Principles of Animation

Question 1

Two types of animation

Answer 1

Artist driven animation
Data driven animation

Question 2

What is Squash

Answer 2

Flatten an object or character by pressure or its own power

Question 3

What is Stretch

Answer 3

Increase the sense of speed and emphasise the squash by contrast

Question 4

What is timing

Answer 4

Affects the weights of objects
Light objects move quickly
Heavy objects move slower
Timing completely changes the interpolation of the motion

Question 5

Two different concepts to describe and generate motion of objects in the scene

Answer 5

1) Production
Offline rendering like movies
Better because there is more time to generate

2) interactive
Like gaming, simulators, VR
much trickier - haven’t got to the same level of realism

Question 6

What is Key framing

Answer 6

When we specify the scene only at some instances of time
and then generate the in-between frames automatically

Question 7

What is Tweening

Answer 7

The generation of in-between frames in key-framing

Question 8

What is Procedural animation

Answer 8

Describing the motion algorithmically
Expressing the animation as a function of a small number of parameters
Eg when a clocks hands move, when grass moves in the wind

Question 9

Physical-based animation

Answer 9

Means we assign physical properties to the animation: masses, forces
Eg wind

Question 10

Low Dimensional controls

Answer 10

Use modeling of a joint angle or a bone transformation to mimic animation
These low level controls are mapped to semantically meaningful high level controls
Eg A character walking

Question 11

What is Rigging

Answer 11

Builidng the animation controls of a 3D object

Question 12

What is an “articulated skeleton”

Answer 12

A model that carries a topology: which parts are connected, the geometric relation of joints, and the tree structure of the body parts (in absence of loops)
Note: angles are the low-level dimensional controls in an articulated skeleton

Question 13

What are Forward kinematics

Answer 13

The first stage of rigging
Describes the position of a body part as a function of joint angles
The animator provides angles and the computer determines the position of the end effector
So the animation is describe as an angle parameter with respect to time

Question 14

What are “bones” in animation

Answer 14

Body parts

Question 15

What are Inverse Kinematics

Answer 15

Given the end effector position, finding the joint angles
Animator provides the position of the end effector, and the computer determines the joint angles that satisfies that constraint
The goal for this step is to keep the end of the limb fixed whilst the body moves
Can change end of limb by direct manipulation
More generally, can define any constraint over the skeleton
The resulting animation may not agree entirely with physics but it is a very effective technique of animating

Question 16

Why is inverse kinematics hard

Answer 16

There may be multiple solutions in a configuration space
A solution may not even exist due to some constraints like humans cannot bend their arms 360 degrees

Question 17

Motion Capture

Answer 17

The data-driven approach to rigging instead of direct manipulation
Records real-world performance and extracts pose as a function of time
Uses optical markers and high-speed cameras with triangulation for positions
Very perfect realism

Question 18

What is re-targeting

Answer 18

Animation uses bone transformation but during motion capture we cannot place markers directly on bones
So we require re-targeting - the translation into character controls

Question 19

What is Skinning

Answer 19

Modelling skin surface based on the bone movement

Question 20

What is Skeleton Subspace Deformation (SSD)

Answer 20

The most popular technique for skinning
(other aliases like vertex blending)
Each bone has a deformation of the space around in defined by the rotation matrices
At the bone, skin is attached directly (quite rigid) But at the joint, skin is attached to multiple bones using a weighted combination depending on the movement

Question 21

Weight combination of skin at a joint or bone

Answer 21

We assign a weight for each vertex, vi and for each bone, bj
We define weight Wij
which defines how much vertex i should move the bone j
When Wij = 1
The vertex i is rigidly attached to the bone j

Question 22

2 properties of weighted bones

Answer 22

Weight must be non-negative
Sum over all bones should be equal to 1 for every vertex

Question 23

What is bind pose

Answer 23

joints and bones in rest position

Question 24

Why is SSD complicated on a joint

Answer 24

Take an elbow bend
It may work well for the bottom skin of the arm but not for the top half that is compressing

Question 25

Mapping pi from bind pose

Answer 25

p’ij = TjBj^(-1)pi
T is the conversion from coordinate bone space to world space of the object