Facial Animation Techniques
Mr. K. Gnanamuthu Prakash
Dr. S. Balasubramanian
Anna University Coimbatore
Anna University Coimbatore
A major unsolved problem in computer graphics is the construction and animation of realistic human facial models. Traditionally, facial models have been built painstakingly by manual digitization and animated by ad hoc parametrically controlled facial mesh deformations or kinematic approximation of muscle actions. Fortunately, animators are now able to digitize facial geometries through the use of scanning range sensors and animate them through the dynamic simulation of facial tissues and muscles. However, these techniques require considerable user input to construct facial models of individuals suitable for animation.
Facial Modeling Techniques
Polygonal – Polygonal modeling specifies exactly each 3d point, which connected to each other as polygons. This is an exacting way to get topology (points) where you need it on a face and not where you don’t. Advantages: Specific control and placement (of topology and mapping). Disadvantages: Hard to work with polygon data as you must work with it directly.
Patches (NURBs) Patches (or a set of splines) indirectly defines a smooth curve surface from a set of control points. A small amount of control points (called CVs in Maya) can define a complex surface. One type of spline is called NURBs which stands for Non-Uniform Rational B-Splines. This type of batch allows each control point to have its own weight that can affect the “pinch” of the curve at the point. So they are considered the most versatile of batches. They work very well for organic smooth objects so hence they are well suited for facial modeling however several issues arise. Disadvantages:
To create a face from NURBS you must use topology (control points) evenly across the face but the face tends to have areas of high detail (eyes, mouth, ears) and areas of low detail (forehead, cheeks, …). Modeling with batches deals with this in two (not perfect) ways: either you 1) make one large patch with high density everywhere because it is needed in the high detail areas or 2) make a low density patch for the face and use higher density patches specific to facial areas ( ie. Eyes) and stitch them in to the overall face batch. The two online tutorials demonstrate modeling heads with patches. The first shows creating a even density head gravitating the extra splines into the mouth for its needed detail and the stitching in eyes, at the end it shows how to make one (lame) patch over the whole head.. Other issues are that with faces have many creases (around the eyes, mouth, ears, nose, and wrinkles) and patches present a challenge to do creases in a non-adhoc way.
Sub-Division surfaces is a fairly new modeling technique that gives you the control and flexibility of polygons with the ease of use and smoothness of patches. Tony DeRose (who wrote the paper on Sub-D surfaces and created a working version for Pixar, first used in Geri’s Game) has slides on the advantages on sub-d surface
Sub-D surfaces gives you the detail only where you need it.. Paul Aichele discussed this on our Pixar trip with Geri’s head. “Rather then fighting the curvature with NURBs”, he was able to put the topology where the curvature of the sculpture called for it. See. Sub-D surfaces also gives you better control of texturing, creases and local sharpness. Maya now has Sub-D surfaces in the unlimited version and Sub-D emulation (with polygons) in there complete version.
Facial models can be created by digitizing live humans or physical models. There are several techniques. One that does need an expensive digitizer is using fudical points to reconstruct 2D photographs into a 3D model such as I did for the Kraftwerk music video animation.. Now however, automatic...
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