学步园

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。