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CSCE 552 Fall 2012. Animations. By Jijun Tang. Animation terms. frame – an image that is displayed on the screen, usually as part of a sequence. pose – an orientation of an objects or a hierarchy of objects that defines extreme or important motion. - PowerPoint PPT Presentation
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CSCE 552 Fall 2012
Animations
By Jijun Tang
Animation terms
frame – an image that is displayed on the screen, usually as part of a sequence.
pose – an orientation of an objects or a hierarchy of objects that defines extreme or important motion.
keyframe – a special frame that contains a pose.
tween – the process of going “between” keyframes.
secondary motion – an object motion that is the result of its connection or relationship with another object.
Skeletal Hierarchy
The Skeleton is a tree of bones Modelling characters Often flattened to an array in practice Each bone has a transform, stored relative to its
parent’s transform Top bone in tree is the “root bone”
Normally the hip May have multiple trees, so multiple roots
Transforms are animated over time Tree structure is often called a “rig”
Example
The Transform
“Transform” is the term for combined: Translation Rotation Scale Shear
Can be represented as 4x3 or 4x4 matrix But usually store as components Non-identity scale and shear are rare
Examples
Homogeneous coordinates
Four-dimensional space Combines 3 3 matrix and translation
into one 4 4 matrix
Translation
Translation matrix
Translates the origin by the vector T
translate
1 0 0
0 1 0
0 0 1
0 0 0 1
x
y
z
T
T
T
M
Scale
Scale matrix
Scales coordinate axes by a, b, and c If a = b = c, the scale is uniform
scale
0 0 0
0 0 0
0 0 0
0 0 0 1
a
b
c
M
Rotation (Z)
Rotation matrix
Rotates points about the z-axis through the angle
-rotate
cos sin 0 0
sin cos 0 0
0 0 1 0
0 0 0 1
z
M
Rotations (X, Y)
Similar matrices for rotations about x, y
-rotate
1 0 0 0
0 cos sin 0
0 sin cos 0
0 0 0 1
x
M
-rotate
cos 0 sin 0
0 1 0 0
sin 0 cos 0
0 0 0 1
y
M
Euler Angles
Three rotations about three axes Intuitive meaning of values
Euler Angles
This means that we can represent an orientation with 3 numbers
A sequence of rotations around principle axes is called an Euler Angle Sequence
Assuming we limit ourselves to 3 rotations without successive rotations about the same axis, we could use any of the following 12 sequences:
XYZ XZY XYX XZXYXZ YZX YXY YZYZXY ZYX ZXZ ZYZ
Using Euler Angles
To use Euler angles, one must choose which of the 12 representations they want
There may be some practical differences between them and the best sequence may depend on what exactly you are trying to accomplish
Interpolating Euler Angles
One can simply interpolate between the three values independently
This will result in the interpolation following a different path depending on which of the 12 schemes you choose
This may or may not be a problem, depending on your situation
Note: when interpolating angles, remember to check for crossing the +180/-180 degree boundaries
Problems
Euler Angles Are Evil No standard choice or order of axes Singularity “poles” with infinite number of
representations Interpolation of two rotations is hard Slow to turn into matrices
Use matrix rotation
Rotation Matrix
3x3 Matrix Rotation
Easy to use Moderately intuitive Large memory size - 9 values Interpolation is hard
Introduces scales and shears Need to re-orthonormalize matrices after
Quaternions
Quaternions are an interesting mathematical concept with a deep relationship with the foundations of algebra and number theory
Invented by W.R.Hamilton in 1843 In practice, they are most useful to use as a
means of representing orientations A quaternion has 4 components
3210 qqqqq
Quaternions on Rotation
Represents a rotation around an axis Four values <x,y,z,w> <x,y,z> is axis vector times sin(θ /2) w is cos(θ/2) Interpolation is fast
Illustration
Quaternions (Imaginary Space)
Quaternions are actually an extension to complex numbers
Of the 4 components, one is a ‘real’ scalar number, and the other 3 form a vector in imaginary ijk space!
3210 kqjqiqq q
jiijk
ikkij
kjjki
ijkkji
1222
Quaternions (Scalar/Vector)
Sometimes, they are written as the combination of a scalar value s and a vector value v
where
321
0
qqq
qs
v
vq ,s
Unit Quaternions
For convenience, we will use only unit length quaternions, as they will be sufficient for our purposes and make things a little easier
These correspond to the set of vectors that form the ‘surface’ of a 4D hypersphere of radius 1
The ‘surface’ is actually a 3D volume in 4D space, but it can sometimes be visualized as an extension to the concept of a 2D surface on a 3D sphere
123
22
21
20 qqqqq
Quaternions as Rotations
A quaternion can represent a rotation by an angle θ around a unit axis a:
If a is unit length, then q will be also
2sin,
2cos
2sin,
2sin,
2sin,
2cos
aq
q
or
aaa zyx
Quaternions as Rotations
11
2sin
2cos
2sin
2cos
2sin
2cos
2sin
2sin
2sin
2cos
22222
22222
2222222
23
22
21
20
a
q
zyx
zyx
aaa
aaa
qqqq
Quaternion to Matrix
22
2110322031
103223
213021
2031302123
22
2212222
2222122
2222221
qqqqqqqqqq
qqqqqqqqqq
qqqqqqqqqq
To convert a quaternion to a rotation matrix:
Matrix to Quaternion
Matrix to quaternion is doable It involves a few ‘if’ statements, a
square root, three divisions, and some other stuff
Search online if interested
Animation vs. Deformation
Skeleton + bone transforms = “pose” Animation changes pose over time
Knows nothing about vertices and meshes Done by “animation” system on CPU
Deformation takes a pose, distorts the mesh for rendering Knows nothing about change over time Done by “rendering” system, often on
GPU
Pose
Model
Describes a single type of object Skeleton + rig One per object type Referenced by instances in a scene Usually also includes rendering data
Mesh, textures, materials, etc Physics collision hulls, gameplay data, etc
Instance
A single entity in the game world References a model Holds current states:
Position & orientation Game play state – health, ammo, etc
Has animations playing on it Stores a list of animation controls Need to be interpolated
Animation Control
Links an animation and an instance 1 control = 1 anim playing on 1 instance
Holds current data of animation Current time Speed Weight Masks Looping state
Animation Storage
The Problem Decomposition Keyframes and Linear Interpolation Higher-Order Interpolation The Bezier Curve Non-Uniform Curves Looping
Storage – The Problem
4x3 matrices, 60 per second is huge 200 bone character = 0.5Mb/sec
Consoles have around 256-512Mb 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 But human characters may have higher requirement
Muscle move, smiling, etc. Cloth under winds
Don’t store constant values every frame, use index instead
Keyframes
Motion is usually smooth Only store every nth frame (key frames) Interpolate between keyframes
Linear Interpolate Inbetweening or “tweening”
Different anims require different rates Sleeping = low, running = high Choose rate carefully
Linear Interpolation
Higher-Order Interpolation
Tweening uses linear interpolation Natural motions are not very linear
Need lots of segments to approximate well So lots of keyframes
Use a smooth curve to approximate Fewer segments for good approximation Fewer control points
Bézier curve is very simple curve
Bézier Curves (2D & 3D)
Bézier curves can be thought of as a higher order extension of linear interpolation
p0
p1
p0
p1p2
p0
p1
p2
p3
The Bézier Curve
(1-t)3F1+3t(1-t)2T1+3t2(1-t)T2+t3F2
t=0.25
F1
T1
T2
F2
t=1.0
t=0.0
The Bézier Curve (2)
Quick to calculate Precise control over end tangents Smooth
C0 and C1 continuity are easy to achieve C2 also possible, but not required here
Requires three control points per curve (assume F2 is F1 of next segment)
Far fewer segments than linear
C0/C1/C2
The curves meet
the tangents are shared
the"speed" is the same before and after
Catmull-Rom Curve
Defined by 4 points. Curve passes through middle 2 points.
P = C3t3 + C2t2 + C1t + C0
C3 = -0.5 * P0 + 1.5 * P1 - 1.5 * P2 + 0.5 * P3 C2 = P0 - 2.5 * P1 + 2.0 * P2 - 0.5 * P3 C1 = -0.5 * P0 + 0.5 * P2 C0 = P1
Non-Uniform Curves
Each segment stores a start time as well Time + control value(s) = “knot” Segments can be different durations Knots can be placed only where needed
Allows perfect discontinuities Fewer knots in smooth parts of animation
Add knots to guarantee curve values: Transition points between animations
Looping and Continuity
Ensure C0 and C1 for smooth motion At loop points At transition points: walk cycle to run
cycle C1 requires both animations are
playing at the same speed: reasonable requirement for anim system
Playing Animations
“Global time” is game-time Animation is stored in “local time”
Animation starts at local time zero Speed is the ratio between the two
Make sure animation system can change speed without changing current local time
Usually stored in seconds Or can be in “frames” - 12, 24, 30, 60 per
second
Scrubbing
Sample an animation at any local time Important ability for games
Footstep planting Motion prediction AI action planning Starting a synchronized animation
Walk to run transitions at any time
Avoid delta-compression storage methods Very hard to scrub or play at variable speed
Delta Compression
Delta compression is a way of storing or transmitting data in the form of differences between sequential data rather than complete files.
The differences are recorded in discrete files called deltas or diffs.
Because changes are often small (only 2% total size on average), it can greatly reduce data redundancy.
Collections of unique deltas are substantially more space-efficient than their non-encoded equivalents.
Animation Blending
The animation blending system allows a model to play more than one animation sequence at a time, while seamlessly blending the sequences
Used to create sophisticated, life-like behavior Walking and smiling Running and shooting
Blending Animations
The Lerp Quaternion Blending Methods Multi-way Blending Bone Masks The Masked Lerp Hierarchical Blending
The Lerp
Foundation of all blending “Lerp”=Linear interpolation Blends A, B together by a scalar weight
lerp (A, B, i) = iA + (1-i)B i is blend weight and usually goes from 0 to 1
Translation, scale, shear lerp are obvious Componentwise lerp
Rotations are trickier – normalized quaternions is usually the best method.
Quaternion Blending
Normalizing lerp (nlerp) Lerp each component Normalize (can often be approximated) Follows shortest path Not constant velocity Multi-way-lerp is easy to do Very simple and fast
Many others: Spherical lerp (slerp) Log-quaternion lerp (exp map)
Which is the Best
No perfect solution! Each missing one of the features All look identical for small interpolations
This is the 99% case Blending very different animations looks
bad whichever method you use Multi-way lerping is important So use cheapest - nlerp
Multi-way Blending
Can use nested lerps lerp (lerp (A, B, i), C, j) But n-1 weights - counterintuitive Order-dependent
Weighted sum associates nicely (iA + jB + kC + …) / (i + j + k + … ) But no i value can result in 100% A
More complex methods Less predictable and intuitive Can be expensive
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
The Masked Lerp
Two-way lerp using weights from a mask Each bone can be lerped differently
Mask value of 1 means bone is 100% A Mask value of 0 means bone is 100% B Solves weighted-sum problem
(no weight can give 100% A) No simple multi-way equivalent
Just a single bone mask, but two animations
Hierarchical Blending
Combines all styles of blending A tree or directed graph of nodes Each leaf is an animation Each node is a style of blend
Blends results of child nodes Construct programmatically at load time
Evaluate with identical code each frame Avoids object-specific blending code Nodes with weights of zero not evaluated
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