The face is a complicated structure to recreate in 3D and more so if you want it to distort realistically, At first glance the facial distortion seems very complex with distortion happening all over its surface in the form of skin sliding over bone, muscles directly affecting the surface or skin creasing & stretching due to muscle activity near by.
A tried and tested method for dealing with most of this seemingly complex movement of the face is the facial edge loop system, originally derived by studying the musculature of the face. 3 muscle sphincters around the eyes and mouth who’s only purpose is to close the opening it surrounds and the radiating muscle groups designed only to open the hole within the sphincter.
In our experience with building, rigging and, more importantly, animating faces, we have learned of some limitations with this muscle derived structure. while the basic principle is sound, it is based on muscle movement and even seems to be influenced by its aesthetics and doesn’t take into account the surface movement that we are actually trying to simulate.
2 significant problem areas with muscle derived loops:
1. the eye: there is way too much attention given to the eyelid. have a look at your eyelids and the area surrounding it, now make this area move around by just moving your eyeball and opening and closing your eyelids. Do not use your cheeks or jaw to do this. you will notice that the eyelids have a very subtle effect on its surroundings, the movement you get in the cheek is of the skin being drawn up towards the eye, about 0.5 – 1.0 cm. now in comparison move the corner of your mouth. Notice the effect that has on your face, we have movement of the entire cheek, extending from the jaw just past the chin to the nose to directly under the eyelid and all the way to the side of the eye socket and even as far as the temple. All this from a simple movement of the mouth
2. the cheek: following from the point above, noting the amount of cheek movement and the direction of that movement. Notice how the entire cheek within the area described before, all move as one mass along an arc which pivots somewhere near the inside corner of your eye. now lets look at that area on the muscle derived loops.
notice how the two sets of loop, one originating from the eye and the other radiating from the mouth both intersect at this point creating quads and edges flowing in very different directions. this lay out is not desirable for the kind of distortion we need for the cheek. the movement of the cheek in this area will force us to bring diagonally opposite points of the quad closer together. This is very undesirable. to explain, lets look at a quad.
a quad is basically two triangle sharing an invisible edge, there are good ways and bad ways to distort a quad along its UV (or XY) surface,
this distortion is acceptable the texture stretches evenly across the surface
if you are not able to control the direction of your invisible edge you may end up with something like this. on one side you have a lot of distortion which is highlighted even more by the lack of distortion in the other triangle.
Even if you are able to flip your hidden edge of your diamond shaped quad this still causes undesirable streatching in the perpendicular direction when ever the skin moves. It can be massaged into reducing all these unwanted distortion but it is a an avoidable headache with the right loops.
Now that we are aware of some problem areas in the standard Edge Loop system we can look at ways of fixing this and even improving on the simulation of skin based on some lessons learned from age, Wrinkles!. They are the key to solving our allusive layout because they are perfect markers as to how the face moves.
We will cover this in Part 2 of our study on Facial Edge Loops.
