Human Facial Animation: Thinking about Muscles instead of Bones: The whole face is often in a state of constriction or relaxation. Most emotions lie along a continuum between these two states. Many emotions have essentially the same muscle groups in action, but in different combinations. Often the intensity of the emotion is determined by the EXTENT of the constriction. The extent of each muscular groupÕs contraction is generally not linear, either Ð it starts with very fine muscle movements, for use in speech, and subtle emotion, and progresses upwards along an exponential curve to the maximum contractions used for screaming, snarling, and Jim Carrey movies. One way to visualize this is as if there was a cam with an eccentric lobe driving the animation. At the bottom of the cam the animator can move the controls for the face more to get subtle movements. As the cam lobe rises the animatorÕs movements have an increasing effect on the face. The Third Rule: facial expressions are not a linear blend, they are a more complex mix that accentuates muscle action towards the most extreme poses and allows for fine control of subtle emotion at the least extreme poses.