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09.流信息与动态切换

丁毅桂2026/1/21大约 4 分钟gstreamernotesplaybinstreams

GStreamer学习笔记：09.流信息与动态切换

本示例尝试使用 playbin 播放多媒体文件，并提取流信息（视频、音频、字幕）以及动态切换不同的流。

核心概念

1. Playbin 基础

Playbin 是 GStreamer 提供的高级播放元素，内部自动创建完整的播放 pipeline。

设置播放源

data.playbin = gst_element_factory_make("playbin", "playbin");
g_object_set(data.playbin, "uri", "https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_cropped_multilingual.webm", NULL);

添加外部字幕

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);
  • subtitle-font-desc 格式：[FAMILY-LIST（字体）] [STYLE-OPTIONS（字体属性）] [SIZE（字号）]

2. Playbin Flags

Playbin 使用 flags 控制哪些流应该被输出。

typedef enum
{
    GST_PLAY_FLAG_VIDEO = (1 << 0), // 我们要视频输出
    GST_PLAY_FLAG_AUDIO = (1 << 1), // 我们要音频输出
    GST_PLAY_FLAG_TEXT = (1 << 2)   // 我们要字幕输出
} GstPlayFlags;

配置 Flags

gint flags;
g_object_get(data.playbin, "flags", &flags, NULL); // 获取默认值

// 使用音频和视频，取消字幕，其他值保留默认值
flags |= GST_PLAY_FLAG_VIDEO | GST_PLAY_FLAG_AUDIO;
flags &= ~GST_PLAY_FLAG_TEXT;

g_object_set(data.playbin, "flags", flags, NULL);

3. 网络连接速度

// 网络连接速度，单位 kbps。默认值：0（未知）
g_object_set(data.playbin, "connection-speed", 56, NULL); // 56kbps

这会告诉 playbin 当前网络连接的最大速度，playbin 会选择最合适版本的视频流。

4. 流信息提取

获取流数量

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);

获取流标签（Metadata）

视频流标签
g_signal_emit_by_name(data->playbin, "get-video-tags", idx, &tags);
if (tags)
{
    g_print("video stream %d:\n", idx);
    if (gst_tag_list_get_string(tags, GST_TAG_VIDEO_CODEC, &str))
    {
        g_print("\t codec: %s\n", str); // 例如：codec: On2 VP8
        g_free(str);
    }
    gst_tag_list_free(tags);
}
音频流标签
g_signal_emit_by_name(data->playbin, "get-audio-tags", idx, &tags);
if (tags)
{
    g_print("audio stream %d:\n", idx);

    // 获取音频编码器名称
    if (gst_tag_list_get_string(tags, GST_TAG_AUDIO_CODEC, &str))
    {
        g_print("\t codec: %s\n", str); // 例如：codec: Vorbis
        g_free(str);
    }

    // 获取语言缩写代码
    if (gst_tag_list_get_string(tags, GST_TAG_LANGUAGE_CODE, &str))
    {
        g_print("\t language: %s\n", str); // 例如：language: pt
        g_free(str);
    }

    // 获取比特率
    guint rate;
    if (gst_tag_list_get_uint(tags, GST_TAG_BITRATE, &rate))
    {
        g_print("\t bitrate: %d\n", rate); // 例如：bitrate: 160000
    }

    gst_tag_list_free(tags);
}
字幕流标签
g_signal_emit_by_name(data->playbin, "get-text-tags", idx, &tags);
if (tags)
{
    g_print("subtitle stream %d:\n", idx);
    if (gst_tag_list_get_string(tags, GST_TAG_LANGUAGE_CODE, &str))
    {
        g_print("\t language: %s\n", str);
        g_free(str);
    }
    gst_tag_list_free(tags);
}

获取当前播放的流

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);

5. 动态切换流

g_object_set(data->playbin, "current-audio", index, NULL);

注意：

  • 这种切换不是立即生效的
  • 一些之前解码好的音频数据将仍然在 pipeline 中流动
  • 虽然新的流已经开始解码
  • 延迟取决于容器中流的特定多路复用和 playbin 的内部 queue 的长度（这取决于网络状况）

6. 事件监听

Bus 监听

bus = gst_element_get_bus(data.playbin);
gst_bus_add_watch(bus, (GstBusFunc)handle_message, &data);

键盘输入监听

#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);

7. 状态变化处理

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)
        {
            // 一旦进入播放状态，分析流信息
            analyze_streams(data);
        }
    }
}
break;

8. 键盘输入处理

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 = g_ascii_strtoull(str, NULL, 0);
        if (index < 0 || index >= data->n_audio)
        {
            g_printerr("Index out of bounds\n");
        }
        else
        {
            g_print("Setting current audio stream to %d\n", index);
            g_object_set(data->playbin, "current-audio", index, NULL);
        }
    }
    g_free(str);
    return TRUE;
}

输出示例

1 video stream(s), 2 audio stream(s), 3 text stream(s)

video stream 0:
	 codec: On2 VP8

audio stream 0:
	 codec: Vorbis
	 language: pt
	 bitrate: 160000

audio stream 1:
	 codec: Vorbis
	 language: en
	 bitrate: 160000

subtitle stream 0:
	 language: pt
subtitle stream 1:
	 language: es
subtitle stream 2:
	 language: gr

Currently playing video stream 0, audio stream 0 and text stream -1
Type any number and hit ENTER to select a different audio stream

完整代码

#include <gst/gst.h>
#include <stdio.h>

typedef struct _CustomData
{
    GstElement *playbin;
    gint n_video, n_audio, n_text;
    gint current_video, current_audio, current_text;
    GMainLoop *main_loop;
} CustomData;

typedef enum
{
    GST_PLAY_FLAG_VIDEO = (1 << 0),
    GST_PLAY_FLAG_AUDIO = (1 << 1),
    GST_PLAY_FLAG_TEXT = (1 << 2)
} GstPlayFlags;

static gboolean handle_message(GstBus *bus, GstMessage *msg, CustomData *data);
static gboolean handle_keyboard(GIOChannel *source, GIOCondition cond, CustomData *data);
static void analyze_streams(CustomData *data);

int main(int argc, char *argv[])
{
    CustomData data;
    GstBus *bus;
    GstStateChangeReturn ret;
    gint flags;
    GIOChannel *io_stdin;

    gst_init(&argc, &argv);

    data.playbin = gst_element_factory_make("playbin", "playbin");
    if (!data.playbin)
    {
        g_printerr("Not all elements could be created.\n");
        return -1;
    }

    g_object_set(data.playbin, "uri", "https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_cropped_multilingual.webm", NULL);
    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);

    g_object_get(data.playbin, "flags", &flags, NULL);
    flags |= GST_PLAY_FLAG_VIDEO | GST_PLAY_FLAG_AUDIO;
    flags &= ~GST_PLAY_FLAG_TEXT;
    g_object_set(data.playbin, "flags", flags, NULL);

    g_object_set(data.playbin, "connection-speed", 56, NULL);

    bus = gst_element_get_bus(data.playbin);
    gst_bus_add_watch(bus, (GstBusFunc)handle_message, &data);

#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);

    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;
    }

    data.main_loop = g_main_loop_new(NULL, FALSE);
    g_main_loop_run(data.main_loop);

    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;
}

编译和运行

gcc main.c -o main.out $(pkg-config --cflags --libs gstreamer-1.0)
./main.out

总结

本示例展示了：

  1. Playbin 高级功能：

    • 播放多媒体文件
    • 添加外部字幕
    • 配置 flags 控制输出流
    • 设置网络连接速度

  2. 流信息提取：

    • 获取流数量
    • 提取流标签（编解码器、语言、比特率）
    • 获取当前播放的流

  3. 动态切换流：

    • 切换音频流、视频流、字幕流
    • 理解切换延迟的机制

  4. 事件监听：

    • Bus 消息监听
    • 键盘输入监听
    • 状态变化处理

Playbin 提供了完整的媒体播放解决方案，可以处理复杂的多媒体场景，是构建媒体播放器的理想选择。

贡献者: YiguiDing
MIT Licensed | Copyright © 2019-present Yigui-Ding