[Bug 761251] textoverlay: Expose text rendering dimensions to applications and remove absolute positioning limit

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=761251

--- Comment #17 from Lubosz Sarnecki <lubosz at gmail.com> ---
So I came up with a patch that introduces a new alignment without the clamp, so
the old alignment mode will behave like before:
https://git.collabora.com/cgit/user/lubosz/gst-plugins-base.git/commit/?h=textoverlay-pitivi2&id=caa677447abe18d2a93a2e5477459ab5afe48cd1

But what I understand now is that the property range is not compatible with how
 GstController uses it. Can't GstController handle +/-MAX_DOUBLE values at all,
or is just changing the range of an existing property breaking compatibility?

==
About the current mapping + clamping:
==

I can't understand why the property gets clamped and not mapped to the range it
wants to achieve:
For example the current upper left position for an example text is ~ [0.17,
0.02]
http://i.imgur.com/RZmu7nB.png

This is dependend on canvas and text aspect ratios. The LEGAL properties
between [0,0] and [0.17, 0.02] are clamped to the same position (upper left
corner). This is a hack to achieve that 0,0 is the upper left corner.
The same problem occurs at the bottom left, where [0.83, 0.98] is bottom right,
and legal values between [0.83, 0.98] - [1.0,1.0] are useless. 

This mapping will lead to weird animations with GstController.

I don't know why this mapping is used, maybe because of the padding, but a
valid range [0,0]-[1,1] for inside the canvas could be mapped like this without
clamping:

http://i.imgur.com/uhMrj11.jpg

And without restriction it could actually be moved out of canvas, to make
animations where text slides in etc.

TL;DR: The current mapping does not make sense to me.

==
About how to solve the incompatibility of the new range:
==

I think thibault's solution to introduce 2 new properties (btw, are there
vector / list properties?) for positioning with infinite range.
Should this property be only used when the alignment mode is changed to
absolute, like in the patch above?
Will an infinite range be usable in GstController?

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