Hello world

Base on basic-tutorial-1.c

初始化GStreamer

gst_init (&argc, &argv)

&argc, &argv是給命令列后綴留下的引數.

創建一個管道示例播放一個視頻

pipeline = gst_phrase_launch("playbin uri=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm", NULL);

gst_phrase_launch() 一個簡單的管道創建方法, 將文本形式管道名詞轉化為實際的管道.

playbin 一個特殊的元素型別, 用來連接元素和播放內容.

設定element播放狀態

/* Start playing */
gst_element_set_state (pipeline, GST_STATE_PLAYING);

每一個element都有一個狀態, 這里的作用是設定pipeline為播放狀態.

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);

gst_bus_timed_pop_filtered()為u等待程式結束時回傳,這里是回傳錯誤GST_MESSAGE_ERROR或者結束GST_MESSAGE_EOS.

Clean 清理

/* Free resources */
if (msg != NULL)
  gst_message_unref (msg);
gst_object_unref (bus);
gst_element_set_state (pipeline, GST_STATE_NULL);
gst_object_unref (pipeline);

閱讀函式檔案以查詢是否需要釋放物件, 在這里gst_bus_timed_pop_filtered的函式解釋Return引數中有寫道需要用gst_object_unref清除.

編譯basic-tutorial-1.c

gcc basic-tutorial-1.c -o basic-tutorial-1 `pkg-config --cflags --libs gstreamer-1.0`

基本上是在gcc編譯器命令后加入pkg-config --cflags --libs gstreamer-1.0

GStramer 概念

base on basic-tutorial-2.c

Creat element 創建元素

用gst_element_factory_make('引數1','引數2')創建新元素.

引數1是元素型別, 例如playbin/uridecodebin/decodebin/filesrc.

引數2是我們為其取的名稱, 如果定義為NULL那么GStreamer會自動生產一個名稱.

source = gst_element_factory_make ("videotestsrc", "source");
sink = gst_element_factory_make ("autovideosink", "sink");

這里有兩個元素型別.

videotestsrc - 提供測驗畫面(Produces data).

autovideosink - 接識訓面(consumes data), 自動選擇最好的sink.

Create pipeline 創建管道

創建管道用gst_pipeline_new("引數").

/* 創建空管道 */
pipeline = gst_pipeline_new ("test-pipeline");

Add element to pipeline 把元素加入進管道

/* 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;
}

GST_BIN (pipeline)就是上面創建的空管道, 由于管道是一種特殊的Bin, 所以添加Bin的方法GST_BIN (Bin)也適用于管道.

gst_bin_add_many()添加多個元素到Bin, 結尾為NULL.

gst_bin_add()添加單個元素到Bin.

gst_element_link (source, sink)連接同一個Bin中的元素, source是源, sink是目的地, 順序很重要.

Properties 屬性設定

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

g_object_set用來設定source(videotestsrc)元素的屬性, 對應的還有g_object_get()讀取屬性.

"pattern"屬性是元素的輸出型別, 對于videotestsrc來說不同數字會使測驗視頻樣式發生變化.

錯誤檢查

/* 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;
}

We check gst_element_set_state() return value for errors.

另外一段錯誤檢查代碼

/* 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);
  }

涉及到的幾個方法:

gst_message_parse_error()

gst_element_get_bus()

動態管道

base on basic-tutorial-3.c

定義結構體

typedef struct _CustomData
{
  GstElement *pipeline;
  GstElement *source;
  GstElement *convert;
  GstElement *resample;
  GstElement *sink;
} CustomData;

這里將所有所需資訊作為區域變數放入一個分組結構中, 這種結構有利于函式回呼.

創建元素

/* Create the elements */
  data.source = gst_element_factory_make ("uridecodebin", "source");
  data.convert = gst_element_factory_make ("audioconvert", "convert");
  data.resample = gst_element_factory_make ("audioresample", "resample");
  data.sink = gst_element_factory_make ("autoaudiosink", "sink");

這里用到4個元素:

1. uridecodebin用來實體化幾個必要元素(Sources, demuxers and decoders), 用來將URL視頻轉化為原始音頻/視頻流.
2. audioconverter用來轉換音頻格式.
3. audioresample用來轉換采樣率.
4. autoaudiosink將音頻呈現到聲卡.

創建管道

  /* Build the pipeline. Note that we are NOT linking the source at this
   * point. We will do it later. */
  gst_bin_add_many (GST_BIN (data.pipeline), data.source, data.convert,
      data.resample, data.sink, NULL);
  if (!gst_element_link_many (data.convert, data.resample, data.sink, NULL)) {
    g_printerr ("Elements could not be linked.\n");
    gst_object_unref (data.pipeline);
    return -1;
  }

這里我們將除源以外的所有元素鏈接好.

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