Problems defining application specific caps

[Tiago Katcipis]

On Tue, Jun 3, 2014 at 4:09 AM, Sebastian Dröge <sebastian at centricular.com>
wrote:

> On Mo, 2014-06-02 at 18:23 -0300, Tiago Katcipis wrote:
> > On Sun, Jun 1, 2014 at 5:14 AM, Sebastian Dröge <
> sebastian at centricular.com>
> > wrote:
> >
> > > On Sa, 2014-05-31 at 17:51 -0300, Tiago Katcipis wrote:
> > > > Hi,
> > > >
> > > > Sorry if I'm asking something stupid, but i was unable to find
> anything
> > > > about this subject on the docs (about caps it always explaining how
> the
> > > > different negotiation scenarios happens, but not this).
> > > >
> > > > I want to use a application specific caps on a plugin that I'm
> writing,
> > > it
> > > > will basically receive audio and produce a proprietary binary format
> > > > (neither audio or video).
> > > >
> > > > So i create my caps like:
> > > >
> > > > static GstStaticPadTemplate src_factory = GST_STATIC_PAD_TEMPLATE
> ("src",
> > > >     GST_PAD_SRC,
> > > >     GST_PAD_ALWAYS,
> > > >     GST_STATIC_CAPS ("application/vnd.mydata+octet-stream")
> > > >     );
> > > >
> > > > And i run a pipeline with fakesink or filesink (both accepts "any"
> caps),
> > > > but it fails to link it:
> > > >
> > > > WARNING: erroneous pipeline: could not link myplugin to fakesink0
> > > >
> > > > if i just change the caps to:
> > > >
> > > > static GstStaticPadTemplate src_factory = GST_STATIC_PAD_TEMPLATE
> ("src",
> > > >     GST_PAD_SRC,
> > > >     GST_PAD_ALWAYS,
> > > >     GST_STATIC_CAPS ("audio/x-raw")
> > > >     );
> > > >
> > > > It works perfectly fine. It makes no sense to me since
> fakesink/filesink
> > > > should accept any type of data.
> > > >
> > > > Is Gstreamer limited to a fixed set of caps ? I cant define a new
> > > > application specific caps ?
> > >
> > > GStreamer does not limits itself to a fixed set of caps. The only
> reason
> > > why custom caps might not be accepted by another element is if that
> > > element does not accept these custom caps.
> > >
> >
> > I tested a little further and i got this:
> >
> > gst-launch-1.0 audiotestsrc ! appspecificcaps ! fakesink
> > WARNING: erroneous pipeline: could not link appspecificcaps0 to fakesink0
> >
> > gst-launch-1.0 fakesrc ! appspecificcaps ! fakesink
> > works fine.
> >
> > It seems that changing the source causes the problem, which seems odd
> since
> > a intersection between the source pad and the sink pad is only required
> > (and automatically done/checked) on plugins that inherits from
> > GstBaseTransform. But my plugin inherits from GstElement (actually i have
> > an empty template that reproduces the problem).
> >
> >
> > >
> > > But you say you directly link your element to a fakesink? That should
> > > work just fine, and if it doesn't please provide a debug log with
> > > GST_DEBUG=6. It will contain the reason why it doesn't work.
> > > If changing the caps to audio/x-raw makes it work in exactly the same
> > > pipeline, then this suggests that you have an element between your
> > > element and fakesink that only supports raw audio.
> > >
> >
> > I attached the log. From what i can see it seems that a intersection
> > between the sink pad and the src pad of my element is being made, is this
> > the default behaviour ?
>
> It's only the default behaviour if you set the GST_PAD_FLAG_PROXY_CAPS
> flag on the sinkpad, or of course if your query handler on the sinkpad
> does that.
>
> Your debug log suggests that you have the PROXY_CAPS flag set on both
> pads.
>

Yeah, the docs make it very clear, since the query is being proxied down,
when the sink pad has a audio caps it will fail to negotiate, when the caps
is ANY (the fakesrc) it will work.

Maybe it would be cool to add a /* TODO: MESSAGE */ on the gst-template to
help future developers see that they need to change this if they are
writing a plugin like a decode/encoder.

I'm starting to rewrite some plugins from gstreamer 0.10 to 1.0 and i don't
remember to have any problems with this proxy caps stuff.

But I'm also a little confused if i should invest on gst-template since the
tutorials talk about it and then redirects you to a project on gst-plugins-bad
(an element maker).

Thanks for the help Sebastian.


>
> --
> Sebastian Dröge, Centricular Ltd - http://www.centricular.com
> Expertise, Straight from the Source
>
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