Part 2: Advanced methods

Chapter 3: Animating complex objects

1. Descriptive models

2. Physically-based models

3. Layered models for complex objects

– Principle of layered models

– Case study: character animation

– Case study: natural phenomena

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Animating Complex Objects

• Grass blowing in the wind, interacting with the feet

• Trees, clouds…

• Characters

Procedural model?

Descriptive animation?

Geometry / physics?

3

Animating Complex Objects

Solution : « layered model»

Successive animation layers

each one models a specific feature

– Eases conception & control

– Best model for each layer

– Possible retro-action

4

Layered models

General methodology

1. Observe & identify the sub-phenomena to reproduce

2. Represent them independently

– Choose the best model for each feature

Physics, kinematics, geometry, textures…

– Use different time & space sampling if necessary

3. Couple them together

Animation loop Successive update of each layer + possible retroaction

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layered models: case studies

1. Natural phenomena

Examples

• Lava flow

• Grass blowing in the wind

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Layered models for Natural Phenomona

Lava flows

Aim: visual realism

Difficulties

• Viscous liquid

– Separation, fusion

• Varying behavior

– Viscosity function of temperature

• Two important scales

– Global trajectories

– Details of the crust, moving with the flow

7

Layered models for Natural Phenomona

Lava flows

Sub-models & layers

• Global trajectory

– Smoothed particles, heat equation

• Implicit surface

• Details of the crust

– Animated displacement texture

4000

particles

8

Layered models for Natural Phenomona

Lava flows

Coupling sub-models

[Stora Agliati Cani Neyret 99]

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Layered models for Natural Phenomona

Lava flows

[Stora Agliati Cani Neyret 99]

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Layered models for Natural Phenomona

Prairies blowing in the wind

View of a walker in real-time?

Difficulties

• Number of blades of grass

– Rendering: aliasing problems

• Control of the wind

– Breeze, gusp of wind, wirld wind

• Plausible action

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Layered models for Natural Phenomona

Prairies blowing in the wind

Sub-models

• Grass: 3 levels of detail

• Wind model : mask + action

– Breeze, gusp of wind, wirld wind

• Receever : blade of grass

– deformations : pre-simulation …

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Layered models for Natural Phenomona

Prairies blowing in the wind

Transitions between levels of details

• 3D blades of grass / texture 2D1/2

• texture 2D1/2 / texture

…

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Layered models for Natural Phenomona

Prairies blowing in the wind

[Perbet Faure Cani 02]

Animating Ocean Waves

• Aims

– Tunable compromise realism/efficiency

– Camera motion

– Unbounded ocean

• Difficulties

– Complex deformations

– Close to far view

– Aliasing

Animating Ocean Waves

Sub-models

• Receivers

– Sampled surface

– Projection of screen pixels

• Wave trains

– mask + action

Animating Ocean Waves

Animation : Levels of detail

• Filtering wave trains with the distance

– Increases efficiency and reduces aliasing

Without

filtering

Our method

Animating Ocean Waves

[Hinsinger Neyret

Cani, SCA’02]

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Case study 2: animated characters

Need of different layers for

1. Brain, decision taking

2. Moving the skeleton (walking, gesturing)

3. Deforming flesh & skin

4. Hair

5. Clothing

Exo: Which models would you use?

Is retro-action necessary?

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Layer 1: Behavioral model (brain, decision taking)

Example: crowd animation

Particle systems

• Attraction towards an objective

• Repulsive obstacles

• Avoid inter-collisions (fluids)

Techniques from artificial intelligence (AI)

• Individual behavior : rules, emotions, personality

• Social behavior for crowds

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Layer 2 : animating the skeleton

From the behavioral model

1. Coordinate the different actions (finite automata)

2. Call elementary motions

Choose a model for elementary motions

– Descriptive methods

• Direct and inverse kinematics

• Motion capture

– Procedural models

• Physically-based animation + control

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Motion capture A well suited method for humans

• Capture of markers on an actor

– Magnetic or optic: set of synchronized cameras

Difficulty: reconstruct motion despite of occlusions

• Replay similar motion

– curves of angle values over time

Examples of use:

- Feature films

- sport video games

(library of typical motion)

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Motion capture

Problems to be solved

To make it generally applicable

• Adapt to different morphologies

– monsters, aliens…

• Combining different motions

– walk while raising arms

– motion graphs for transitions (walk, fall, get up, run….)

• Editing at various levels of detail

– walk on uneven ground

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Layer 3: Flesh & skin deformation

• Existing methods

– Object hierarchies

– Shape interpolation

– Smooth skinning

– Anatomical simulation

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3. Flesh & skin deformation Object hierarchies

• Character = union of rigid links

assembled in a hierarchy

Exo: How would you avoid holes when

joints articulate?

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3. Flesh & skin deformation

Object hierarchies

Advantages : Low memory cost

Store transformations

Un-bounded motion

• Drawbacks

– Joints are visible

Answer to the exercise: add spheres at joints

– No visual realism: too rigid

Use a single mesh ?

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• Single mesh

• Deformed by the skeleton

(hierarchy of joints)

3. Flesh & skin deformation

Smooth skinning

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• For each mesh point

– skinning weight ki with respect to each Si

– combine positions in the different frames

P = ∑ ki Pi

+ Almost no memory cost

+ Real-time computation

+ Skin motion created independently for each frame

• Exo: Draw the mesh for a bended arm to identify problems:

What happens for a high bent and points near the joint?

3. Flesh & skin deformation

Smooth skinning

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• Drawbacks

– Choose the weights? (painted by a computer artist!...)

– Artifacts for large deformations: loss of volume

3. Flesh & skin deformation

Smooth skinning

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Constant volume skinning: based on post-correction

3. Flesh & skin deformation

Smooth skinning: example of solution

[Rohmer, Hahmann, Cani 2008-2009]

3. Flesh & skin deformation

Key frames vs Blend Shapes

Example of an animated face

• Key frames = Temporal interpolation

– Model and store all successive key- faces

• Blend shapes = Multi-target interpolation

– Model a few « extreme faces » from a « neutral face »

– Animate a trajectory in this space

For each mesh point,

compute successive barycenters on the fly

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3. Flesh & skin deformation

Multi-target interpolation

Advantages

– Fast interpolation

– No need to model something repetitive

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3. Flesh & skin deformation

Adding dynamics to the flesh

Using finite elements

[Capell et al. SIGGRAPH 03]

• Associate each cell with a bone

• Linear elasticity for local models

• Constraints to paste cells together

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• Realistic model for each layer

skeleton, flesh, skin

3. Flesh & skin deformation

Anatomical simulation

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• Advantage : realism, possibility to simulate muscles

• Drawback : computational time!

3. Flesh & skin deformation

Anatomical simulation

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4. Clothes and hair

Physically-based models

1. Difficulties for clothes

– Collisions between thin objects

– Non-extensible: should fold!

– Numerical integration with stiff springs?

2. Difficulties for hair

– 100 000 strands

Exploit spatial coherency!

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4. Clothes

Ease formation of folds

[Choi and Ko 02] Stable but responsive cloth

– Rotation when compression force in the plane of the cloth

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Layered model for clothes

[Rohmer, Popa, Cani,

Hahmann, Sheffer,

SIGGRAPH Asia 2010]

Coarse mesh

deformed by

convolution skeletons

to add folds

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5. Hair

Hair animation

Physically-based models

• Rigid sticks

• Mass-springs

Geometry

• Hair wisps

• Interpolate between guide hair

– Not realistic without collisions

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5. Hair

• New mechanical model for a strand

“Super helices” : Piece-wise helices, constant lenght

[Bertails, Audoly, Cani et al Siggraph 2006]

• Geometric hair strands

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5. Hair Super-Helices

[Bertails, Audoly, Cani, Querleux,

Lévèque, Leroy, SIGGRAPH’06]

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Layered models

Exercice: animate a jumping snow-man

Sub-phenomena

1. Jumping motion

2. Efficient collision processing

3. Deformations of a smooth surface

• Discuss different models for each layer

• Write the animation loop. Is there retroaction ?

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Possible solutions?

1. Skeleton points

• Main skeleton: physics of a ball, user control

• Others: vertical mass-spring chain?

2. Implicit or spline surface, mesh?

3. Collision processing

• Detection with the implicit surface or spheres?

• Deformations due to contact modeling

• Response from the amount of deformation