Caps is empty - can anyone explain why?

[Sebastian Dröge]

On Sa, 2015-09-05 at 22:26 +0200, Graham Leggett wrote:
> Hi all,
> 
> I am currently developing an element that exposes the following src:
> 
> static GstStaticPadTemplate gst_xxx_src_template =
> GST_STATIC_PAD_TEMPLATE ("src",
>     GST_PAD_SRC,
>     GST_PAD_ALWAYS,
>     GST_STATIC_CAPS ("text/x-raw, format=(string)utf8")
>     );
> 
> I am trying to link my element to fakesink with “xxx ! fakesink”, but
> am having no luck at all:
> 
> 0:00:22.440949542  3551    0x21880 INFO                GST_PADS
> gstpad.c:2186:gst_pad_link_prepare: trying to link xxx0:src and
> fakesink0:sink
> 0:00:22.441148030  3551    0x21880 INFO                GST_PADS
> gstpad.c:3745:gst_pad_peer_query:<xxx0:sink> pad has no peer
> 0:00:22.441339851  3551    0x21880 INFO                GST_PADS
> gstpad.c:2242:gst_pad_link_prepare: caps are incompatible
> 0:00:22.441522089  3551    0x21880 INFO                 default
> gstutils.c:1891:gst_element_link_pads_filtered: Could not link pads:
> xxx0:(null) - fakesink0:(null)
> 0:00:22.441684170  3551    0x21880 ERROR           GST_PIPELINE
> ./grammar.y:616:gst_parse_perform_link: could not link xxx0 to
> fakesink0
> WARNING: erroneous pipeline: could not link xxx0 to fakesink0
> 
> Picking apart the gstreamer code we fail inside
> gst_caps_can_intersect() as follows:
> 
>   /* empty caps on either side, return empty */
>   if (G_UNLIKELY (CAPS_IS_EMPTY (caps1) || CAPS_IS_EMPTY (caps2)))
>     return FALSE;
> 
> How does a caps become empty?
> 
> Looking at gst-inspect-1.0 we have the following caps:
> 
> Pad Templates:
>   SRC template: 'src'
>     Availability: Always
>     Capabilities:
>       text/x-raw
>                  format: utf8
> 
> linking to
> 
> Pad Templates:
>   SINK template: 'sink'
>     Availability: Always
>     Capabilities:
>       ANY
> 
> Can anyone give me a clue why these two caps are interpreted as
> “empty"?

While the template caps are certainly compatible, maybe one of the pads
is returning EMPTY caps in the CAPS query, which is done during
linking.

Can you provide a full debug log with GST_DEBUG=6 from the time
gst_pad_link() is called to the lines you pasted above? That will tell
the exact caps it is checking for compatibility.

-- 
Sebastian Dröge, Centricular Ltd · http://www.centricular.com

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