Skeletal Animation and Skinning

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Skeletal Animation and Skinning

• A (hardware friendly) software approach

By Brandon Furtwangler
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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
• Pros
• Easier to understand
• Quicker
• Cons
• More memory usage
• Less flexible

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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

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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)

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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?

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Demo 1

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

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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?

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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

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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)

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Questions?

• demos at http//www.msu.edu/furtwan1