Caps is empty - can anyone explain why?

[Graham Leggett]

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"?

Regards,
Graham
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