，文件里本身包含了多个字幕流了，这用suburi来设置的字幕会加入这个列表一起列出，并且是默认选择的。

      注意，在文件里面包含的字幕流是有元数据的（比如字幕的语言），然而外部的字幕流没有元数据。当运行这个例子时你会看到第一个字幕流没有语言的标签。

      subtitle-font-desc属性允许设置字幕的文本字体。因为使用了Pango库来进行字体的渲染，所以具体可以查询相关文档。

      简单概括一下，字符串字体的描述是根据[FAMILY-LIST][STYLE-OPTIONS][SIZE]来的，其中FAMILY-LIST是用逗号隔开的一系列可选字体，STYLE-OPTIONS是用空格来分开的一系列字体样式，SIZE则是字体大小。比如：

      sans bold 12

      serif, monospace bold italic condensed 16

      normal 10

      常见的字体包括：Normal，Sans，Serif和Monospace。

      常用的样式包括：Normal，Oblique，Italic

      常见的粗细包括：Ultra-Light，Light，Normal，Bold，Ultra-Bold，Heavy

      常见的变化包括：Normal，Small_Caps

      常见的拉伸包括：Ultra-Condensed，Extra-Condensed，Condensed，Semi-Condensed，Normal，Semi-Expanded，Extra-Expanded，Ultra-Expanded

  /* Set flags to show Audio, Video and Subtitles */
  g_object_get (data.playbin2, "flags", &flags, NULL);
  flags |= GST_PLAY_FLAG_VIDEO | GST_PLAY_FLAG_AUDIO | GST_PLAY_FLAG_TEXT;
  g_object_set (data.playbin2, "flags", flags, NULL);

      我们设置flags属性，把音频，视频和字幕的开关都打开。

      剩下的部分和上一篇里面的例子是一样的，除了键盘输入是改变current-text的属性而不是current-audio的属性。这里再强调一下，切换流不会马上起作用，因为缓冲了许多解码好的数据了。

Conclusion

This tutorial showed how to handle subtitles from playbin, whether they are embedded in the container or in a different file:

• Subtitles are chosen using the current-text and n-text properties of playbin.

• External subtitle files can be selected using the suburi property.

• Subtitle appearance can be customized with the subtitle-font-desc property.

The next playback tutorial shows how to change the playback speed.

Remember that attached to this page you should find the complete source code of the tutorial and any accessory files needed to build it. It has been a pleasure having you here, and see you soon!