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Implemented 2D stabilization:
- Based on average point of several tracks.
- Tracks for stabilization are controllable from
"2D stabilization" panel.
- Implemented autoscaling. Could fail if average point is
too close to frame boundary.
- Background clip in 3D viewport and MovieClip input
compositor nodes are using stable shot.
Some internal weak things:
- Stable shots aren't cached in moviecache. There is
only one "slot" for stable shot inside stabilization
structure.
- This could fail if several threads are requested different
frames (for example when starting playback, pause to make
Nodes Editor trigger refresh, continue playback).
- Due to it shouldn't crash i decided to implement better
caching for stable shots a bit later.
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