In some
cases, you may want the object to remain stationary and track only changes
in scale. For example, assume that a camera is looking down a straight
road at a car approaching. You need to track the license plate as
the car approaches.
The Tracker requires
two tracker boxes to measure scaling. The changing distance between
the centers of the two reference boxes determines the scale.
In our example, you want
to lock a new license plate to the old one. You would position one
reference box over one corner of the license plate and the other
reference box over the diagonally opposite corner.
The license plate is
a layer. It assumes you have a Reaction node with two layers—the
background layer and the source layer. In this example, the background
is the car footage and the source is the license plate footage or
object.
NoteYou can also use
the 2D Transform, and the scale parameter controls of the Garbage
Mask, Remove Dust and Warp2D tools.
To track changes in scale only:
- Determine a good reference frame and
position the background footage at that frame.
- Select the Tracker Analyzer tool from
the Tools tab and drag it to the Schematic. You do not need to attach
it directly to the dependency graph as it is not going to be modifying
the image but rather, you will be using data associated with that
tracker.
- Attach the composition to the input of
the tracker.
- Add a second Tracker Analyzer.
- Resize and reposition each Tracker, so
that they track the scaling change. (In this example, the Trackers
are placed over the upper-right and lower-left corners of the license
plate.)
- Adjust the settings for each Tracker.
- Select the first analyzer from the File
browser and click Analyze to start the analysis.
- Return to the first frame.
- Select the second analyzer from the File
browser and click Analyze to start the analysis.
- Drag a Reaction tool from the Tools tab
and place it in the dependency graph between the footage node and
the Output node.
- With the cursor over the Reaction node,
right-click and select Add Source.
- At the bottom of the Layer Editor, select
Bilinear and click Create.
A new layer is created.
- Attach the second composition (or object)
as source 1.
- Go back to the first frame.
- Click the Reaction tab.
- Right-click Scale and select Track Scale.
- Select both Tracker Analyzers using Ctrl + click, and click Link.
An expression is automatically
created linking the tracking data to the scaling of the new license
plate.
- Play the footage to view the result.
NoteWhen tracking data
needs to applied to more than one layer in a composition, add an
extra axis node to the dependency graph and connect it to the appropriate
layers. Linking the axis to several layers facilitates panning multiple objects
inside a reaction. In this case you would then click the axis node
instead of the Layers node at step 15, and right-click the Scale
label and continue the procedure.
