c - 将管道设置为播放状态后，Gstreamer 回调未命中

Question

我正在编写一个媒体应用程序来从视频文件中抓取视频帧。为此，我想在从管道中提取样本之前获取视频属性。因此，我在解码器处添加了 auto-plug 信号的回调并尝试获取属性。即使我将管道置于播放状态后，这些回调也不会被调用，但如果我尝试使用 gst_app_sink_pull_sample 从管道中提取样本，则会调用这些回调。

我在这里遗漏了什么吗？我的理解是，当我们将管道置于播放状态时，这些回调将被调用。

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

static void bus_callback (GstBus *bus, GstMessage *msg, gpointer data) 
{

  switch (GST_MESSAGE_TYPE (msg)) 
  {
    case GST_MESSAGE_ERROR: {
                  GError *err;
                  gchar *debug;
                  gst_message_parse_error (msg, &err, &debug);
                  g_print ("Error: %s\n", err->message);
                  g_error_free (err);
                  g_free (debug);
                  break;
                }
    default:
                /* Unhandled message */
                break;
  }
}

static void 
on_pad_added (GstElement *element, GstPad *pad, gpointer data)
{
  GstPad *sinkpad;
  GstElement *decoder = (GstElement *) data;

  /* We can now link this pad with the decoder sink pad */
  sinkpad           =  gst_element_get_static_pad (decoder, "sink");
  gst_pad_link (pad, sinkpad);
  gst_object_unref (sinkpad);
}

static void
auto_plug_select (GstElement *decoder, GstPad *pad, GstCaps *caps,
    GstElementFactory *factory, int *width )
{
  const gchar *klass   =  gst_element_factory_get_klass (factory);
/*  MW_customData *cdata =  (MW_customData*) data;*/
  GstCaps *scaps       =  gst_pad_query_caps (pad, NULL);
  GstStructure *str    =  gst_caps_get_structure (scaps, 0);
  const gchar *type    =  gst_structure_get_name (str);
  printf (" Pad cap: %s\n", type);

  if (g_strrstr(type,"video"))
  {
   gst_structure_get_int (str, "width", width);
   printf(" Width: %d\n", *width);
  }
}

int main (gint   argc,
      gchar *argv[])
{
  GstElement *pipeline, *filesrc, *decoder, *fakesink;
  GstBus *bus;

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

  /* check args */
  if (argc != 2) {
    g_print ("Usage: %s <filename>\n", argv[0]);
    return -1;
  }

  /* create a new pipeline to hold the elements */
  pipeline = gst_pipeline_new ("pipeline");

  /* Bus call back*/
  bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline));
  gst_bus_add_watch (bus, bus_callback, NULL);
  gst_object_unref (bus);

  /* create file source and typefind element */
  filesrc = gst_element_factory_make ("filesrc", "source");
  g_object_set (G_OBJECT (filesrc), "location", argv[1], NULL);
  decoder = gst_element_factory_make ("decodebin", NULL);
  fakesink = gst_element_factory_make ("fakesink", "sink");

  int width = 0;
/* Connect the sink pad when decoder completes the operation */
  g_signal_connect (decoder, "pad-added", G_CALLBACK (on_pad_added), &width);
  g_signal_connect (decoder, "autoplug-select", G_CALLBACK (auto_plug_select), fakesink);

  /* setup */
  gst_bin_add_many (GST_BIN (pipeline), filesrc, decoder, fakesink, NULL);
  gst_element_link (filesrc, decoder);
  gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_PLAYING);

  printf(" Width: %d\n", width);

  gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_NULL);

  return 0;
}

Answer 1

任何时候都不要让管道处于运行状态。您可能会在数据触发解码器的回调之前停止它。

为了便宜，尝试一下:

gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_PLAYING);

g_usleep(100000000);

printf(" Width: %d\n", width);

gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_NULL);

但更正确的方法是使用真正的 GMainLoop 并根据特定事件执行操作以再次停止管道。

编辑:P.S.为什么不使用GstDiscoverer？ https://gstreamer.freedesktop.org/documentation/pbutils/gstdiscoverer.html?gi-language=c