How to add source and sink with nvstreammux and nvstreamdemux dynamically

Accelerated Computing Intelligent Video Analytics DeepStream SDK
10

I found a pretty simple workaround to this limitation!
Since the nvstreamdemux element only allows pads to be added in the NULL state, you can add as many pads as you need before starting the pipeline. Then while the pipeline is playing, you can link and unlink these existing pads instead of adding and removing them.
Here is this idea, in snip-its of code:

  1. Decide on the maximum number of outputs you expect to need out of nvstreamdemux:
#define MAX_NUM_OUTPUTS 8
  1. Add pads to the nvstreamdemux element when constructing the original pipeline:
// make the demux element
demux = gst_element_factory_make ("nvstreamdemux", "stream-demuxer");
// at some point BEFORE PLAYING, add all the output pads to demux
for (guint i=0; i<MAX_NUM_OUTPUTS; i++)
{
  gchar demux_src_pad_name[8];
  g_snprintf (demux_src_pad_name, 7, "src_%u", i);
  GstPad *demux_src_pad = gst_element_get_request_pad (demux, demux_src_pad_name);
  gst_object_unref (demux_src_pad);
}
  1. Any time you need another output from your nvstreamdemux element, you will need to link the demux output (“source”) pad to the receiving element’s input (“sink”) pad:
// first prepare your sink element
// in my pipeline, after demux, I have a bin with:
// queue | nvvideoconvert | nvdsosd
GstElement *sink_element = your_sink_element;
// decide which output stream you want
guint index = 3; // suppose you want the fourth output stream

// get the sink pad from sink_element
// in my code demux links to a queue
// since queue takes one input, it has a static pad simply named "sink"
GstPad *sink_pad = gst_element_get_static_pad (sink_element, "sink");
// get the source pad from demux
gchar demux_src_pad_name[8];
g_snprintf (demux_src_pad_name, 7, "src_%u", index);
GstPad *demux_src_pad = gst_element_get_static_pad (demux, demux_src_pad_name);
// link the pads
if (gst_pad_link (demux_src_pad, sink_pad) != GST_PAD_LINK_OK) {
  g_printerr ("Failed to link sink_element to demux!\n");
  // handle errors in any way you see fit (maybe exit)
}
// don't forget to unref your pad pointers
gst_object_unref (demux_src_pad);
gst_object_unref (sink_pad);
  1. Any time you need to remove an output from your nvstreamdemux element, you will need to unlink the demux source pad from the receiving element’s sink pad:
// stopping sink_element by changing the state is normal and is not part of the workaround

// after you have stopped the source that feeds into the nvstreammux
// now you can stop the sink that is fed out of nvstreamdemux, using the same method:
GstStateChangeReturn state_change_return =
    gst_element_set_state (sink_element, GST_STATE_NULL);
switch(state_change_return) {
  case GST_STATE_CHANGE_ASYNC:
    // wait for the state change to complete
    gst_element_get_state (sink_element, NULL, NULL, GST_CLOCK_TIME_NONE);
  case GST_STATE_CHANGE_SUCCESS:
    // DO NOT DO THIS (do not remove demux's source pad):
    // gst_element_release_request_pad (linkedElement, linkedPad);
    
    // WORKAROUND:
    // instead leave demux's source pad and unlink it from sink_element's sink pad
    // first get the demux source pad using its index
    gchar demux_src_pad_name[8];
    g_snprintf (demux_src_pad_name, 7, "src_%u", index);
    GstPad *demux_src_pad = gst_element_get_static_pad (demux, demux_src_pad_name);
    // then get sink_element's sink pad
    // here this is done by getting the peer of demux's source pad
    GstPad *sink_pad = gst_pad_get_peer (demux_src_pad);
    // now unlink the pads
    gst_pad_unlink (demux_src_pad, sink_pad);
    // and don't forget to unref the pads
    gst_object_unref (sink_pad);
    gst_object_unref (demux_src_pad);
    // END OF WORKAROUND

    // remove sink_element
    gst_bin_remove (GST_BIN (pipeline), sink_element);
    gst_object_unref (sink_element);

    break;
  case GST_STATE_CHANGE_FAILURE:
    g_printerr ("STATE CHANGE FAILURE\n");
    break;
  case GST_STATE_CHANGE_NO_PREROLL:
    g_print ("STATE CHANGE NO PREROLL\n");
    break;
  default:
    break;
}

I hope anyone who needs to dynamically change the outputs from nvstreamdemux finds this workaround code usable. Feel free to use this code for your purposes.
If anyone (Nvidia people?) see some error in this code, or have any other information about this (feature addition timeline?), please let me know.

Credit where credit is due: this solution to the dynamic changes in feeds with demux problem is laid out by another user in this comment on another post:

Thanks!

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