Character Animation

1. Character Animation

2. Contents • Keyframe animation • Mocap • Smooth skin by vertex blending • Rigging & retargeting • Gait analysis • (Ragdoll physics)

3. Keyframe & Inbetweening

4. Keyframe Interpolation1 Periodic motion, with n keys, period T Each key contains the pose-defining parameters At time t, subtract integer multiples of T (so that 0  t* < T) Use t* to determine the interpolating keys the max i with t(i) < t* Linearly interpolate key(i) and key(i+1) Key1 t1 Key2 t2 Key3 t3 Keyn tn … T

5. Keyframe Interpolation2 Key1 t1 Key(i) t(i) Key(i+1) t(i+1) Keyn tn … … T

6. Quake II（雷神之錘II） • First-person shooter game by id software in 1997 • Md2: the model format used by Quake II • Model archive: http://planetquake.gamespy.com/quake2/

7. Better than linear interpolation

8. MOCAP • Motion Capture • File format: ASF/AMC • Database, mixamo • Sample program

10. ASF/AMC (ref) • ASF (Acclaim Skeleton File) • AMC (Acclaim Motion Capture data) • :root • :bonedata • :hierarchy

11. Root • "axis" keyword in the root section defines the rotation order of the root object. • "order" keyword specifies the channels of motion that are applied to the root and in what order they will appear in the AMC file. • "position" and "orientation" keywords are each followed by a triplet of numbers indicating the starting position and orientation of the root. These are typically, but not always, zero.

12. Bonedata (ref) • "id" unique id for the segment • "name" alphabetic identifier for bone. • "direction" the direction of the segment in (world coordinate system) • "length" The length of the segment. • "axis" an axis of rotation for the segment. The axis line indicates the initial orientation of a bone’s axes relative to fixed, global axes • "dof"  The dof line tokens determine: • what degrees-of-freedom exist between a bone and its parent • the order in which these degrees-of-freedom are written in the AMC file • the sequence in which rotations are applied to the bone • "limits" For each channel that appears there will be a pair of numbers inside parenthesis indicating the minimum and maximum allowed value for that channel

13. Example

14. In-Depth Bonedata

15. lfemur

16. lfoot

17. In v’=Mv convention From Lee and Koh (1995) Euler angles in ASF Fall 2012 17

18. Transformation Code

19. Skinning References: Rotenburg(UCSD), Frost (UCI), code

20. Skinning Introduction • 3D character models play an important role in gaming • Organic models are more complex to render than rigid models • Organic: humans, animals, etc. • Rigid: building, rock, etc. • The mesh that defines the shape of the model constantly changes as the models animate

21. Skinning Introduction (cont) • The animating mesh is referred to as a “skin” • Skinning is the process of animating the mesh • Skinning has been traditionally done on the CPU • As 3D character models complexity increases, skinning is done on the video cards using vertex shaders

22. Methods to Animate Skinned Characters • Keyframe animation (as in MD2) • Save characters in “keyframes” and blend • Suitable for low polygon models • On high-detail models, it can be memory intensive • Matrix palette skinning • Weighting individual points to the bones • Just the skeletal animation needs to be saved (rather than the whole models in keyframes) • Not mutually exclusive • Face/hand for keyframes; matrix palette for the rest

23. Vertex Blending • Also known as: • Matrix palette, skeleton-subspace deformation

24. How Bones are Created • Obtain the skin model • Duplicate the skin • Scale down fractionally • Cut the smaller skin into smaller body parts which are used as bones

25. Skinning Simple Skin • every vertex of the continuous skin mesh is attached to a joint. • every vertex is transformed exactly once. Smooth Skin • a vertex can be attached to more than one joint with adjustable weights that control how much each joint affects it • Vertices rarely need to be attached to more than three joints • Each vertex is transformed a few times and the results are blended

26. Smooth Skin – Binding Matrix • we use a binding matrix B for each joint that defines where the joint was when the skin was attached and premultiply its inverse with the world matrix:

27. 0.5/0.5 0.0/1.0 1.0/0.0 0.7/0.3

28. To compute shading, we need to transform the normals to world space also • If the matrices have non-rigid transformations, then technically, we should use:

29. Limitation • Smooth skin is very simple and quite fast, but its quality is limited • Joints tend to collapse as they bend more • Very difficult to get specific control • Unintuitive and difficult to edit • Still, it is common in games and commercial animation!

31. Other Topics Rigging, retargeting, gait analysis, ragdoll physics

32. What is rigging (ref) • A character rig is essentially a digital skeleton bound to the 3D mesh. Like a real skeleton, a rig is made up of joints and bones, each of which act as a "handle" that animators can use to bend the character into a desired pose.

33. Automatic Rigging (Baran & Popovic, 2007)

34. Automatic Rigging (cont)

35. Retargeting Animation from Rig to Rig (ref) • When you retarget animation between rigs, the retargeting operator figures out which rig elements match based on their tags. Then it maps and generates the animation that is transferred to the target rig. • Online motion retargeting 1999 • Youtube (havok)

36. Mixamo • Auto-Rigger (youtube)

37. Gait • Gait is the pattern of movement of the limbs of animals, including humans, during locomotion over a solid substrate. Most animals use a variety of gaits, selecting gait based on speed, terrain, the need to maneuver, and energetic efficiency. Different animal species may use different gaits due to differences in anatomy that prevent use of certain gaits, or simply due to evolved innate preferences as a result of habitat differences. 

38. Ragdoll physics • Verlet integration • IK postprocessing