[Bug 768160] qtplugins: How to implement qmlglsrc.

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=768160

--- Comment #44 from Matthew Waters (ystreet00) <ystreet00 at gmail.com> ---
(In reply to Haihua Hu from comment #39)
> How can qmlglsrc get to know if downstream support the transformation meta?
> The only method that comes to me is to use GST_TAG_IMAGE_ORIENTATION to
> inform downstream the orientation which is support by videoflip, glvideoflip
> and glimagesink, but need set property to let them get tag event. If use
> GST_TAG_IMAGE_ORIENTATION, we should remove transformation meta from output
> buffer in case that glimagesink will do flip twice.

The transformation meta negotiation is exposed through the allocation query
that you will receive from upstream (glimagesink) proposed by
https://cgit.freedesktop.org/gstreamer/gst-plugins-bad/tree/ext/gl/gstglimagesink.c#n1933
and an example of checking for it's existence:
https://cgit.freedesktop.org/gstreamer/gst-plugins-bad/tree/ext/gl/gstgltransformation.c#n644

The affine transformation meta should be preferred over the image-orientation
tag.

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