Design and Implementation of Motion Synthesis System Based on Support Phase

Design and Implementation of Motion Synthesis System Based on Support Phase Quan Yu

Outline • Background and Related Work • My Work • System Design and Implementation • Design Idea and System Framework • Torso Coordinate and BVHFile • Support Phase Detecting • Dynamic Time Warping • Motion Blending • Skinning • Result • Conclusion and Future Work

Background • Motion Capture System • Two Defects • Only available data that can be used. • The data can only be applied to the character who shares the same skeleton structure with the actor. • Solution: motion synthesis and retargeting motion transition

Related Work Support Phase: Make a transition between two support phases Motion Graph： Make a transition between two frames with the most similar postures. computation consuming can be smoother

My Work • Implement an interactive motion synthesis system based on support phase detecting, dynamic time warping and motion blending • Skinning Mesh Animation

System Design and Implementation • Design Idea and System Framework • Torso Coordinate and BVHFile • Support Phase Detecting • Dynamic Time Warping • Motion Blending • Skinning

Design Idea • Support Phases Detecting: • Find the idealist pair of support phases • Synchronize the motion and narrow the searching scope • Dynamic Time Warping • Find the start frame and end frame for blending • The smaller distance, the smoother transition • Motion Blending • Generate the transition with linear or spherical linear interpolation

Design Idea(Cont.) a general motion transition a transition based on support phases

System Framework System Flowchart

Torso Coordinate and BVH File • Torso Coordinate：a tree structure • BVH File：skeleton structure and motion data HIERACHY ROOT hip { OFFSET 0.00 0.00 0.00 CHANNELS 6 Xpositon Ypos. Zpos. Xrotation Zrot. Yrot. JOINT chest { OFFSET 0.000000 5.018153 -1.882228 CHANNELS Xrotation Zrotation Yrotation …… End Site { OFFSET 0.000000 -6.699570 0.000000 } } } MOTION Frames: 100 Frame Time: 0.033333 0.000000 42.014908 0.240040 14.929800 0.146550 ……

Torso Coordinate and BVH File(Cont.) • Transformation Matrix • Compute joints positions in the world coordinate E.g. : transformation matrix for left ankle The position of left ankle:

Torso Coordinate and BVH File(Cont.) • Recursive Program by utilizing matrix stack of OpenGL void calJointsPositionAtFrame(Joint* joint,int frame_no) { GLfloat M[16]; glPushMatrix(); glTranslatef(x_offset,y_offset,z_offset); glRotatef(radX, 1.0f, 0.0f, 0.0f ); glRotatef(radY, 0.0f, 1.0f, 0.0f ); glRotatef(radZ, 0.0f, 0.0f, 1.0f ); glGetFloatv(GL_MODELVIEW_MATRIX,M); Vector3 pos=Vector3(M[12],M[13],M[14]); mapJointsPositionAtFrame[frame_no]->push_back(pos); for (int i=0; i<joint->numOfChildren(); i++ ) calJointsPositionAtFrame( joint->getChild(i), frame_no ); // recursion glPopMatrix(); }

Support Phase Detecting • Find the idealist pair of support phases • Support Phase • The constrain between the feet and the ground • Four Categories • no support(NS), right leg support(RS), left leg support(LS), double leg support(DS) • Classification criteria • The height and velocity of feet “ > ” ：larger than the threshold “ < ” ：smaller than the threshold ” * ” ：unrelated

Support Phase Detecting(Cont.) • Choose the candidate pairs Rules: e.g. “ O ” : possible “ X “ : impossible

Support Phase Detecting(Cont.) • Choose the idealist pair of support phases • Selection Criteria • The lengths of A_sp and B_sp are nearly the same. • Both the lengths of A_sp and B_sp are close to the ideal length • Final criterion • Choose the pair with minimal w.

Dynamic Time Warping • Find the start frame for A_sp and end frame for B_sp • Feature Vector • Position, velocity and acceleration of Joints in WCS. • Align the root joint of frame B_sp_j with that of frame A_sp_i. Alignment Matrix: Velocity: Acceleration:

Dynamic Time Warping(Cont.) • Frame distance • Measure the similarity of a pair of frames(postures) • Choose the pair with minimal frame distance position distance: velocity distance: acceleration distance： frame distance:

Dynamic Time Warping(Cont.) • Criteria for blending length (1) Use the ideal length (threshold) if possible; (2) Or else, use the length as long as possible; (3) If the length is smaller than minimal threshold, use minimal length, and adjust the start fame and the end frame.

Motion Blending • Motion Alignment • Align the position of the root joint of B_sp_i with that of A_sp_i • Align the velocity direction of the root joint of B_sp_i with that of A_sp_i

Motion Blending (Cont.) • Interpolation • root position: linear interpolation • root orientation: linear interpolation with small angle • other channels: spherical linear interpolation Also and

Skinning • Linear blend skinning • Skin: obj file • Weight : xml file • Binding: 3dmax skinmodifier • The position of a vertex in WCS vertex position in WCS at binding pose Inverse transformation matrix of bone i at the binding pose transformation matrix of bone i at a frame weight of vertex related to bone i the number of bones influencing this vertex vertex position in WCS at a frame

Result • User interface • VC2005、OpenGL and Qt

Result(Cont.) • parameter dialog

Result(Cont.) • Support phase detecting result • Adjust the threshold manually if there is a need

Result(Cont.) a transition from walking to running

Result (Cont.) a transition from walking to jumping

Result (Cont.) • skinning result

Conclusion and Future Work • Design and implement a motion synthesis system based on support phases, dynamic time warping and motion blending. • Future Work • motion path synthesis • improve skinning algorithm and result

Thank you!

Design and Implementation of Motion Synthesis System Based on Support Phase