此博客是在gstreamer官网学习并总结的学习概要，具体参考gstreamer官网教程：基础教程 2：GStreamer 概念

实例

依然先运行官网给的实例，代码在下面（也可以在gstreamer代码中找到）：

示例代码

#include <gst/gst.h>

#ifdef __APPLE__
#include <TargetConditionals.h>
#endif

int
tutorial_main (int argc, char *argv[])
{
    
  GstElement *pipeline, *source, *sink;
  GstBus *bus;
  GstMessage *msg;
  GstStateChangeReturn ret;

  /* Initialize GStreamer */
  gst_init (&argc, &argv);

  /* Create the elements */
  source = gst_element_factory_make ("videotestsrc", "source");
  sink = gst_element_factory_make ("autovideosink", "sink");

  /* Create the empty pipeline */
  pipeline = gst_pipeline_new ("test-pipeline");

  if (!pipeline || !source || !sink) {
    
    g_printerr ("Not all elements could be created.\n");
    return -1;
  }

  /* Build the pipeline */
  gst_bin_add_many (GST_BIN (pipeline), source, sink, NULL);
  if (gst_element_link (source, sink) != TRUE) {
    
    g_printerr ("Elements could not be linked.\n");
    gst_object_unref (pipeline);
    return -1;
  }

  /* Modify the source's properties */
  g_object_set (source, "pattern", 0, NULL);

  /* Start playing */
  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");
    gst_object_unref (pipeline);
    return -1;
  }

  /* Wait until error or EOS */
  bus = gst_element_get_bus (pipeline);
  msg =
      gst_bus_timed_pop_filtered (bus, GST_CLOCK_TIME_NONE,
      GST_MESSAGE_ERROR | GST_MESSAGE_EOS);

  /* Parse message */
  if (msg != NULL) {
    
    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);
        break;
      case GST_MESSAGE_EOS:
        g_print ("End-Of-Stream reached.\n");
        break;
      default:
        /* We should not reach here because we only asked for ERRORs and EOS */
        g_printerr ("Unexpected message received.\n");
        break;
    }
    gst_message_unref (msg);
  }

  /* Free resources */
  gst_object_unref (bus);
  gst_element_set_state (pipeline, GST_STATE_NULL);
  gst_object_unref (pipeline);
  return 0;
}

int
main (int argc, char *argv[])
{
    
#if defined(__APPLE__) && TARGET_OS_MAC && !TARGET_OS_IPHONE
  return gst_macos_main (tutorial_main, argc, argv, NULL);
#else
  return tutorial_main (argc, argv);
#endif
}

运行结果

概念

元素（elements）

元素（elements）是gstreamer里面的基础模块，一个最简单的pipeline如下：

source元素解析视频数据，通过filter元素，发送给sink模块播放

元素类型可参考链接： 简易元素

GstBin

bin是一个可以包含其他元素的元素（它的类型就是GstElement），
如何派生出来的：

GObject
    ╰──GInitiallyUnowned
        ╰──GstObject
            ╰──GstElement
                ╰──GstBin
                    ╰──GstPipeline

从上面可以看出来，pipeline是bin的一种，同时它们也都是特殊的element。它们本质上都是由GObject派生出来的。

BUS

它是由元素生成的，用来向应用程序传递message的对象，它传递的message会依次发送给应用程序。应用程序需要一直关注bus，来获取对应的消息，可以通过函数gst_bus_timed_pop_filtered()来获取，也可以使用信号来获取。

代码解析

元素创建

  /* Create the elements */
  source = gst_element_factory_make ("videotestsrc", "source");
  sink = gst_element_factory_make ("autovideosink", "sink");

通过调用函数gst_element_factory_make()可以创建一个元素，第一个入参是元素类型，第二个参数是元素名，如果元素名是NULL，系统会自动指定一个唯一的名字。

创建pipeline

  /* Create the empty pipeline */
  pipeline = gst_pipeline_new ("test-pipeline");

调用函数gst_pipeline_new()创建一个新的pipeline

  /* Build the pipeline */
  gst_bin_add_many (GST_BIN (pipeline), source, sink, NULL);
  if (gst_element_link (source, sink) != TRUE) {
    
    g_printerr ("Elements could not be linked.\n");
    gst_object_unref (pipeline);
    return -1;
  }

pipeline是一种特殊的bin，所以适用于bin的接口也适用于pipeline。
函数gst_bin_add_many()用来给bin加入多个元素，如果只加入一个元素，则可以调用函数gst_bin_add()。
函数 gst_element_link()用来关联两个元素，需要注意的是：要用来关联的两个元素必须在同一个bin里面，所以要先调用gst_bin_add()再关联两个元素。

配置元素

  /* Modify the source's properties */
  g_object_set (source, "pattern", 0, NULL);

配置source元素的属性，属性名为pattern，属性值为0。
原生代码配置的属性结果已经放在上面的图里，修改这个属性值为1，则可以得到另一种结果：

如图所示，修改属性值之后，显示的图案变成了纯雪花。
元素公开的所有属性的名称和可能值可以使用 Basic tutorial 10: GStreamer tools 中描述的 gst-inspect-1.0 工具找到。

错误检查

 /* Start playing */
  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");
    gst_object_unref (pipeline);
    return -1;
  }

这条语句开启播放并检查返回值，如果pipeline返回失败则退出。

  /* Wait until error or EOS */
  bus = gst_element_get_bus (pipeline);
  msg =
      gst_bus_timed_pop_filtered (bus, GST_CLOCK_TIME_NONE,
      GST_MESSAGE_ERROR | GST_MESSAGE_EOS);

  /* Parse message */
  if (msg != NULL) {
    
    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);
        break;
      case GST_MESSAGE_EOS:
        g_print ("End-Of-Stream reached.\n");
        break;
      default:
        /* We should not reach here because we only asked for ERRORs and EOS */
        g_printerr ("Unexpected message received.\n");
        break;
    }
    gst_message_unref (msg);
  }

通过gst_bus_timed_pop_filtered()获取对应的事件，如果是ERROR，可以调用gst_message_parse_error()函数获取具体错误。

课后练习

添加VertigoTV模块，这个模块具有旋转和缩放功能，此处，还加入了videoconvert模块，防止出现错误。

添加上述两个模块之后，显示的视频有了动态模糊效果。通过修改VertigoTV模块的speed参数，还可以修改动态模糊的速度。代码如下：

#include <gst/gst.h>

#ifdef __APPLE__
#include <TargetConditionals.h>
#endif

int
tutorial_main (int argc, char *argv[])
{
    
  GstElement *pipeline, *source, *sink, *vto, *vct;
  GstBus *bus;
  GstMessage *msg;
  GstStateChangeReturn ret;

  /* Initialize GStreamer */
  gst_init (&argc, &argv);

  /* Create the elements */
  source = gst_element_factory_make ("videotestsrc", "source");
  sink = gst_element_factory_make ("autovideosink", "sink");

  /* my elements */
  vto = gst_element_factory_make ("vertigotv", "myvto");
  vct = gst_element_factory_make ("videoconvert", "myvct");

  /* Create the empty pipeline */
  pipeline = gst_pipeline_new ("test-pipeline");

  if (!pipeline || !source || !sink || !vto || !vct) {
    
    g_printerr ("Not all elements could be created.\n");
    return -1;
  }

  /* Build the pipeline */
  //gst_bin_add_many (GST_BIN (pipeline), source, sink, NULL);
  gst_bin_add(GST_BIN (pipeline), source);
  gst_bin_add(GST_BIN (pipeline), sink);
  gst_bin_add(GST_BIN (pipeline), vto);
  gst_bin_add(GST_BIN (pipeline), vct);
  if (gst_element_link (source, vto) != TRUE) {
    
    g_printerr ("Elements[source, vto] could not be linked.\n");
    gst_object_unref (pipeline);
    return -1;
  }
  if (gst_element_link (vto, vct) != TRUE) {
    
    g_printerr ("Elements[vto, vct] could not be linked.\n");
    gst_object_unref (pipeline);
    return -1;
  }
  if (gst_element_link (vct, sink) != TRUE) {
    
    g_printerr ("Elements[vct, sink] could not be linked.\n");
    gst_object_unref (pipeline);
    return -1;
  }

  /* Modify the source's properties */
  g_object_set (source, "pattern", 0, NULL);

  /* test: modify vto's speed */
  g_object_set (vto, "speed", 0.1, NULL);

  /* Start playing */
  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");
    gst_object_unref (pipeline);
    return -1;
  }

  /* Wait until error or EOS */
  bus = gst_element_get_bus (pipeline);
  msg =
      gst_bus_timed_pop_filtered (bus, GST_CLOCK_TIME_NONE,
      GST_MESSAGE_ERROR | GST_MESSAGE_EOS);

  /* Parse message */
  if (msg != NULL) {
    
    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);
        break;
      case GST_MESSAGE_EOS:
        g_print ("End-Of-Stream reached.\n");
        break;
      default:
        /* We should not reach here because we only asked for ERRORs and EOS */
        g_printerr ("Unexpected message received.\n");
        break;
    }
    gst_message_unref (msg);
  }

  /* Free resources */
  gst_object_unref (bus);
  gst_element_set_state (pipeline, GST_STATE_NULL);
  gst_object_unref (pipeline);
  return 0;
}

int
main (int argc, char *argv[])
{
    
#if defined(__APPLE__) && TARGET_OS_MAC && !TARGET_OS_IPHONE
  return gst_macos_main (tutorial_main, argc, argv, NULL);
#else
  return tutorial_main (argc, argv);
#endif
}