Modeling Skin Separate object(s) per limb (and joints) Continuous Skin Rigidly transform vertices Transform vertices between bisectors Blend using multiple bones Pose space deformations Wrap skin around model of internal structure

Separate objects Overlapping gives appearance of continuous surface - somewhat Add element at joint to smooth between limbs Works best with uniform color

Bind skin vertex to skeleton Continuous skin Design character skin Define underlying skeleton from skin Bind skin vertex to skeleton

Rigidly bind vertex to closest bone Skin vertex Closest bone Associate vertex with closest ‘bone’ Rigidly transform vertex as bone transforms

Problems with overlap and stretching Rigidly transform vertices Problems with overlap and stretching

Bind vertex to bone - bisectors Locate vertex relative to closest bone Joint bisectors Normal of bisecting plane Skin vertex Line parallel to bone through vertex s t s – distance from vertex to first bisector t – distance between bisectors v Distance from vertex to bone d p2 p0 p1

Use d, s, t to reconstruct location of vertex relative to bone Map vertices to relative location Use d, s, t to reconstruct location of vertex relative to bone

Bind vertex to multiple bones Locate vertex relative to closest bone Skin vertex 20% 80% User ‘paints’ which bones affect what vertices Relative distance used to compute weights

Vertices around joint are affected by both bones Blend transformation of vertices Vertices around joint are affected by both bones

So add addition joints or use alternatives to linear blending But twists can collapse vertices So add addition joints or use alternatives to linear blending

See: www.cs.wisc.edu/graphics/Courses/cs-838-2001/Students/lx/p2.htm Pose space deformations User ‘sculpts’ skin in various poses For each vertex solve weights to use with Radial Basis Functions for scattered data interpolation See: www.cs.wisc.edu/graphics/Courses/cs-838-2001/Students/lx/p2.htm

Use anatomy to model deep structure What artists do Surface geometry affected by what’s happening underneath Skin wraps bones, muscles, tendons, subcutaneous tissue

Warp skin based on model of internal structure Skeletal articulation modifies shape of muscle Muscle shape modifies skin shape Add model of tendons, fatty tissue

Reference material Artistic Anatomy Anatomy related to surface appearance versus Medical Anatomy Well-developed literature Forensic Science Facial Reconstruction Plastic Surgery Surface Appearance Reconstruction

Layered: Bone-Muscle-Skin Muscle &Tendons Fatty Tissue Bone Skin

Modeling - Layered FFD muslces John Chadwick, ABD CSE OSU ‘90

Modeling - Thalmann, EPFL Implicit surfaces From http://ligwww.epfl.ch/

Modeling Embed polyhedra in implicit surface Implicits used for approx. collision detect & respond Polyhedra used for display Use M.-P. Cani technique to distort implicits based on collisions Karan Singh, PhD CSE OSU ‘95

Human Figure Ferdi Scheepers, PhD CSE OSU ‘97 Use artistic anatomy to design muscles

Human Figure

Human Figure

Figures - Wilhelms, UCSB http://www.cse.ucsc.edu/~wilhelms/fauna/Monkeys/5monk_parts.gif

Use Visible Human dataset Skeletal Muscle Modeling Using a NURBS-Based Finite Element Method www.digital-humans.org/Report2004/Documents/13-MuscleStressAndStrain.htm http://graphics.stanford.edu/~fedkiw/