1. Computer Animation Ying Zhu Georgia State University
Soft Bodies and Rigid Bodies

2. Soft body and clothes
In older versions of Blender, cloth is part of Soft Body simulation
But in the latest version, cloth simulation has its own parameters and controls
So now soft bodies are mainly used for simulating closed meshes

3. Background Dampened Mass Spring model
Soft bodies are simulated by connecting neighbor vertices with dampened mass springs
The same applies to cloth simulation too
Many Soft Body related parameters are better understood in the context of a dampened mass spring
Mass
Stiffness
Damping: a friction force that resists the motion

4. Dampened Mass Spring http://www.myphysicslab.com/spring1.html

5. Dampened Mass Spring model
The behavior of a simple spring is governed by Hook’s law : Fspring = −k x
K is spring stiffness
X is stretch
Add damping force then we have
acceleration x'' = − k⁄m x − b⁄m x‘
m: mass
b: damping coefficient
X’: velocity

6. Dampened Mass Spring model
Increase object mass will reduce the acceleration of the object
Increase object mass will reduce the frequency of spring oscillation (i.e. soft body will be less bouncy)
Stiffness:
Increase spring stiffness will increase the restoring force generated by the spring
Increase stiff will increase the frequency of spring oscillation (i.e. soft body will be bouncier)

7. Dampened Mass Spring model
Damping:
A force that reduces the amplitude of spring oscillations 
Increase damping makes a soft body less bouncy

8. How to create a soft body in Blender?
Create a soft body object
In “Object”, “Physics buttons” panel, “Soft Body” tab, press “Soft Body” button

9. Soft body parameters

10. Soft body parameters Friction: “media friction” Mass: mass of vertices
Friction with the surrounding air or liquid
Increase this friction make the soft body move slower in the air
Mass: mass of vertices
Larger mass slows down motion
Speed: internal timing of physical simulation
Use values less than one to simulate dense air or very light fabric dropping

11. Use Goal By default the “Use Goal” button is pressed
This is similar to “Pinning of cloth”
Part of the soft body is pinned and doesn’t move
How to set goal for soft body?
Create a vertex group on the soft body object
In “Soft Body” tab, press “Use Goal”
Click on the (up and down arrow) button next to the “Use Goal” button, select the vertex group from the drop down menu

12. “Use Goal” parameters
G Stiff: the stiffness of the connection between the pinned vertex group and rest of the mesh
If set to 0.0, the object may swing
G Damp: the damping of the connection between the pinned vertex group and rest of the mesh
“G Min” and “G Max”: usually keep the default values
Setting “G Max” below 1.0 and the goal (pinning) would have no effect

13. Why doesn’t the soft body fall under gravity?
By default “Use Goal” is enabled
If no vertex group is selected as a “goal”, the entire soft body object is pinned
To make soft body object fall under gravity, release the “Use Goal” button

14. Soft body parameters Use Edges: connect neighbor vertices with springs
Stiff Quads
For quad faces, also connect diagonal vertices with springs

15. Soft body parameters Stiffness
Pull Stiffness: spring stiffness when pulled
Push stiffness: spring stiffness when pushed (usually the same as Pull Stiffness)
Bending stiffness: spring stiffness when bended
Shear stiffness: spring stiffness when twisted
Increasing stiffness generally creates stiffer and bouncier soft bodies (if everything else remains the same)

16. Soft body parameters Damping: Friction for edge springs
Higher damping means higher resistance to motion
Higher damping means that the spring will go back to its rest position quicker (i.e. object will calm down quicker)

17. Collision
In the Collision tab, press “Self Collision” if you see soft body faces penetrating each other during animation

18. Baking
Baking happens automatically the first time you press Alt + A to start the animation
When you make changes and press Alt + A, baking automatically restarts
A cache folder is automatically created under the “output” folder you set in the “Scene” panel
Set baking start and end frames to match your animation
Note that each soft body has its own baking start and end frames

19. Create a deflector Create an object (e.g. a flat plane)
Under “Object”, “Physics buttons” panel, “Collision” tab, press “Collision” button
There is a group of parameters under “Soft Body and Cloth Interaction”

20. Collision parameters
You can define a collision layer surrounding the actual mesh
Inner: thickness of the collision layer inside the object
Outer: thickness of the collision layer outside the object
Damping
Low damping: hard surface
High damping: soft surface

21. Soft Body example: a soccer ball
Add an Icosphere
Subdivision: 3
Radius: 1
Set Soft Body parameter as follows

22. Soft Body example: a Jell-O
Create a cube
Set Soft Body parameters as follows:

23. Soft Body example

24. Rigid bodies Rigid bodies are handled in the Logic (F4) panel
Select an object
Go to Logic Panel
Select “Rigid body” from the Object type menu
The default is “Static”
Move mouse cursor to 3D View, press P to start the physics simulation

25. Rigid body A “Rigid body” object will fall under gravity
A “Static” object will not fall under gravity
A rigid body object will collide with other rigid body objects and static objects
Unlike soft bodies, there is no need to explicitly make an object “collide-able” with rigid body

26. Rigid body parameters
Ghost: if enabled, the rigid body will not collide with other objects
Mass
Damp: general movement damping
Reducing damping makes the object bouncier
RotDamp: rotation damping

27. Bounds Blender uses bounding boxes for collision detection
Bounds: bounding box shape
Choose a bounding box shape that is close to the shape of the mesh
For example, choose box for cube, sphere for ball, etc.
Choose “Convex Hull” for more complex objects
“Triangle Mesh” is the most accurate but also the slowest

28. Baking
Unlike soft bodies, there is no explicit baking button for rigid bodies
The process
In Game menu, select “Record Game Physics to IPO”
In “Scene” panel, adjust start and end frames in “Anim” tab
Move mouse cursor to 3D View, and press P to start physics simulation
The physics simulation will be “baked” into IPO curves
Next time you can press Alt + A to play back the rigid body simulation

29. User interaction with rigid bodies
There are three set of parameters under the Logic panel
Sensors: event trigger
Controllers: link sensors with actuators
Actuators: how to manipulate the rigid body obj
Together they make it possible for users to control a rigid body object with keyboard, mouse, or joystick
They can also make the rigid body move without user interaction

30. Sensors Press the “Add” button under “Sensors” to add a new sensor
Always: always on (no user interaction)
Keyboard: triggered by keyboard input
Mouse: triggered by mouse input
Joystick: triggered by joystick input
Collision: triggered by collision

31. Controllers
Press the “Add” button under “Controllers” to add a new controller
Usually use the default “AND” controller type
Use mouse to connect a Sensor with a Controller

32. Actuators
Press the “Add” button under “Actuators” to add a new actuator
Actuator types
Motion
Visibility
Camera
Sound
and many more types
Use mouse to connect a Controller with an Actuator

33. Motion Actuators Can transform the rigid body object
Loc
Rot
Can apply forces to the rigid body object
Force
Torque
Can change its velocity
LinV: linear velocity
AngV: angular velocity

34. Simple game
With Sensors, Controllers, and Actuators, you can create a simple game
For examples see