Description
The Vector Corner Pin modifier is very similar to the UV Map generator and shares the same workflow.
In order to place the image to match-move correctly, the Vector Corner Pin offers a Corner Pin widget right in the viewport on which you can move the To Points.

Whenever moving the To Points, a keyframe will be created on the "User To" parameters in the timeline. Each keyframe serves as a new reference frame for the UV maps: at a keyframe, the deformation caused by the UV Map will be pass-through and only the Corner Pin you adjust is used as transformation on the image.
Whenever adding new keyframes or editing keyframes, you need to re-track the UV-Maps in between the previous and next keyframes. To do so, you can use the horizontal toolbar in the viewer contextual interface.

The first 4 buttons are used to launch tracking over different ranges of frames. The next 4 buttons are used to remove keyframes on the Corner Pin. The Motion-Vectors and UV-Maps menus are useful to remove generated files on disk.
When Overwrite Files is left unchecked in the menu, any existing file will be preserved when re-generating, thus skipping computations. Turning it on will force a re-generation of the motion-vectors/UV-Maps.
Changing these parameters require a recomputation of the motion-vectors:
- Frame Distance
- Motion Estimation parameters
- EXR Disk Cache Path
- Filename Identifier
- Track Region Mode or Track Region rectangle
- Motion source
Changing these parameters require a recomputation of the UV-Maps:
- Any parameters that affect the motion-vectors
- Any change made on the Corner Pin
Setting new keyframes
When the match-move starts to drift, you can place a new keyframe by moving any To point in the viewport. You will then need to relaunch a track, using the viewer toolbar. Make sure "Overwrite Files" in the UV-Maps settings is checked.
Smoothing the motion
You may want to reduce the strength of the deformation caused by the motion vectors. To do so, the following parameters can be used:
-
Frame Distance: Increase it to compute vectors across a longer range of time so that motion is more smooth. For fast motions, this can lead to drifts
-
UV-Map Blur: Slightly blur the map in order to avoid artefacts. A high blur can lead to drift in areas with complexe motions, such as the border of the lips of a human face
-
Motion-Vectors influence: Reducing it will give more weight to the Corner Pin transformation, reducing the visible deformation applied by the motion vectors.