【GStreamer开发】GStreamer播放教程02—

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

[objc] view
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#include <gst/gst.h>
/* Structure to contain all our information, so we can pass it around */
typedef struct _CustomData {
; /* 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 {
<< 0), /* We want video output */
<< 1), /* We want audio output */
<< 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, charchar *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");
;
}
/* 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
2_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);
;
}
/* 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);
;
}
/* 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 */
, "n-video", &data->n_video, NULL);
, "n-audio", &data->n_audio, NULL);
, "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");
; i < data->n_video; i++) {
tags = NULL;
/* Retrieve the stream's video tags */
, "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");
; i < data->n_audio; i++) {
tags = NULL;
/* Retrieve the stream's audio tags */
, "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");
; i < data->n_text; i++) {
tags = NULL;
/* Retrieve the stream's subtitle tags */
g_print ("subtitle stream %d:\n", i);
, "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");
}
}
, "current-video", &data->current_video, NULL);
, "current-audio", &data->current_audio, NULL);
, "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 (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);
|| 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);
, "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。

[objc] view
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/* 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

[objc] view
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/* 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。

巴特西

【GStreamer开发】GStreamer播放教程02——字幕管理

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