Articulated Mattes

Articulated Mattes

An “articulated” matte is one in which individual mask
points are adjusted to detail a shape in motion.
Each adjustment that you make to an animated mask
requires the entire frame to redraw, whether in the Comp
or Layer viewer. Good rotoscoping requires eliminating
this delay, as details are clear only when seen in motion.
Doing so is simple:
1. Create a comp containing only the source plate.
2. Add a solid layer above the plate.
3. Turn off the solid layer’s visibility.
4. Lock the plate layer.
5. Select the solid layer and draw the fi rst mask shape,
then press Option+Shift+M/Alt+Shift+M to keyframe it.
Now any changes you make to the masked solid have no
effect on the plate layer or the cache; you can RAM Preview
the entire section and it is preserved, as is each frame
as you navigate to it and keyframe it. When it comes time
to apply the masks, you can either apply the solid as a track
matte or copy the masks and keyframes to the plate layer
itself. Genius!
Specifi c strategies make effective rotoscoping of complex
organic shapes possible, regardless of tools:
. Use multiple overlapping masks instead of a single
mask on a complex, moving shape.

One key to working quickly is to intentionally create
overlapping, separate mask shapes for different parts
of your masked area that you know will move independently,
or be revealed and concealed (in this case,
because of a simple perspective shift). A shape with 20
or more points will be diffi cult to manage over time.
. When working with multiple masks, you can lock, hide,
or show unselected masks to get them out of the way
(and bring them back): context-click > Mask > Lock
Other Masks/Hide Locked Masks/Show All Masks.
. Defi ne a shape beginning on a frame where it can be
drawn with the fewest possible points, adding more
points as needed as you go. As a rule of thumb, no
articulated mask should contain more than a dozen or
so points.
. Go through the clip and look for the natural keyframe
points, where a change of direction, speed, or shape
begins or ends, and block in keyframes before you animate
them.
. To replace a Mask shape, in Layer view, select the shape
from the Target menu and start drawing a new one;
whatever you draw replaces the previous shape. Beware:
The fi rst vertex point of the two shapes may not match,
creating strange in-between frames. To set the fi rst vertex
point, context-click on any mask vertex.
. Use a mouse—a pen and tablet system makes exact
placement of points diffi cult.
. Use the arrow keys to move points. The increments
change according to the zoom level of the current view.
. Select a set of points and use the Tranform Box to
offset, scale, or rotate them together instead of moving
them individually . Most objects shift perspective
more than they fundamentally change shape,
and this method uses that fact to your advantage.
Block in and refi ne the fi rst shape, then refi ne as necessary.
Check the result at full speed against a contrasting
background, just as you do with color keys. Then move on
to the next shape.