Creating both ghost source as well as sink pads

Application Development
1

I was working on a transcoding pipeline. I added and linked all the elements like decoding, scaling, encoding into a bin called videoTranscodeBin. Let us assume only 1 media type - video. I could create a ghost sink pad as follows:

 // sink
  GstPad* padSink = gst_element_get_static_pad(binFirstElement, "sink");
  GstPad* ghostPadSink = gst_ghost_pad_new("sink", padSink);
  gst_pad_set_active(ghostPadSink, TRUE);
  gst_element_add_pad(videoTrancodeBin, ghostPadSink);
  gst_object_unref(padSink);

Here binFirstElement is the first element in the videoTranscode bin. How does one create ghost source pad ? I tried as in above with “source” replacing “sink” but I get an error.

2

What error do you get, and can you share the exact code you tried?

3

Here are the errors I get upon execution:


GSTREAMER VERSION (Major, Minor, Macro, Nano): 1 24 3 0 Release

(gstMediaTranscoder:17643): GStreamer-**CRITICAL** **: 15:58:54.578: gst_ghost_pad_new: assertion 'GST_IS_PAD (target)' failed

(gstMediaTranscoder:17643): GStreamer-**CRITICAL** **: 15:58:54.579: gst_pad_set_active: assertion 'GST_IS_PAD (pad)' failed

(gstMediaTranscoder:17643): GStreamer-**CRITICAL** **: 15:58:54.579: gst_element_add_pad: assertion 'GST_IS_PAD (pad)' failed

(gstMediaTranscoder:17643): GStreamer-**CRITICAL** **: 15:58:54.579: gst_object_unref: assertion 'object != NULL' failed

Elements audEnc and mux could not be linked.

The application involves a media (both audio and video) transcoding chain from filesrc to filesink. I am attaching the whole code here.

#include <gst/gst.h>

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

typedef struct CustomData_s {
  // overall pipeline
  GstElement *pipeline;
  // filesource - demux
  GstElement *source;
  GstElement *demux;
  // audio transcode bin:
  // queue - mpegaudioparse - mpg123audiodec - audioconvert - audioresample - opusenc - mux
  GstElement *audQueueD;
  GstElement *audParse;
  GstElement *audDec;
  GstElement *audConv;
  GstElement *audResample;
  GstElement *audEnc;
  GstElement *audioTranscodeBin;
  // video transcode bin:
  // queue - h264parse - avdec_h264 - videoscale - capsfilter - x264enc - h264timestamper - muxer - queue - filesink
  GstElement *vidQueueD;
  GstElement *h264parse;
  GstElement *h264dec;
  GstElement *scale;
  GstElement *capsfilter;
  GstElement *x264enc;
  GstElement *h264ts;
  GstElement *videoTrancodeBin;
  // mux onward to sink
  GstElement *mux;
  GstElement *queueM;
  GstElement *sink;
} CustomData_t;

/* This function will be called by the pad-added signal for video */
static void on_pad_added_video (GstElement * src, GstPad * new_pad, GstElement * dest) {
  GstPad *sink_pad = gst_element_get_static_pad (dest, "sink");
  GstPadLinkReturn ret;
  GstCaps *new_pad_caps = NULL;
  GstStructure *new_pad_struct = NULL;
  const gchar *new_pad_type = NULL;

  g_print ("Received new pad '%s' from '%s':\n", GST_PAD_NAME (new_pad),
      GST_ELEMENT_NAME (src));

  // If our converter is already linked, we have nothing to do here
  if (gst_pad_is_linked (sink_pad)) {
    g_print ("We are already linked. Ignoring.\n");
    goto exit;
  }

  /* Check the new pad's type */
  new_pad_caps = gst_pad_get_current_caps (new_pad);
  new_pad_struct = gst_caps_get_structure (new_pad_caps, 0);
  new_pad_type = gst_structure_get_name (new_pad_struct);
  if (g_str_has_prefix(new_pad_type, "audio")) {
    g_print ("It has type '%s' which is audio. Ignoring. \n", new_pad_type);
    goto exit;
  } else if (g_str_has_prefix (new_pad_type, "video")) {
    g_print ("It has type '%s' which is video. \n", new_pad_type);
  } else {
    g_print ("It has type '%s' which is unknown. Ignoring. \n", new_pad_type);
    goto exit;
  }

  /* Attempt the link */
  ret = gst_pad_link (new_pad, sink_pad);
  if (GST_PAD_LINK_FAILED (ret)) {
    g_print ("Type is '%s' but link failed.\n", new_pad_type);
  } else {
    g_print ("Link succeeded (type '%s').\n", new_pad_type);
  }

exit:
  /* Unreference the new pad's caps, if we got them */
  if (new_pad_caps != NULL)
    gst_caps_unref (new_pad_caps);

  /* Unreference the sink pad */
  gst_object_unref (sink_pad);
}

/* This function will be called by the pad-added signal for audio */
static void on_pad_added_audio (GstElement * src, GstPad * new_pad, GstElement * dest)
{
  GstPad *sink_pad = gst_element_get_static_pad (dest, "sink");
  GstPadLinkReturn ret;
  GstCaps *new_pad_caps = NULL;
  GstStructure *new_pad_struct = NULL;
  const gchar *new_pad_type = NULL;

  g_print ("Received new pad '%s' from '%s':\n", GST_PAD_NAME (new_pad),
      GST_ELEMENT_NAME (src));

  // If our converter is already linked, we have nothing to do here
  if (gst_pad_is_linked (sink_pad)) {
    g_print ("We are already linked. Ignoring.\n");
    goto exit;
  }

  /* Check the new pad's type */
  new_pad_caps = gst_pad_get_current_caps (new_pad);
  new_pad_struct = gst_caps_get_structure (new_pad_caps, 0);
  new_pad_type = gst_structure_get_name (new_pad_struct);
  if (g_str_has_prefix(new_pad_type, "audio")) {
    g_print ("It has type '%s' which is audio. \n", new_pad_type);
  } else if (g_str_has_prefix (new_pad_type, "video")) {
    g_print ("It has type '%s' which is video. Ignoring. \n", new_pad_type);
    goto exit;
  } else {
    g_print ("It has type '%s' which is unknown. Ignoring. \n", new_pad_type);
    goto exit;
  }

  /* Attempt the link */
  ret = gst_pad_link (new_pad, sink_pad);
  if (GST_PAD_LINK_FAILED (ret)) {
    g_print ("Type is '%s' but link failed.\n", new_pad_type);
  } else {
    g_print ("Link succeeded (type '%s').\n", new_pad_type);
  }

exit:
  /* Unreference the new pad's caps, if we got them */
  if (new_pad_caps != NULL)
    gst_caps_unref (new_pad_caps);

  /* Unreference the sink pad */
  gst_object_unref (sink_pad);
}

int
tutorial_main (int argc, char *argv[])
{
  CustomData_t data;
  GstBus *bus;
  GstMessage *msg;

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

  // Print version
  guint major, minor, micro, nano;
  const gchar* nano_str;
  gst_version(&major, &minor, &micro, &nano);
  nano_str = nano == 1 ? "(CVS)" : nano == 2 ? "Prerelease" : "Release";
  g_print("GSTREAMER VERSION (Major, Minor, Macro, Nano): %u %u %u %u %s \n", major, minor, micro, nano, nano_str);

  // Init the pipeline
  data.pipeline = gst_pipeline_new("test-pipeline");

  // Create the elements from source to demux
  data.source = gst_element_factory_make("filesrc", "source");
  data.demux = gst_element_factory_make("qtdemux", "demux");

  // Create the audio transcoding elements
  data.audQueueD = gst_element_factory_make("queue", "auQueueD");
  data.audParse = gst_element_factory_make("mpegaudioparse", "audParse");
  data.audDec = gst_element_factory_make("mpg123audiodec", "audDec");
  data.audConv = gst_element_factory_make("audioconvert", "audConv");
  data.audResample = gst_element_factory_make("audioresample", "audResample");
  data.audEnc = gst_element_factory_make("opusenc", "audEnc");

  // Create the video transcoding elements
  data.vidQueueD = gst_element_factory_make("queue", "vidQueueD");
  data.h264parse = gst_element_factory_make("h264parse", "h264parse");
  data.h264dec = gst_element_factory_make("avdec_h264", "h264dec");
  data.scale = gst_element_factory_make("videoscale", "scale");
  data.capsfilter = gst_element_factory_make("capsfilter", "capsfilter");
  data.x264enc = gst_element_factory_make("x264enc", "x264enc");
  data.h264ts = gst_element_factory_make("h264timestamper", "h264ts");

  // Create the elements from mux to sink
  data.mux = gst_element_factory_make("mp4mux", "mux");
  data.queueM = gst_element_factory_make("queue", "queueM");
  data.sink = gst_element_factory_make("filesink", "sink");

  // sanity check
  if (  !data.pipeline || 
        // common front end
        !data.source ||
        !data.demux ||
        // audio 
        !data.audQueueD ||
        !data.audParse ||
        !data.audDec ||
        !data.audConv ||
        !data.audResample ||
        !data.audEnc ||
        // video
        !data.vidQueueD || 
        !data.h264parse ||
        !data.h264dec ||
        !data.scale ||
        !data.capsfilter ||
        !data.x264enc ||
        !data.h264ts ||
        // common back end
        !data.mux ||
        !data.queueM ||
        !data.sink ) {
    g_printerr ("Not all elements could be created.\n");
    return -1;
  }

  // create audio bin
  data.audioTranscodeBin = gst_bin_new("audioTranscodeBin");
  gst_bin_add_many(GST_BIN(data.audioTranscodeBin), data.audQueueD, data.audParse, data.audDec, data.audConv, 
                                                    data.audResample, data.audEnc, NULL);
  if (!gst_element_link_many( data.audQueueD, data.audParse, data.audDec, data.audConv, 
                              data.audResample, data.audEnc, NULL)) {
    g_printerr ("Elements could not be linked into the audio bin.\n");
    gst_object_unref(data.audioTranscodeBin);
    gst_object_unref(data.pipeline);
    return -1;
  }

  // add ghost pads to audio bin - sink
  GstPad* padAudSink = gst_element_get_static_pad(data.audQueueD, "sink");
  GstPad* ghostPadAudSink = gst_ghost_pad_new("sink", padAudSink);
  gst_pad_set_active(ghostPadAudSink, TRUE);
  gst_element_add_pad(data.audioTranscodeBin, ghostPadAudSink);
  gst_object_unref(padAudSink);
  // add ghost pads to audio bin - source
  GstPad* padAudSrc = gst_element_get_static_pad(data.audEnc, "source");
  GstPad* ghostPadAudSrc = gst_ghost_pad_new("sink", padAudSrc);
  gst_pad_set_active(ghostPadAudSrc, TRUE);
  gst_element_add_pad(data.audioTranscodeBin, ghostPadAudSrc);
  gst_object_unref(padAudSrc);

  // create video bin
  data.videoTrancodeBin = gst_bin_new ("videoTranscodeBin");
  gst_bin_add_many(GST_BIN(data.videoTrancodeBin), data.vidQueueD, data.h264parse, data.h264dec, 
                                                   data.scale, data.capsfilter, data.x264enc, data.h264ts, 
                                                   NULL);
  if (!gst_element_link_many(data.vidQueueD, data.h264parse, data.h264dec, data.scale, data.capsfilter, 
                             data.x264enc, data.h264ts, NULL)) {
    g_printerr ("Elements could not be linked into the video bin.\n");
    gst_object_unref(data.audioTranscodeBin);
    gst_object_unref(data.videoTrancodeBin);
    gst_object_unref(data.pipeline);
    return -1;
  }

  // add ghost pads to video bin - sink
  GstPad* padVidSink = gst_element_get_static_pad(data.vidQueueD, "sink");
  GstPad* ghostPadVidSink = gst_ghost_pad_new("sink", padVidSink);
  gst_pad_set_active(ghostPadVidSink, TRUE);
  gst_element_add_pad(data.videoTrancodeBin, ghostPadVidSink);
  gst_object_unref(padVidSink);

  // Add all elements to pipeline
  gst_bin_add_many(GST_BIN(data.pipeline),  data.source, data.demux,
                                            data.audioTranscodeBin,
                                            data.videoTrancodeBin, 
                                            data.mux, data.queueM, data.sink, NULL);

  // set the source element
  g_object_set(G_OBJECT(data.source), "location", 
               "/Users/kumarsnathan/Movies/youtube/1920x1080/AhalyaShortFilm.m4v", NULL);

  // audio bin settings
  g_object_set(G_OBJECT(data.audQueueD),  "max-size-bytes", (guint) 0,
                                          "max-size-buffers", (guint) 0,
                                          "max-size-time", (guint64) 0,
                                          NULL);

  // video bin settings
  // set the caps filter element
  GstCaps* caps = gst_caps_new_simple("video/x-raw",
                                      "format", G_TYPE_STRING, "I420",
                                      "width", G_TYPE_INT, 1280,
                                      "height", G_TYPE_INT, 720,
                                      NULL);
  g_object_set(G_OBJECT(data.capsfilter), "caps", caps, NULL);

  // set the x264enc element
  g_object_set(G_OBJECT(data.x264enc),  "bitrate", (guint) 4000, 
                                        "dct8x8", (gboolean) TRUE,
                                        "noise-reduction", (guint) 2,
                                        "ref", (guint) 4,
                                        "rc-lookahead", (gint) 25,
                                        "vbv-buf-capacity", (guint) 1000,
                                        NULL);

  // set the sink element
  g_object_set(G_OBJECT(data.sink), "location", "./data/AhalyaShortFilm-video-try0.mp4", NULL);

  // start linking all pipeline elements
  if (!gst_element_link(data.source, data.demux)) {
    g_printerr ("Elements source and demux could not be linked.\n");
    gst_object_unref(data.pipeline);
    return -1;
  }
  if (!gst_element_link(data.audioTranscodeBin, data.mux)) {
    g_printerr ("Elements audEnc and mux could not be linked.\n");
    gst_object_unref(data.pipeline);
    return -1;
  }
  if (!gst_element_link(data.h264ts, data.mux)) {
    g_printerr ("Elements h264ts and mux could not be linked.\n");
    gst_object_unref(data.pipeline);
    return -1;
  }
  if (!gst_element_link(data.mux, data.queueM)) {
    g_printerr ("Elements mux and queueM could not be linked.\n");
    gst_object_unref(data.pipeline);
    return -1;
  }
  if (!gst_element_link(data.queueM, data.sink)) {
    g_printerr ("Elements queueM and sink could not be linked.\n");
    gst_object_unref(data.pipeline);
    return -1;
  }
  g_signal_connect(data.demux, "pad-added", G_CALLBACK (on_pad_added_audio), data.audioTranscodeBin);
  g_signal_connect(data.demux, "pad-added", G_CALLBACK (on_pad_added_video), data.videoTrancodeBin);

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

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

  /* Free resources */
  if (msg != NULL)
    gst_message_unref (msg);
  gst_object_unref (bus);
  gst_element_set_state (data.pipeline, GST_STATE_NULL);
  gst_object_unref (data.pipeline);
  return 0;
}

int
main (int argc, char *argv[])
{
#if defined(__APPLE__) && TARGET_OS_MAC && !TARGET_OS_IPHONE
  return gst_macos_main ((GstMainFunc) tutorial_main, argc, argv, NULL);
#else
  return tutorial_main (argc, argv);
#endif
}
4

Note that in the code above, I was able to successfully use ghost sink pads for both audio and video transcode chains. But when I tried using ghost src pad for either audio or video, I get this error and it says that the audio/video transcode bin could not be linked to muxer. But the last element in either bin can be linked to the muxer successfully. Why is this ?

5 
-  GstPad* padAudSrc = gst_element_get_static_pad(data.audEnc, "source");
-  GstPad* ghostPadAudSrc = gst_ghost_pad_new("sink", padAudSrc);
+  GstPad* padAudSrc = gst_element_get_static_pad(data.audEnc, "src");
+  GstPad* ghostPadAudSrc = gst_ghost_pad_new("src", padAudSrc);

is what you want here.

To debug such critical warnings, you can set the environment varabile G_DEBUG=fatal_warnings and run in a debugger to get a breakpoint on the first of those to know where it comes from. In this case, also the GStreamer debug logs would’ve helped.

6

Thank you for pointing out this error in my code. Yes, the changes you suggest do work. Much appreciated.