[Bug 764667] New: videoaffinetransformationmeta: doesn't define the coordinate space

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=764667

            Bug ID: 764667
           Summary: videoaffinetransformationmeta: doesn't define the
                    coordinate space
    Classification: Platform
           Product: GStreamer
           Version: git master
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: gst-plugins-base
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: ystreet00 at gmail.com
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

i.e. currently it's not defined whether it operates on vertex values between
[0, 1] or [-1, 1] or put another way, where the origin is which is an important
consideration when writing/integrating transformation matrices.

e.g the flip matrix is different between the two above mentioned coordinate
spaces.

[0, 1]:
1, 0, 0, 0,
0, -1, 0, 0,
0, 0, 1, 0,
0, 1, 0, 1,

Origin is in a corner.

[-1, 1]:
1, 0, 0, 0,
0, -1, 0, 0,
0, 0, 1, 0,
0, 0, 0, 1,

Origin is in the centre.

In the wild, both conventions are used in different places.  In OpenGL, vertex
coordinates use [-1, 1], texture coordinates are [0, 1].  DirectX seems to be
the same except the z-axis is [0, 1] and uses a different handedness (another
thing to define) in their coordinate system.

The current assumption by android MediaCodec is that the matrix is used for
texture coordinates, i.e. [0, 1] however changing between them is relatively
easy (another two matrix transformations).

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