[Bug 779787] waylandsink: handling y-flipped dmabuf

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=779787

--- Comment #3 from Damian Hobson-Garcia <dhobsong at igel.co.jp> ---
Please allow me to add some background to this discussion.

The y-flip setting is for dmabuf wayland protocol only.  Shm buffers do not
have this flag.

The need for the flag comes from the fact that OpenGL generally has the origin
(0,0) at the bottom-left corner, while other sources typically, but not
necessarily have it at the top-left, which causes a problem when the two
different types of buffers are exported and imported via dmabuf. (The GL
texture2D functions take this into account for buffers that are imported via a
virtual memory address).  

I have discussed this with Shinya and we both agree with you that the ideally
the user shouldn't have to care about this setting and should be set
automatically by the meta-data.  However we couldn't find any meta-data in use
that seemed to capture the necessary information.  I think that the orientation
tag can be used if the input comes from a device with at top-left origin, but
its also necessary to determine if the buffer has been written by OpenGL (or
another boffom-left origin source) and not modified elsewhere in the pipeline.
We didn't want to propose some new meta-data for just this one case, so we
opted for the property instead.  

Also, the "polarity" of the flag changed with Weston 1.12.  Before that y-flip
being set would mean that the origin was in the top-left, but after 1.12, it
now means that it is in the bottom-left,  (cf.
https://cgit.freedesktop.org/wayland/weston/commit/?id=319397e050e2b4833e10093ccefd8ad77a6ef78d)
so a different setting will be necessary depending on the version of weston
installed on the system.  I suppose thought that this is something that could
be addressed with the configure scripts.

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