Dev:Ref/Release Notes/2.69/Motion Tracker
Blender 2.69: Motion Tracker
Plane track is now supported in movie clip editor and compositor (r59176)
This plane track is used to replace things like billboards and screens on the footage with another image or video. it also might be used for masking.
First of all, there need to be at least 4 feature points tracked across the footage which belongs to the plane one wants to replace. Less tracks are not acceptable, more tracks will give better estimation of plane motion.
Feature points used to estimate plane motion could be used from any place on the plane, meaning it's not necessarily need to be corners. Corners are not always easy to be tracked, they might be occluded and so.
Let's consider example from the right, where we want to replace the window. It's tricky to track window corners (edges in the corners are quite unclear and one of corners is constantly occluding). In this case we've tracked features from the same wall but which are situated far away from the actual window. This gives much more accurate plane motion estimation.
It is also possible to have some tracks appear and disappear during the time, it's only required that two neighbor frames have at least 4 common tracks.
Once feature points are tracked, it's possible to create plane out of them. Template:Button button is used for this. This button will create a plane object which is deforming in the same way as plane defined by all selected point tracks (point track is just a regular old-school track).
It is likely that corner of this plane object need to be manually adjusted. To do this sliding individual corners with mouse or general transform tools (G, R, S) could be used.
Adjusting plane corners will keep it following the plane defined by tracks it was originally created from.
Left bottom corner of the plane does have X/Y axis (X is red, Y is green) to help distinguishing orientation of the plane in space.
It is now possible to parent mask points to plane track. To do this, you might either manually set parent to Plane Track and choose which plane to parent to, or you might just select Plane Track and all mask points you want to parent to the plane and hit CtrlP.
New compositor node called Plane Track Deform was added. This node gets a single input, which is an image which need to be warped to the plane track. And this node will output image warped to the plane and will also give a mask created from the plane.
Internally plane track is implemented pretty much straightforward. Homography between two frames is getting estimated from the point tracks. This is why 4 common tracks at neighbor frames is the minimum requirement.
Then this homography is applying on a corners of plane object, which makes this object follow the plane.