[Bug 738914] video: Add a linear/affine transformation matrix meta

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=738914

--- Comment #2 from Nicolas Dufresne (stormer) <nicolas.dufresne at collabora.co.uk> ---
The merged affine transform is not really "negotiated" as describe. Element
needs to support any value in the 4x4 matrix, or not support it at all. That
basically mean, videoscale cannot advertise support for the scaling matrix, and
videoflip cannot advertise support for the flipping matrix. So there is no
pipeline that can guaranty a ISOMP4 with an AffineMatrixMeta can be rendered
unless you have GL (I didn't see that patches for that either, but I might have
missed it).

So unless we have an element (software and GL) that can handle that meta, I'm
not sure this solution helps in enabling the matrix on ISOMP4, as that would
likely break things. Best effort for those matrix is kind of ugly. We also need
to figure-out and document the interaction with other metas. An example, when
the transform is from ISOMP4, it's clear it should be applied before placing
the text, but if the transform was added later by an element, then maybe it
should be applied later. How do we define the ordering of all meta that are
applied on a buffer ?

I think more work is needed, the Meta is just a peace of the puzzled thrown in.

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