Huh?: Gstreamer Basic Tutorial / API (3: Dynamic pipelines)

[Matthew Waters]

On 16/5/19 1:11 pm, R C wrote:
>
> so while messing with it,  I forgot to change teh second part, and
> even though it seems incosistent, can someone explain why this works? 
> (I am just putting the declarations here,  rewritting to ugly got's
> into something nicr is not the problem)
>
>
> in the main part the declarations I had are:
>
> // Create the elements
> data.source = gst_element_factory_make("uridecodebin", "source");
> data.audioconvert = gst_element_factory_make("audioconvert",
> "audioconvert");
> data.audiosink = gst_element_factory_make("autoaudiosink", "audiosink");
> data.videoconvert = gst_element_factory_make("videoconvert",
> "videoconvert");
> data.videosink = gst_element_factory_make("autovideosink", "videosink");
>
>
> and I forgot to make the changes in the second part
> (pad_added_handler), surprised it worked, and seems counter-intuitive?
>
> GstPad *audiosink_pad = gst_element_get_static_pad(data->audioconvert,
> "sink");
> GstPad *videosink_pad = gst_element_get_static_pad(data->videoconvert,
> "sink");
>

"sink" is the name of the pad on the element you're retrieving from
(audio/videoconvert in this case) not the name of the element.

> but  ..  why??  How can it even find the right GstElement, since none
> of the are named "sink"?
>

The element is the 'data->audio/videoconvert' parameters you pass into
'gst_element_get_static_pad'.

Cheers
-Matt

> docs:
>
> ******************************************************************
>
>
>       gst_element_get_static_pad ()
>
> GstPad <https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/GstPad.html> *
> gst_element_get_static_pad (/|GstElement
> <https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/GstElement.html>
> *element|/,
>                             /|const gchar
> <https://gstreamer.freedesktop.org/usr/share/gtk-doc/html/glib/glib-Basic-Types.html#gchar>
> *name|/);
>
> Retrieves a pad from /|element|/ by name. This version only retrieves
> already-existing (i.e. 'static') pads.
>
> *Parameters*
>
> element a GstElement
> <https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/GstElement.html>
> to find a static pad of.
>
> name the name of the static GstPad
> <https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/GstPad.html>
> to retrieve.
>
>
>         Returns
>
> the requested GstPad
> <https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/GstPad.html>
> if found, otherwise |NULL|
> <https://gstreamer.freedesktop.org/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#NULL:CAPS>.
> unref after usage.
>
> MT safe.
>
> ************************************************************************
>
>
>
> thanks,
>
>
> Ron
>
>
>
> On 5/15/19 8:33 PM, R C wrote:
>>
>> Hello,
>>
>> I am trying to get a little more familiar with gstreamer, and thought
>> I'd do some
>>
>> of the "basic" programming tutorial examples.
>>
>>
>> I did play with the 3rd example:
>>
>> https://gstreamer.freedesktop.org/documentation/tutorials/basic/dynamic-pipelines.html?gi-language=c
>>
>>
>> If I copy the example, and compile it, it works, no surprise there.
>> But when I change it and do the "exercise", I get some weirdness.
>>
>>
>> What I noticed is (well I probably missed something)...  but the
>> function call:
>>
>> |data.sink = gst_element_factory_make ("autoaudiosink", "sink");
>> According to the docs, the 2nd parameter can be 'whatever' you want
>> to name it. Since I wanted to do the video part too, I figure I
>> should change the name to "audio-something", and changed the second
>> function call to: ||data.convert = gst_element_factory_make ("audioconvert", "convert");
>> (original) data.sink = gst_element_factory_make ("autoaudiosink",
>> "audiosink"); (changed name)|
>>
>> of course in the even handler (pad_added_handler), the name needs to be changed in to:
>>
>> |GstPad *sink_pad = gst_element_get_static_pad (data->convert,
>> "audiosink");Browsing through the code, that should be it, BUT it seems "||sink_pad|"|||| always is NULL
>>  and the pipeline elements don't/can't be linked. (see errors below)
>>
>> since this is supposed to be a simple example,  what am I missing?
>>
>> thanks,
>>
>> Ron
>>
>>
>> errors:
>> # ./basic-tutorial-3
>> Pipeline state changed from NULL to READY:
>> Received new pad 'src_0' from 'source':
>>
>> (basic-tutorial-3:14679): GStreamer-CRITICAL **: 20:22:54.991: gst_pad_is_linked: assertion 'GST_IS_PAD (pad)' failed
>> It has type 'video/x-raw' which is not raw audio. Ignoring.
>>
>> (basic-tutorial-3:14679): GStreamer-CRITICAL **: 20:22:54.992: gst_object_unref: assertion 'object != NULL' failed
>> Received new pad 'src_1' from 'source':
>>
>> (basic-tutorial-3:14679): GStreamer-CRITICAL **: 20:22:54.992: gst_pad_is_linked: assertion 'GST_IS_PAD (pad)' failed
>>
>> (basic-tutorial-3:14679): GStreamer-CRITICAL **: 20:22:54.992: gst_pad_link_full: assertion 'GST_IS_PAD (sinkpad)' failed
>> Type is 'audio/x-raw' but link failed.
>>
>> (basic-tutorial-3:14679): GStreamer-CRITICAL **: 20:22:54.992: gst_object_unref: assertion 'object != NULL' failed
>> Error received from element source: Internal data stream error.
>> Debugging information: gstbasesrc.c(2950): gst_base_src_loop (): /GstPipeline:test-pipeline/GstURIDecodeBin:source/GstSoupHTTPSrc:source:
>> streaming stopped, reason not-linked (-1)
>>
>
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