Gstreamer将decodebin2链接到autovideosink

Question

I'm trying to add some processing logic to a program that chugs away on a local video file, but I'm having some trouble understanding how to translate the following (successfull) gst-launch command into code (and supply it with a "pad-added" callback):

gst-launch filesrc location=/path/to/my/video.avi ! decodebin2 ! autovideosink

I've tried my hand at gstreamer's basic-tutorial-3 , using decodebin2 in place of audioconvert :

data.source = gst_element_factory_make("filesrc", "source");
data.convert = gst_element_factory_make("decodebin2", "uridecoder");
data.sink = gst_element_factory_make("autovideosink", "autodetect");

However, I am never able to link data.convert to data.sink , as it is outlined in the example; the gst_element_link(data.convert, data.sink) always fails. I suspect there's some special treatment for decodebin2 . Some gstreamer users have metioned using ghostpads and seperate bins , which after a swift attempt, also yielded no success:

data.bin = gst_bin_new("processing-bin");
gst_bin_add_many(GST_BIN(data.bin), data.decoder, data.sink, NULL);
gst_element_add_pad(data.bin, 
                    gst_ghost_pad_new("bin_sink",
                                      gst_element_get_static_pad(data.decoder,"sink")));

I'm a little confused as how to continue debugging. Does anyone else have any pointers?

Here is a boiled down gist of the current code: ( gist )

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Update: My callback is now firing, I think it was because I had an incorrect filename for the filesrc location (whoops)

Now, after following the advice below, I am able to confirm that I'm getting audio and video caps types and check against them in the pad-added callback. However, I'm now getting the following "not-linked" error after one frame of pad processing:

Debugging information: gstavidemux.c(5187): gst_avi_demux_loop (): /GstPipeline:gstreamer-test/GstBin:processing-bin/GstDecodeBin2:uridecoder/GstAviDemux:avidemux0:
streaming stopped, reason not-linked

Answer 1

If your file contains both audio and video, your callback will be called for both audio and video pads. Hence in the callback you should check the caps of the pad and ensure you are trying to link only the video pad to the video sink

Answer 2

I think the main issue I was running into is that I was listening to the decoder for new pads, when I should've been getting the static pad from the video sink. I've updated the gist with my callback function to illustrate the difference.

Basically, it boiled down to this change:

I changed

GstPad *sink_pad = gst_element_get_static_pad(data->decoder, "sink");  

to the following

GstPad *sink_pad = gst_element_get_static_pad(data->sink, "sink");