Character Animation and Skinningtheshark/courses/cs354/... · 2018-10-22 · Other 3D Animation...
Transcript of Character Animation and Skinningtheshark/courses/cs354/... · 2018-10-22 · Other 3D Animation...
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Character Animation and Skinning
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Motion over time
Animation
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Lead animator draws sparse key frames
Secondary artists fill in (by hand) the intermediate frames: inbetweening
Traditional Character Animation
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Computer Character Animation
How to inbetween automatically on a 2D sprite?
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Surround object with animation cage
Moving the cage moves interior points
Cage-Based Animation
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Simplest Cage: Triangle
Use barycentric interpolation
Matches points’ pixels between triangles
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Polygonal Cages
Must generalize barycentric coordinates to arbitrary polygons
Many ways to do this: generalized barycentric coordinates not unique
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Generalized Barycentric Coordinates
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Polygonal Cages
Other properties:1. Weights must be
positive inside the polygon (or get leaks)
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Polygonal Cages
Other properties:1. Weights must be
positive inside the polygon (or get leaks)
2. Weights must unique (or get flips)
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Polygonal Cages
Other properties:1. Weights must be positive inside the
polygon (or get leaks)2. Weights must unique (or get flips)3. Smooth4. Easy to compute
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Some Possible Schemes
Wachspress CoordinatesMean-value CoordinatesGreen CoordinatesBounded Biharmonic Weightsetc…
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Cage-Based Animation
Extends to 3D from 2D naturally
Full control, but not intuitive
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Handle-Based Animation
Pick special points (handles) on object
Moving handles moves nearby points
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Character Rigs
Skeletons inside the geometry
the industry standardfor character animation
moving bones movessurrounding geometry
how to build rig?
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Building a RigUsually done by hand using Maya etc.
Expressiveness/complexity tradeoff
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Rigging in Practice
https://www.youtube.com/watch?v=WxZz-yH-mKU
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Building a Rig
Some automatic tools exist…
[Pinocchio][Mixamo]
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Mixamo Demo
https://www.mixamo.com/
Automatic rigging can work well for humans/humanlike objects• Assumes bipedal with standard
placement and orientation of joints
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Not so impressive for arbitrary characters…
https://www.youtube.com/watch?v=-5jWtz3rzco
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Representing a Rig
Tree of bones connected by joints
bones have two endpoints• first attached to parent
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Bone Local Coordinates
Origin O One natural direction: tangent axis
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Bone Local Coordinates
Origin O One natural direction: tangent axis Two perpendicular directions:
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Bone Local Coordinates
Origin O One natural direction: tangent axis Two perpendicular directions:
second endpoint:
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Bone Local Coordinates
Child bone can be expressed in terms of parent coordinate system
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In local coordinates:
Bone to World Coordinates
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In local coordinates: In world coordinates:
Bone to World Coordinates
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What About the Base?
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What About the Base?
write origin & axes in world coordinates, then
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Forward Kinematics
changing also changes child coordinate systems
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Additional Reading
https://www.gamedev.net/resources/_/technical/graphics-programming-and-theory/skinned-mesh-animation-using-matrices-r3577
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Skinning
Moving bones moves the character
Closer bones have more influence
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Nearest-Bone Skinning
Given: undeformed (rest) skeleton and deformed skeleton
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Coordinate Systems
undeformed bone
world
deformed bone
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Coordinate Systems
undeformed bone
world
deformed bone
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Coordinate Systems
undeformed bone
world
deformed bone
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Coordinate Systems
undeformed bone
world
deformed bone
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Coordinate Systems
undeformed bone deformed bone
Key (and confusing) point: • maps from undeformed local to
world coords (doesn’t move point) • Identity maps undeformed to deformed
bone coords (and does move point)
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Nearest-Bone Skinning
Undeformed to deformed skin position (world coordinates):
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Nearest-Bone Skinning
Undeformed to deformed skin position (world coordinates):
changes during animation
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Problems with Nearest-Bone
Which bone does point belong to?
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Problems with Nearest-Bone
Which bone does point belong to? One solution: average -1
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Linear-Blend Skinning
Each vertex feels weighted average of each bone’s transformations
Nearby bones have higher
weight
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Linear-Blend Skinning
How to determine skinning weights w?
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Linear-Blend Skinning
How to determine skinning weights w?
• Use only nearest bone
• Spatially blend the weights
• In practice: paint weights by hand
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Painting Weights
https://www.youtube.com/watch?v=dD5adx9SdH0
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The “Arm Twist” Problem
(Why does this happen?)
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Blending Transformations
Each individual bone undergoes a rigid transformation• Combination rotation and translation
• Linear blend of rigid motions not rigid
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Blending Transformations
Translations alone: trivial to blend Rotations alone: blend using SLERP • use quaternions • do not use Euler angles!
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Separate TransformsBlending Transformations
Idea: separately blend translation and rotation components of rigid motion
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Separate Transforms: Problem
where is the child bone half way in between the motion?
(where it the origin?)
T1
T2
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Separate Transforms: Problem
where is the child bone half way in between the motion?
(where it the origin?)
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Separate Transforms: Problem
where is the child bone half way in between the motion?
(where it the origin?)
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Separate Transforms: Problem
Blended transformation not coordinate-independent
• different origin positions in bone hierarchy result in different blends
• unify translation and rotation into single state
• blend centers of rotation
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Dual Quaternion Skinning• No more arm twisting issues• Less deformation• The industry standard (used in Maya, etc)
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Dual Quaternion Skinning• Expresses a rotation (quaternion) and
translation• Blended dual quaternions represent a
rotation and translation as quaternions
·q =∑n
i=1 wi·qi
∥ ∑ni=1 wi
·qi∥
·q = qr + qdεwhere
qr = rqd =
12
tr
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Dual Quaternion Skinning
Prevents loss of volume during rigid motion
https://cs.gmu.edu/~jmlien/teaching/cs451/uploads/Main/dual-quaternion.pdf
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Other Skinning Challenges
Volume Conservation
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Other 3D Animation Techniques
Anatomy-based animation• simulate the tendons and muscles
Most correct motionRestricted to “real” animalsSlow
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Other 3D Animation Techniques
Faces are hard to rig: many muscles
instead, precomputea small set of basisdeformations
blend between them(blendshapes)
knobs not alwaysintuitive
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Other 3D Animation Techniques
SCAPE: data-driven statistical model of human body shapes
rig a “mean” template human once by hand
for an arbitrary human,transfer the rig by fittingto a SCAPE model
works well only for (nearly) naked humans
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Simplifying Keyframing
Simulation (“inverse kinematics”)
https://www.youtube.com/watch?v=dEvE0oCJpUA
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Motion Capture
https://www.youtube.com/watch?v=CmrXK4fNOEo
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Machine Learning and Motion Synthesis
• Machine learning can allow for good-looking animations
• https://www.youtube.com/watch?v=urf-AAIwNYk
• Can lower cost of artistic pipeline• Downside is that learned function is mostly
unmodifiable/incomprehensible• Difficult to correct in pathological cases
• Also realism is not necessarily as appealing as artistic interpretation
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Animation RecapMost common pipeline:• build a 3D model of the character• rig the 3D model (build a skeleton
inside)• skin the model (determine bone-skin
weights)• animate the bones by specifying
keyframes; skin moves with them
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Animation Recap
Most common pipeline:• model, rig, skin, animate
Automatic approaches exist for each step• not great, but getting better
Still a grand challenge: use Kinect to build a fully rigged and skinned digital avatar