GStreamer学习2--动态link实现播放mp4

在官方文档基础教程 1：Hello world！ (gstreamer.freedesktop.org) 中，我们看到可以使用playbin来实现播放，

在基础教程 2：GStreamer 概念中，可以看到element的link来实现播放，即使用api程序来实现类似gst-launch-1.0 videotestsrc ! autovideosink 的播放效果。

那怎么播放mp4文件呢，搜一下，可以看到这样的link方式

gst-launch-1.0 filesrc location=test.mp4 ! decodebin ! autovideosink

仿照基础教程2中的例子，似乎就可以实现了，

如果真这么做了，会发现，link失败，因为有时候需要动态连接，选择合适的pad去进行连接，

参考基础教程 3：动态管道 (gstreamer.freedesktop.org)，

里面的代码可以再添加一个autovideosink的element连接来显示视频。

关键的地方在于对pad-added信号的监听，及相应的回调处理

/* Connect to the pad-added signal */
  g_signal_connect (data.source, "pad-added", G_CALLBACK (pad_added_handler), &data);

知道了这个思路，就可以让AI来实现了，以下是通义生成的修改后的例子

#include <gst/gst.h>
//#include <gst/app/gstappsink.h>

#define FILE_PATH "D:\\2024\\06\\Test\\test.mp4"

static gboolean bus_call(GstBus* bus, GstMessage* msg, gpointer data) {
  GMainLoop* loop = (GMainLoop*)data;
  switch (GST_MESSAGE_TYPE(msg)) {
  case GST_MESSAGE_EOS:
    g_print("End-of-stream reached.\n");
    g_main_loop_quit(loop);
    break;
  case GST_MESSAGE_ERROR: {
    gchar* debug;
    GError* error;
    gst_message_parse_error(msg, &error, &debug);
    g_free(debug);
    g_printerr("Error received from element %s: %s\n", GST_OBJECT_NAME(msg->src), error->message);
    g_clear_error(&error);
    g_main_loop_quit(loop);
    break;
  }
  default:
    break;
  }
  return TRUE;
}

static void pad_added_callback(GstElement* decodebin, GstPad* pad, gpointer user_data);

int main(int argc, char* argv[]) {
  GstElement* pipeline, * source, * decodebin, * videosink;
  GstBus* bus;
  GMainLoop* loop;
  GstPad* src_pad, * sink_pad;
  GstCaps* caps;
  GstStateChangeReturn ret;

  /* 初始化GStreamer */
  gst_init(&argc, &argv);

  /* 创建管道 */
  pipeline = gst_pipeline_new("playback-pipeline");

  /* 创建source元素 */
  source = gst_element_factory_make("filesrc", "file-source");
  g_object_set(source, "location", FILE_PATH, NULL);

  /* 创建decodebin元素 */
  decodebin = gst_element_factory_make("decodebin", "decoder");

  /* 创建视频sink元素 */
  videosink = gst_element_factory_make("autovideosink", "video-sink");

  /* 将元素添加到管道 */
  gst_bin_add_many(GST_BIN(pipeline), source, decodebin, videosink, NULL);

  /* 链接source和decodebin */
  if (gst_element_link(source, decodebin) != TRUE) {
    g_printerr("Elements could not be linked.\n");
    gst_object_unref(pipeline);
    return -1;
  }

  /* 监听decodebin的pad-added信号 */
  g_signal_connect(decodebin, "pad-added", G_CALLBACK(pad_added_callback), videosink);

  /* 创建总线并添加监听 */
  bus = gst_pipeline_get_bus(GST_PIPELINE(pipeline));
  loop = g_main_loop_new(NULL, FALSE);
  gst_bus_add_watch(bus, G_CALLBACK(bus_call), loop);
  gst_object_unref(bus);

  /* 运行管道 */
  ret = gst_element_set_state(pipeline, GST_STATE_PLAYING);
  if (ret == GST_STATE_CHANGE_FAILURE) {
    g_printerr("Unable to set the pipeline to the playing state.\n");
    goto cleanup;
  }

  /* 运行主循环 */
  g_print("Press Ctrl+C to stop playback...\n");
  g_main_loop_run(loop);

cleanup:
  /* 清理 */
  gst_element_set_state(pipeline, GST_STATE_NULL);
  gst_object_unref(GST_OBJECT(pipeline));
  g_main_loop_unref(loop);
  return 0;
}

/* pad-added回调函数 */
static void pad_added_callback(GstElement* decodebin, GstPad* pad, gpointer user_data) {
  GstElement* videosink = (GstElement*)user_data;
  GstPad* sink_pad;
  GstCaps* pad_caps;
  GstStructure* pad_struct;
  const gchar* pad_type;

  /* 获取sink pad */
  sink_pad = gst_element_get_static_pad(videosink, "sink");

  /* 获取pad的caps */
  pad_caps = gst_pad_get_current_caps(pad);
  pad_struct = gst_caps_get_structure(pad_caps, 0);
  pad_type = gst_structure_get_name(pad_struct);

  g_print("Received new pad '%s' with type '%s'.\n", GST_PAD_NAME(pad), pad_type);

  /* 如果是视频类型，链接pad */
  if (g_strcmp0(pad_type, "video/x-raw") == 0) {
    /* 尝试链接pad */
    if (gst_pad_link(pad, sink_pad) == GST_PAD_LINK_OK) {
      g_print("Linked pad '%s' to sink pad '%s'.\n", GST_PAD_NAME(pad), GST_PAD_NAME(sink_pad));
    }
    else {
      g_print("Link failed between pad '%s' and sink pad '%s'.\n", GST_PAD_NAME(pad), GST_PAD_NAME(sink_pad));
    }
  }

  /* 清理 */
//  gst_object_unref((pad_caps));
  gst_caps_unref(pad_caps);
  gst_object_unref((sink_pad));
}

GStreamer学习2--动态link实现播放mp4

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