The Nitty Gritty

The Nitty Gritty

It’s normal for a track to be slightly more complicated than
this, usually due to motion or perspective shifts within the
track area. This can be the result of foreground objects
passing across the track region or the appearance of the
region itself changing over time.
 shows an otherwise straightforward track of a
handheld game unit with the following challenges: fl ares
and refl ections play across the screen, the hands move
back and forth across the unit, and the perspective of the
screen changes from fl at to face-on.

There are two standard solutions to any track that slips:
. Sudden slippage is often the result of foreground
motion or light changing the appearance of the tracked
area; the solution is to mask out the area containing the
disturbance.
. Small, gradual slippage is often the result of shifts in
perspective and can be keyframed.
The clip shown in Figure 8.23 requires both techniques.
A track of the entire face of the unit shifts slightly as it is
tilted and a lot as the thumbs move across the track area
and refl ections play across the screen.
Big shifts in the track region are caused by changes in the
track area, so I fi x those fi rst, adding an additional spline
(or splines) containing the interruptive motion. The Add
X-spline and Add Bézier Spline create a subtractive
shape (or shapes) around the areas of the fi rst region that
contain any kind of motion. Figure 8.24 shows that these
can be oddly defi ned; they track right along with the main
planar track.

Retracking with these additional hold-out masks improves
the track a good amount; all that is required to perfect
this track is a keyframe at one point where the unit is tilted
about 15 degrees toward camera. I look for the frame where
the plane settles into its new, offset position and highlight
one corner of the main track mask (not the hold-outs).

When I highlight the point, zoomed views with crosshairs
appear at the upper-left or right corner (Figure 8.25).
At the right under Keyframe Controls, Autokey is on by
default. This is where it becomes helpful to have aligned
mask points with recognizable features, as the idea is to
move the points to match the adjacent position. This creates
a green keyframe along the main Timeline. Mocha
uses these keyframes as extra points of comparison, rather
than simply averaging their positions.
You need not rely on luck and manual labor to get points
to match over time. Open the AdjustTrack panel. Under
Nudge are controls allowing Mocha to automatically guess
where the point should move to match the Master, along
with controls to move it in fi ne increments. If Mocha isn’t
fi nding your point automatically, you can try adjusting the
neighboring AutoNudge controls. If it seems as though
you’ve started with a reference point that isn’t representative,
you can reset the Master under Reference Point.