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PA199
Advanced Game Design
Lecture 12
Character Modeling
Dr. Fotis Liarokapis
18th May 2017
3D Animation
• Rendering
–3D Scene and Motion
–Sequence of Frames
• Rates: Video 30fps, Film 24fps
–Persistence of Vision
• Animator must create
–Illusion of Life
–Weight
Animation
• Almost every property of every object in
the scene can be animated changed
through time
–Models, cameras
–Transformations (Move, Rotate, Scale)
• Modifications/Deformation:
– Edits, bends, twists, manipulating a skeleton
• Materials, colors, textures
Animation .
• 3D Scene does not have
– Gravity
– Weight
– Force
– Complete interactions between objects
• Sometimes it has…
• You must make it seem so
Preproduction Phases
• Screen-play
• Storyboards
• Character
development
3D Characters
• Digital actor
– Tin can
– Sack of flower
– Butterfly, beetle
– Bird
– Flower
– Robot
– Humanoid
– Etc…
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Typical Character
• Mechanics of movement must be
convincing
• Skin and clothing moves & bends
appropriately
• This process of preparing character controls
is called rigging (see next slides)
–Fully rigged character has:
• Skeleton joints, surfaces, deformers, expressions,
Set Driven Key, constraints, etc
Typical Character .
Facial Animation Video
https://www.youtube.com/watch?v=z86YsS-pVsQ
Character Resolution
• Use low resolution character that has
surfaces ‘parented’ to skeleton
–Allows interactive animations
–Switch to full resolution character later
Typical Character Animation Workflow
• Character Design
• Model
• Skeleton Rigging
• Binding
• Animation
• Integration
• Rendering
Rigging
• Rigging refers to the construction and
setup of an animatable character
– Similar to the idea of building a puppet
• A ‘rig’ has numerous degrees of freedom
(DOFs) that can be used to control various
properties
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Primary Methods of Animation
• Keyframe
• Procedural
–Expressions
–Scripting
• Dynamics/Simulation
–Physics
• Motion Capture
• Combinations of the above
Keyframe Workflow
• Set Keys
–Usually extreme positions
–Less is more: Keys only the properties being
animated
• Set Interpolation
–Specify how to get from one key to another
–Secondary, but a necessary step
• Scrub Time slider and refine motion curve
Setting Keys
• Start with extreme positions
• Add intermediate positions
– Secondary motion
• Less is more
– Don’t add keys for properties that you are not
animating
– Easier to manage/edit fewer keys
Skeletal Hierarchy
• The Skeleton is a tree of bones
–Often flattened to an array in practice
• Top bone in tree is the “root bone”
–May have multiple trees, so multiple roots
• Each bone has a transform
–Stored relative to its parent’s transform
• Transforms are animated over time
• Tree structure is often called a “rig”
Bone Masks
• Some animations only affect some bones
– Wave animation only affects arm
– Walk affects legs strongly, arms weakly
• Arms swing unless waving or holding something
• Bone mask stores weight for each bone
– Multiplied by animation’s overall weight
– Each bone has a different effective weight
– Each bone must be blended separately
• Bone weights are usually static
– Overall weight changes as character changes
animations
Variable Delta Extraction
• Uses root bone motion directly
• Sample root bone motion each frame
• Find delta from last frame
• Apply to instance pos+orn
– Instance pos+orn is the root bone!
• Root bone is ignored when rendering
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Animation Storage Problem
• 4x3 matrices, 60 per second is huge
–200 bone character = 0.5Mb/sec
• Consoles have around 32-64Mb
• Animation system gets maybe 25%
• PC has more memory
–But also higher quality requirements
Decomposition
• Decompose 4x3 into components
–Translation (3 values)
–Rotation (4 values - quaternion)
–Scale (3 values)
–Skew (3 values)
• Most bones never scale & shear
• Many only have constant translation
• Don’t store constant values every frame
Interpolation
• Specify how to get from one key to the
other (in between)
• Common types
–Step: stay at the same value, then suddenly
switch
–Linear: change at constant rate
–Spline/Smooth: make it smooth
• All of these (and more) are useful and
appropriate in the right circumstance
Mesh Deformation
• Find Bones in World Space
• Find Delta from Rest Pose
• Deform Vertex Positions
• Deform Vertex Normals
Find Bones in World Space
• Animation generates a “local pose”
– Hierarchy of bones
– Each relative to immediate parent
• Start at root
• Transform each bone by parent bone’s worldspace
transform
• Descend tree recursively
• Now all bones have transforms in world space
– “World pose”
Find Delta from Rest Pose
• Mesh is created in a pose
– Called the “rest pose”
• Must un-transform by that pose first
• Then transform by new pose
– Multiply new pose transforms by inverse of rest pose
transforms
– Inverse of rest pose calculated at mesh load time
• Gives “delta” transform for each bone
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Deform Vertex Positions
• Deformation usually performed on GPU
• Delta transforms fed to GPU
–Usually stored in “constant” space
• Vertices each have n bones
• n is usually 4
–4 bone indices
–4 bone weights 0-1
–Weights must sum to 1
Deform Vertex Normals
• Normals are done similarly to positions but
use inverse transpose of delta transforms
–Translations are ignored
–For pure rotations, inverse(A)=transpose(A)
–So inverse(transpose(A)) = A
–For scale or shear, they are different
• Normals can use fewer bones per vertex
–Just one or two is common
Motion of characters
• Along with key frame animation we can use
kinematics
– Kinematics = study of motion without regard to the
forces that cause it
Specify fewer degrees of freedom
More intuitive control
FK & IK
• Most animation is “forward kinematics”
– Motion moves down skeletal hierarchy
• But there are feedback mechanisms
– Eyes track a fixed object while body moves
– Foot stays still on ground while walking
– Hand picks up cup from table
• This is “inverse kinematics”
– Motion moves back up skeletal hierarchy
Skeleton Puppet Game Video (FK & IK)
https://www.youtube.com/watch?v=QDwo9d8Fa5M
User Control of Kinematic Characters
• Joint Space
–Position all joints
• Fine level of control
• Cartesian Space
–Specify environmental interactions easily
• Most DOF computed automatically
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Inverse Kinematics
• Balance = keep center-of-mass over support
polygon
• Control
– i.e. position vaulter’s hands on line between
shoulder and vault
– i.e. Compute knee angles that will give runner the
right leg length
Inverse Kinematics is Hard
• Redundancy
Inverse Kinematics is Hard .
• Singularities
Momentum-based Inverse Kinematics
with Motion Capture Video
https://www.youtube.com/watch?v=FJTBMnP6oCM
Single Bone IK
• Orient a bone in given direction
–Eyeballs
–Cameras
• Find desired aim vector
• Find current aim vector
• Find rotation from one to the other
–Cross-product gives axis
–Dot-product gives angle
• Transform object by that rotation
Multi-Bone IK
• One bone must get to a target position
–Bone is called the “end effector”
• Can move some or all of its parents
• May be told which it should move first
–Move elbow before moving shoulders
• May be given joint constraints
–Cannot bend elbow backwards
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Cyclic Coordinate Descent
• Simple type of multi-bone IK
• Iterative
– Can be slow
• May not find best solution
– May not find any solution in complex cases
• But it is simple and versatile
– No pre-calculation or pre-processing needed
Cyclic Coordinate Descent .
• Start at end effector
• Go up skeleton to next joint
• Move (usually rotate) joint to minimize
distance between end effector and target
• Continue up skeleton one joint at a time
• If at root bone, start at end effector again
• Stop when end effector is “close enough”
• Or hit iteration count limit
Cyclic Coordinate Descent ..
• May take a lot of iterations
• Especially when joints are nearly straight
and solution needs them bent
–e.g. a walking leg bending to go up a step
–50 iterations is not uncommon!
• May not find the “right” answer
–Knee can try to bend in strange directions
Two-Bone IK
• Direct method, not iterative
• Always finds correct solution
– If one exists
• Allows simple constraints
– Knees, elbows
• Restricted to two rigid bones with a rotation
joint between them
– Knees, elbows!
• Can be used in a cyclic coordinate descent
Two-Bone IK .
• Three joints must stay in user-specified plane
– e.g. knee may not move sideways
• Reduces 3D problem to a 2D one
• Both bones must remain same length
• Therefore, middle joint is at intersection of two
circles
• Pick nearest solution to current pose
• Or one solution is disallowed
– Knees or elbows cannot bend backwards
Two-Bone IK ..
Allowed
elbow
position
Shoulder
Wrist
Disallowed
elbow
position
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IK by Interpolation
• Animator supplies multiple poses
• Each pose has a reference direction
– e.g. direction of aim of gun
• Game has a direction to aim in
• Blend poses together to achieve it
• Source poses can be realistic
– As long as interpolation makes sense
– Result looks far better than algorithmic IK with
simple joint limits
IK by Interpolation .
• Result aim point is inexact
–Blending two poses on complex skeletons
does not give linear blend result
• Can iterate towards correct aim
• Can tweak aim with algorithmic IK
–But then need to fix up hands, eyes, head
–Can get rifle moving through body
Attachments
• For example character holding a gun
• Gun is a separate mesh
• Attachment is bone in character’s skeleton
–Represents root bone of gun
• Animate character
• Transform attachment bone to world
space
–Move gun mesh to that pos+orn
Attachments .
• For example person is hanging off bridge
• Attachment point is a bone in hand
–As with the gun example
• But here the person moves, not the bridge
• Find delta from root bone to attachment
bone
• Find world transform of grip point on bridge
• Multiply by inverse of delta
–Finds position of root to keep hand gripping
Collision Detection
• Most games just use bounding volume
• Some need perfect triangle collision
–Slow to test every triangle every frame
• Pre-calculate bounding box of each bone
–Transform by world pose transform
–Finds world-space bounding box
• Test to see if bounding box was hit
–If it did, test the this bone influences
NCCA Video 1
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NCCA Video 2 Our Work
Questions