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Chapter 12: Physics Simulation and Dynamics

Haiyue9/1/25About 3 minaws

Chapter 12: Physics Simulation and Dynamics

Learning Objectives

  1. Understand rigid body physics simulation
  2. Master soft body and cloth simulation
  3. Learn the fluid simulation system
  4. Master smoke and fire simulation
  5. Understand collision detection and constraint systems

12.1 Physics Simulation Basics

Physics Simulation Concepts

  • Physics Engine: A system that calculates the motion and interaction of objects
  • Time Step: The time interval for simulation calculations
  • Collision Detection: An algorithm to determine if objects have collided
  • Constraint System: Rules that restrict the motion of objects

Blender Physics System

Properties Panel → Physics Properties
- Rigid Body: Rigid body simulation
- Soft Body: Soft body simulation
- Cloth: Cloth simulation
- Fluid: Fluid simulation
- Collision: Collision detection

World Settings

Scene Properties → Gravity
- Gravity: [0, 0, -9.8] (standard gravity)
- Units: Unit settings affect physics calculations

12.2 Rigid Body Physics Simulation

Basic Concepts

  • Active: Dynamic objects that participate in physics calculations
  • Passive: Static colliders that are not affected by forces
  • Center of Mass: The object's center of gravity

Rigid Body Settings

Physics Properties → Rigid Body:
- Type: Active/Passive
- Shape: Collision shape
  - Box: Fast
  - Sphere
  - Convex Hull
  - Mesh: Precise but slow
- Mass
- Friction
- Bounciness

Constraint System

  1. Fixed: Completely fixes two objects
  2. Point: Connects at a point, allowing free rotation
  3. Hinge: Rotates along one axis
  4. Slider: Slides along one axis
  5. Spring: Elastic connection

Example: Domino Sequence

# Create a domino sequence
import bpy
import bmesh

def create_domino_sequence(count=10, spacing=2.0):
    for i in range(count):
        # Create a cube
        bpy.ops.mesh.primitive_cube_add(
            location=(i * spacing, 0, 1), 
            scale=(0.2, 1, 2)
        )
        
        # Set rigid body
        obj = bpy.context.active_object
        obj.rigid_body.type = 'ACTIVE'
        obj.rigid_body.mass = 1.0
        obj.rigid_body.friction = 0.5

12.3 Soft Body Simulation

Soft Body Principles

  • Spring-Mass Model: Vertices as masses, edges as springs
  • Internal Forces: Elastic forces that maintain shape
  • External Forces: Environmental forces like gravity, wind

Basic Settings

Physics Properties → Soft Body:
- Object → Mass
- Object → Friction
- Object → Speed: Motion damping

Goal:
- Strength: Goal strength
- Default: Default weight

Spring Settings

Edges:
- Pull: Stretching strength
- Push: Compression strength
- Damping
- Plastic: Plastic deformation
- Bending: Bending strength

Self Collision

Self Collision:
- Enable
- Size: Collision ball radius
- Stiffness: Collision strength

12.4 Cloth Simulation

Cloth Physics

  • Fabric Structure: Weft and warp structure
  • Stretching Resistance: Prevents excessive stretching
  • Bending Resistance: Maintains the thickness of the cloth

Basic Settings

Physics Properties → Cloth:
Physical Properties:
- Mass: Mass per area
- Air Viscosity
- Bending Stiffness

Material:
- Structural: Structural strength
- Shear: Shear strength
- Bending: Bending strength

Cloth Presets

  • Cotton: Medium strength
  • Silk: Soft and smooth
  • Denim: Heavy and tough
  • Leather: Hard material
  • Rubber: High elasticity

Pinning Settings

Shape → Pin Group:
1. Select fixed vertices in Edit Mode
2. Create a vertex group
3. Specify the Pin Group in the cloth settings

Wind Effect

Add → Force Field → Wind
- Strength
- Flow: Airflow effect
- Noise: Turbulence noise

12.5 Fluid Simulation

Fluid Types

  1. Domain: The space where the fluid is calculated
  2. Flow: Objects that generate or absorb fluid
  3. Effector: Objects that affect the fluid

Mantaflow Engine

Physics Properties → Fluid:
Type: Domain
Settings:
- Resolution: Calculation precision
- Time Scale
- CFL Number: Stability parameter
- Gravity

Liquid Simulation

Domain → Liquid:
- Viscosity
- Surface Tension
- Density

Flow Object:
- Flow Type: Liquid
- Flow Behavior: Inflow/Outflow
- Initial Velocity

Gas Simulation (Smoke)

Domain → Gas:
- Density
- Temperature
- Vorticity

Flow Object:
- Flow Type: Smoke/Fire/Both
- Density: Smoke density
- Temperature: Temperature difference

12.6 Smoke and Fire Simulation

Quick Setup

Object → Quick Effects:
- Smoke
- Fire
- Smoke + Fire

Advanced Fire Settings

Flow Settings:
- Fuel
- Burn Rate
- Flame Smoke: Smoke generated by fire
- Temperature: Fire temperature

Domain Settings:
- Flame Vorticity
- Burning Rate
- Smoke Color

Material Settings

Shader Editor → Volume:
- Principled Volume: Main volume material
- Density: Connect to smoke density
- Color: Connect to fire color
- Temperature: Connect to temperature attribute

12.7 Collision System

Collision Settings

Physics Properties → Collision:
- Enabled
- Damping
- Thickness: Collision thickness
- Friction
- Permeability

Collision Shape Optimization

  • Simple Shapes: Box, Sphere for performance
  • Precise Shapes: Mesh for accuracy
  • Inside/Outside Detection: Single Sided vs Double Sided

Collision Groups

Relations:
- Collision Group
- Absorb Group
Used to control which objects can collide

12.8 Constraints and Connections

Rigid Body Constraints

Add → Empty → Select constraint type
Rigid Body Constraint:
- Type: Constraint type
- Object 1/2: Connected objects
- Breaking Threshold

Constraint Parameters

  1. Generic Constraint:

    • Location: Position limits
    • Rotation: Rotation limits
    • Springs: Spring settings

  2. Motor Constraint:

    • Target Velocity
    • Max Impulse

12.9 Advanced Physics Techniques

Cache Management

Cache:
- Start/End Frame: Cache range
- Disk Cache
- Compression

Performance Optimization

  1. Lower Resolution: Reduce calculation load
  2. Use Simplified Collisions: Box instead of Mesh
  3. Limit Influence Range: Set Force Field distance
  4. Layered Calculation: Decompose complex scenes

Time Remapping

Scene → Time Remapping:
- Speed: Playback speed
- Frame Remapping
Used to create slow-motion or fast-forward effects

12.10 Practical Cases

Case 1: Destruction Effect

  1. Use the Cell Fracture add-on to break an object
  2. Set fragments as Active rigid bodies
  3. Add an explosion force field
  4. Adjust mass and friction parameters

Case 2: Falling Cloth

  1. Create a cloth plane
  2. Set a Pin Group to fix corners
  3. Add a collision ground
  4. Adjust cloth parameters for realism

Case 3: Liquid Container

  1. Create a container (set as a collider)
  2. Set up a liquid domain
  3. Add a liquid inflow object
  4. Adjust viscosity and surface tension

Practical Exercises

  1. Create a bowling game scene
  2. Animate a waving flag
  3. Simulate pouring water into a glass
  4. Design a destruction scene with an explosion
  5. Animate rising smoke

Key Takeaways

  • Physics simulations require reasonable world settings and units
  • Rigid body simulation is suitable for hard object collisions
  • Soft body simulation can create organic deformations
  • Cloth simulation needs proper pinning and parameters
  • Fluid simulation resolution affects realism and performance
  • Constraint systems can create complex mechanical motions
  • Cache management is crucial for complex simulations

Next Chapter Preview

The next chapter will cover the node system and procedural workflows, including advanced applications of shader nodes, geometry nodes, and composite nodes.

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