In order to correctly project images onto televisions,
bulletin boards, and other quadrilaterals in an image sequence use
the 4-Point tracking workflow.
With four-point tracking
(also known as corner pinning), you use four trackers to generate
tracking data for anchoring the four corners of a bilinear surface
to background footage.
The reference points
you use must be well-defined; it is recommended that you plan them
when shooting the sequence (for example, add markers to the scene).
It is not always possible to do four-point tracking when the reference points
are not well-defined.
In the following example,
the goal is to pin an image of a jet to the screen of a monitor
as the camera moves.
NoteYou can also use
the Reaction tool, and the position and translation parameter controls
of the Warp2D tool, and the position and translation vertices parameter
controls of the Garbage Mask, and Remove Dust tools.
To corner pin and track a bilinear surface
to background footage:
- In the Schematic view, the basic dependency
graph has been set up with the following nodes:
Node |
Purpose |
Input image (front) |
This is the footage that will tracked. |
Input image (back) |
This image will be corner-pinned onto the front footage. |
Keyer |
The Keyer is used to remove the blue portion of the monitor. |
Tracker super tool |
The Tracker will track and analyze the movement of the
four corners of the monitor. The Tracker super tool is used because
multiple Tracker Analyzers are required.
|
Blend & Comp |
The Blend & Comp node will combine the two processed
images.
|
2D Transform |
The 2D transform tool will apply the necessary transformations
to the image to be corner-pinned.
|
Output |
Outputs the final composition. |
- Remove the blue screen from the monitor
with the Keyer.
- Create four Tracker Analyzers and name
them as per their corner positions, then position them in the locations
that will make up the four corners to pin.
- Adjust the Display, Analyze, Reference
box and Tracking box settings.
- Analyze each track (do not forget to
reset the footage back to the start frame after each analysis).
You can also select the first Tracker Analyzer and Shift + click the last one to select
all trackers to analyze at the same time.
- Connect the RGBA output from the Keyer
to the Front input of the Blend & Comp node, and the output
of the second image into the Back input of the Blend & Comp
node.
- Add a 2D Transform tool to the graph
between the second image and the Blend & Comp node.
- With the 2D Transform tool highlighted,
select 4 Point from the Transform Type menu.
- Click Fit To Source
- Right-click on the Destination label
and select Set Trackers.
- Select the trackers in the same sequence
as you created them from the Tracker Selector window and activate
the Use Offset toggle.
- Click Link.
The bilinear image is
pinned to the background image.
- If the corner-pinned image is too small
or too large for the screen it's replacing, create another transform
and adjust its scale while in the SRT transform type.