1.目标

本教程与前一个非常类似，但我们将使用字幕流之间的不同音频流之间切换。这将允许我们学习：

如何选择字幕流
如何增加外部的字幕
如何自定义字幕的字体

2.介绍

我们已经知道(从之前的教程中)容器文件可以容纳多个音频和视频流，我们可以很容易地通过改变playbin的current-audio或current-video属性来选择它们。切换字幕也很简单。
值得注意的是，就像音频和视频一样，playbin负责为字幕选择正确的解码器，GStreamer的插件结构允许添加对新格式的支持，就像复制文件一样容易。所有东西对应用程序开发人员都是不可见的。
除了容器中嵌入的字幕外，playbin还提供了从外部URI添加额外字幕流的可能性。
本教程打开一个已经包含5个字幕流的文件，并从另一个文件中添加另一个字幕流(用于希腊语)。

3.实现

1.compile

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

2.code

#include <stdio.h>
#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 */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;/* playbin 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.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://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.ogv", NULL);/* Set the subtitle URI to play and some font description */g_object_set (data.playbin, "suburi", "https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer_gr.srt", NULL);g_object_set (data.playbin, "subtitle-font-desc", "Sans, 18", NULL);/* Set flags to show Audio, Video and Subtitles */g_object_get (data.playbin, "flags", &flags, NULL);flags |= GST_PLAY_FLAG_VIDEO | GST_PLAY_FLAG_AUDIO | GST_PLAY_FLAG_TEXT;g_object_set (data.playbin, "flags", flags, NULL);/* Add a bus watch, so we get notified when a message arrives */bus = gst_element_get_bus (data.playbin);gst_bus_add_watch (bus, (GstBusFunc)handle_message, &data);/* Add a keyboard watch so we get notified of keystrokes */
#ifdef G_OS_WIN32io_stdin = g_io_channel_win32_new_fd (fileno (stdin));
#elseio_stdin = g_io_channel_unix_new (fileno (stdin));
#endifg_io_add_watch (io_stdin, G_IO_IN, (GIOFunc)handle_keyboard, &data);/* 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;}/* 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.playbin, GST_STATE_NULL);gst_object_unref (data.playbin);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->playbin, "n-video", &data->n_video, NULL);g_object_get (data->playbin, "n-audio", &data->n_audio, NULL);g_object_get (data->playbin, "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->playbin, "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->playbin, "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->playbin, "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->playbin, "current-video", &data->current_video, NULL);g_object_get (data->playbin, "current-audio", &data->current_audio, NULL);g_object_get (data->playbin, "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->playbin)) {if (new_state == GST_STATE_PLAYING) {/* Once we are in the playing state, analyze the streams */analyze_streams (data);}}} 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->playbin, "current-text", index, NULL);}}g_free (str);return TRUE;
}

4.解析

1.属性设置

/* Set the subtitle URI to play and some font description */
g_object_set (data.playbin, "suburi", "https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer_gr.srt", NULL);
g_object_set (data.playbin, "subtitle-font-desc", "Sans, 18", NULL);

设置媒体URI之后，我们设置suburi属性，它将playbin指向包含字幕流的文件。在本例中，媒体文件已经包含多个字幕流，因此suburi中提供的字幕流将被添加到列表中，并且将是当前选中的字幕流。
注意，关于字幕流(像它的语言一样)的元数据驻留在容器文件中，因此，没有嵌入到容器中的字幕将没有元数据。运行本教程时，您会发现第一个字幕流没有语言标记。
subtitle-font-desc属性允许指定显示字幕的字体。由于Pango是用来呈现字体的库，您可以查看它的文档，以了解应该如何指定这种字体，特别是pango-font-description-from-string函数。
简而言之，字符串表示的格式是[FAMILY-LIST] [STYLE-OPTIONS] [SIZE]，其中FAMILY-LIST是用逗号分隔的列表，可以用逗号结束，STYLE_OPTIONS是用空格分隔的单词列表，其中每个单词描述样式、变体、weights或延伸，SIZE是一个小数(以点数表示)。例如，下面的都是有效的字符串表示

sans bold 12
serif,monospace bold italic condensed 16
normal 10
常用的字体有:普通字体(Normal)、无字体(Sans)、衬线字体(Serif)和单色字体(Monospace)。
可用的样式有:Normal(字体是垂直的)，Oblique(字体是倾斜的，但是罗马风格)，Italic(字体是倾斜的，以斜体风格)。
可用的weights是：Ultra-Light(超轻)，Light(轻)，Normal(正常)，Bold(粗体)，Ultra-Bold，Heavy。
可用的变体有:Normal(普通)、Small_Caps小写(一种用小写字符替换为更小的大写字符变体的字体)
可拉伸的款式有:Ultra-Condensed超浓缩，Extra-Condensed超浓缩，Condensed浓缩，Semi-Condensed半浓缩，Normal普通，Semi-Expanded半膨胀，Expanded膨胀，Extra-Expanded超膨胀，Ultra-Expanded超膨胀

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

我们设置了flags属性来允许音频、视频和文本(字幕)。
本教程的其余部分与播放教程1:Playbin使用相同，只是键盘输入改变了当前文本属性而不是当前音频属性。与前面一样，请记住流的更改不是立即的，因为在新流出现之前，有大量的信息流经管道，需要到达管道的末端。

5.讨论

本教程展示了如何处理来自playbin的字幕，无论它们是嵌入在容器中还是不同的文件中

使用playbin的当前文本和n-text属性选择字幕
可以使用suburi属性选择外部字幕文件
字幕外观可以使用subtitle-font-desc属性进行自定义

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