Transcript Skeletal Animation and Skinning

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
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Con’s
 More memory usage
 Less flexible
Why is skeletal animation better?
 Allows you to ‘skin’ meshes
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Matrix Palette Skinning (hardware friendly)
Multiple bones per vertex (weights)
 Greater flexibility
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Play multiple animations on a single mesh
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combine run animation on legs with salute
animation on arms
Mix animations together
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overlap walk with run to get a smooth transition
Implementing MPS
 Things we need:
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Model
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Vertex
 Bone IDs
 Weights
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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:
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Update Dynamic Transforms from animations
Recalculate matrix palette
Transform vertex positions/normals by matrix
palette entries
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Multiple bones per vertex?
How to handle normals?
Demo 1
 Single bone per vertex
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Limited rotations
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Joints overlap
Ugly
Quick
What if we want to model an elbow?
Demo 2
 2 bones per vertex
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linear combination
Outside edge looks great
Inside edge is pinched
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Why?
How do we fix it?
 More bones?
 Any other ideas?
Demo 3
 4 bones/weights per vertex
 4 bones for a single joint
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Limitation for hardware (constant registers)
 Evenly blend between 4 bones with Bernstein
polynomials
 Both outside and inside of joint look
reasonable
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More bones for complicated muscle simulation
Ideas for an animation system
 Quaternion based rotations
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Nice interpolation
 Animation State
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Multiple animations with blend factor
Animation
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Multiple Tracks (one per bone in animation)
Track
 Quaternion Spline (for joint rotations)
Questions?
 demo’s at http://www.msu.edu/~furtwan1