Skeletal Animation and Skinning A (hardware friendly) software approach By: Brandon Furtwangler

What is skeletal animation? The idea: Define a hierarchy of ‘bones’ Define animations that transform the bones Deform the vertices of a mesh based on the bones that they are attached to Common alternative Key frame animation Pro’s  Easier to understand  Quicker Con’s  More memory usage  Less flexible

Why is skeletal animation better? Allows you to ‘skin’ meshes Matrix Palette Skinning (hardware friendly) Multiple bones per vertex (weights) Greater flexibility Play multiple animations on a single mesh combine run animation on legs with salute animation on arms Mix animations together overlap walk with run to get a smooth transition

Implementing MPS Things we need: Model Vertex  Bone IDs  Weights Skeleton  Bones (in a hierarchy)  Parent  Static Transform (bind pose)  Dynamic Transform (animation controlled)  Palette entry (to be multiplied by vertex)

Implementing MPS part 2 The skinning process: Update Dynamic Transforms from animations Recalculate matrix palette Transform vertex positions/normals by matrix palette entries Multiple bones per vertex? How to handle normals?

Demo 1 Single bone per vertex Limited rotations Joints overlap Ugly Quick What if we want to model an elbow?

Demo 2 2 bones per vertex linear combination Outside edge looks great Inside edge is pinched Why? How do we fix it?  More bones?  Any other ideas?

Demo 3 4 bones/weights per vertex 4 bones for a single joint Limitation for hardware (constant registers) Evenly blend between 4 bones with Bernstein polynomials Both outside and inside of joint look reasonable More bones for complicated muscle simulation

Ideas for an animation system Quaternion based rotations Nice interpolation Animation State Multiple animations with blend factor Animation Multiple Tracks (one per bone in animation) Track  Quaternion Spline (for joint rotations)

Questions? demo’s at