Computer Animation Ying Zhu Georgia State University

1. Computer AnimationYing ZhuGeorgia 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. BackgroundDampened 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 • http://en.wikipedia.org/wiki/Damping

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⁄mx − b⁄mx‘ • m: mass • b: damping coefficient • X’: velocity

6. Dampened Mass Spring model • Mass: • 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” • 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 • http://www.blender.org/fileadmin/movies/softboy.avi

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 http://www.continuousphysics.com/ftp/pub/test/index.php?dir=blender/&file=physics_demos-2.42-preview34.zip