Falloff Point System
The blue points pictured around the lips are the source of all falloff deformation. Each vertex is assigned one of these points to guide post-mouth-curvature skin deformations.
How should lip curvature affect the skin around the mouth?
This system is designed to use the curvature of the lips to affect the skin around the face, as is common in human speech. Although it’s a series of muscles that do the deformations in an actual human, lip curvature is used in our system to simulate this movement.
During MTP setup, each vertex on the head mesh is assigned a sibling outer lip point. This is essentially each point’s nearest outer lip point.
Additive and subtractive regions are then defined, which protect certain areas of skin from being unnaturally distorted, like around the eyes.
Sibling Delta Falloff
The further a point is from it’s lip sibling, the less influence it’s sibling likely has.
To determine the maximum amount of influence each lip point can have, each point’s position is stored at the beginning of every frame, but just after the bone weighting has been applied. This is to ensure that we’re capturing the change of the lip curvature. Here’s a quick rundown of the frame loop:
First, the underlying bones are moved to position the coarse features of the head and face.
Then, a snapshot is taken of the current state of each vertex. This is when we capture the ‘start’ position of each lip point.
Next, the lip points are manipulated to their desired shape.
We then take the delta change offset of each lip point. This change is considered the maximum amount to move each sibling falloff point by. Generally, the closer the falloff point is to its lip sibling, the more of this value is used.
As the point falloff reduces (the point is farther from it's sibling or is part of a negative falloff zone), less of this delta change is mixed in.
Point Falloff Zones
There are a number of zones that affect each point’s falloff value. They are illustrated in the video above and broken down below.
