[gstreamer-bugs] [Bug 570144] New: decodebin2 decodes unwanted streams

[GStreamer (bugzilla.gnome.org)]

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  GStreamer | gst-plugins-base | Ver: git
           Summary: decodebin2 decodes unwanted streams
           Product: GStreamer
           Version: git
          Platform: Other
        OS/Version: Windows
            Status: UNCONFIRMED
          Severity: minor
          Priority: Normal
         Component: gst-plugins-base
        AssignedTo: gstreamer-bugs at lists.sourceforge.net
        ReportedBy: lrn1986 at gmail.com
         QAContact: gstreamer-bugs at lists.sourceforge.net
     GNOME version: Unspecified
   GNOME milestone: Unspecified


While application may not link some pads exposed by "pad-added" signal, this
will not prevent decodebin2 from decoding data for these pads.

Because of that decoding time significantly increases, since data from unwanted
(unlinked) pads are still being decoded to a raw form.

In theory, a decoder (or a bin it is placed into) should check whether
decoder's output is required by downstream elements or not, and skip the
decoding when it is not required. In practice this doesn't happen.

Workaround: tap on "autoplug-select" signal do not allow decodebin2 to autoplug
unwanted streams any further, preventing it from attaching decoders to the
pipeline, so no decoding takes place for these streams. Whether a stream is
unwanted or not could only be decided by analyzing proposed GstElementFactory,
because GstCaps may not contain necessary information. For example, for
audio/video in mp4 container first GstCaps that is passed has zero structures
and it's name is "video/quicktime", so it could be identified as video, while
in reality it's just a container which *must* be allowed to be autoplugged for
other streams to appear.

Pipeline that exposes this bug: uridecodebin -> audioconvert -> appsink


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