[Bug 747378] avfvideosrc: orientation property

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=747378

--- Comment #3 from Ilya Konstantinov <ilya.konstantinov at gmail.com> ---
(In reply to Tim-Philipp Müller from comment #2)
> There's a GstVideoOrientation interface fwiw

I actually did look for one, before introducing mine.
GstVideoOrientation deals with hflip, vflip, hcenter, vcenter. Does this
somehow represent 90-cw rotation?

Also, Apple talks in terms of device orientation, in helpful terms of
"portrait" and "landscape", so I'd rather offer this than "int rotation" (or
even worse "float rotation" in radians :) cause, like, rotation from what?

When someone records from an iPhone and says "Portrait", they understand right
away what it means.

> and orientation may also be signalled in tags (don't think we have a meta for that yet)

I think attaching an orientation tag is a job of a GstElement that polls a
position sensor. I think it'll be nice for GStreamer to have one, yes.

(Curiously, in the Apple Technical Q&A linked above, they discuss that on
AVCaptureMovieFileOutput they don't actually rotate buffers but rather set a
transform matrix so the video gets rotated on playback.)

In GStreamer, one could theoretically apply a glshader to rotate on playback
too, but I just wanted to expose this unique hardware-accelerated feature of
rotating the physical video buffer.

We can always still use glshader when it serves our pipelines best.

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