Understanding BVH Animation: Structure and Motion Data Parsing

1. Animation from BVH Andrew Slatton

2. Biovision Hierarchy (BVH) • Contains motion capture data • 2 Major Components: • Hierarchy • Formatted like a scene graph with parents, children, etc. • Motion data • Number of frames, frame time • A list of floating point values associated with “channels” of the hierarchy nodes

3. BVH • Example of Hierarchy: HIERARCHY ROOT Hips { OFFSET -0.347901 96.7718 1.79791 CHANNELS 6 Xposition Yposition Zposition Zrotation Xrotation Yrotation JOINT LeftUpLeg { OFFSET 8.91 -6.27 -2.145 CHANNELS 3 Zrotation Xrotation Yrotation JOINT LeftUpLegRoll { OFFSET 0 -22.7323 0 CHANNELS 3 Zrotation Xrotation Yrotation } … }

4. BVH • Motion data example: MOTION Frames: 119 Frame Time: 0.0416667 -0.347901 96.7718 1.79791 3.45611 -6.10769 -4.18959 -9.22955 -11.0645 -5.78911 6.6312e-019 -6.8208e-017 1.11403 -8.04373e-008 27.9673 4.94512 1.63315e-017 -3.38117e-016 5.53064 1.94131 -9.21066 0.156389 1.74467e-007 -1.06026e-007 3.22852e-016 -6.86092 -9.75453 -1.61311 8.22385e-018 -2.40073e-016 3.92388 -7.54458e-006 75.9784 0.826967 …

5. BVH Motion Data • List of values • Each value corresponds to a rotation or translation channel • Listed in same order as hierarchy was parsed • [Joint0, Channel0] [Joint0, Channel1] [Joint1, Channel0] [Joint1, Channel1] [Joint1, Channel2] … • [X, Y, Z] x [Rotation, Translation]

6. Project Steps • BVH Parser • Needs to read in and handle hierarchy, motion data • Similar to writing any other parser, so no need to discuss • Skeleton drawing • Could handle this mostly in CPU or mostly in GPU • I chose GPU

7. Drawing the Skeleton • High level implementation: • On CPU: • Send skeleton object a time • Skeleton object computes what frame should be displayed at that time • Skeleton object sends appropriate data to shader • Shaders handle everything else

8. Drawing the Skeleton • Vertex Shader • Given motion and hierarchy data • Computes positions of joints for the desired frame • Geometry Shader • Given joint locations • Computes vertex positions and normals for limbs • Fragment Shader • Phong illumination of limbs

9. Drawing the Skeleton • Shader uniform variables: • Motion data for desired frame • Joint offsets • Channel ordering • Could be Xrot, Yrot, Zrot, OR Zrot, Yrot, Xrot (or any other order) • Parent pointers • Must apply ancestors’ transforms to find a joint’s location • Motion data pointers • Some joints span six indices of motion data, some three, and some zero

10. Generating Limbs: Simplest Method: Draw a sphere at the location of each joint Doesn’t give a vivid picture of the motion Drawing the Skeleton

11. Drawing the Skeleton • Generating Limbs: • Could make a cylinder about each ParentJoint-ChildJoint segment • Notice the discontinuities at joints. This is ugly.

12. Drawing the Skeleton • Generating Limbs: • Quick fix for final result • Rotate y = x*x – 1 about each limb axis

13. Odds and Ends • Could implement animation as loop that stops only when animation is complete: while(!skel.last_frame()) { skel.set_frame(elapsed_time); DrawScene(); } • This locks up CPU • No good if we want to allow keyboard/mouse inputs during animation • Also, won’t work with glutPostRedisplay() because we must exit the loop before glutMainLoop can call glutDisplayFunction

14. Odds and Ends • Use glutTimerFunc ! Animate(int delay){ skel.set_frame(elapsed_time); glutPostRedisplay(); if(!skel.last_frame()) { glutTimerFunc(delay,Animate,delay); }} • Will call Animate once every delay milliseconds until we reach end of animation • Allows glutMainLoop to continue execution during this delay time

15. Thank You!