Ryan Marcotte CS 499 (Honours Seminar) March 16, 2011

Transcription

1 Ryan Marcotte CS 499 (Honours Seminar) March 16, 2011

2 Outline Introduction to the problem Basic principles of skeletal animation, tag points Description of animation algorithm Demo Future research 2

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4 Application Most games use 3D models to represent characters, items Some items can be equipped by the player Equipment changes the appearance of the player s avatar 4

5 Example Source: pso-world.com, Phantasy Star Universe 5

6 Problem Many different equipment items = many different (unique) combinations Impractical to model each combination Example 5 swords (must equip one) 4 shields (must equip one) 12 accessories (equip up to three) Result 6

7 Problem How to represent all equipment combinations? Unfeasible to use a unique model for each combination Equipment is subject to player movement and animation Equipment may have own animations independent of the player s avatar Algorithm must be fast 7

8 Proposed Solution Skeletal animation (for animating 3D models via a skeleton) Tag points (for providing attachment points subject to the animation of a model s skeleton) 8

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10 What is skeletal animation? Uses bones and joints to manipulate vertices (the skin) A bone hierarchy exists (analogous to own skeleton) Animation produced by interpolating joint translations and rotations between keyframes 10

11 What is a bone? A bone is a frame of reference for a group of vertices Conceptual in nature 11

12 What is a joint? Joints define points of manipulation in the skeleton Bones are connected by joints Joints may be rotated or translated 12

13 How to construct skeleton? Skeleton is constructed as a tree Individual joints are manipulated by their parent joints 13

14 How to construct skeleton? 14

15 How to construct skeleton? Where should the root node be placed? How many joints are needed? 15

16 Why is this useful? Animators manipulate bones instead of individual vertices Animation appears more realistic Animations can be generated at runtime (rag-doll physics) Less space is required to store animations 16

17 What are potential drawbacks? Additional computations are needed at runtime More complex animations (such as facial animation or those for invertebrates) may be difficult to model using skeletal animation techniques 17

18 Computer Representations No fixed representation The representation used for the MS3D model format will be discussed here C++ structures will be used 18

19 How to represent bones? struct MS3D_Vertex { byte flags; // SELECTED SELECTED2 HIDDEN float vertex[3]; // position char boneid; // -1 = no bone byte referencecount; // editor use only }; 19

20 How to represent joints? struct MS3D_KeyFrame_Rotation { float time; // time in seconds float _3D[3]; // x, y, z angles OR- position }; typedef MS3D_KeyFrame_Position MS3D_KeyFrame_Rotation; struct MS3D_Joint { unsigned char flags; // SELECTED DIRTY char name[32]; // name of joint char parentname[32]; // name of parent joint float rotation[3]; // initial local reference matrix float position[3]; // initial local reference matrix int numkeyframesrot; // number of rotation keyframes int numkeyframestrans; // number of translation keyframes MS3D_KeyFrame_Rotation keyrotation[numkeyframesrot]; // local animation matrices MS3D_KeyFrame_Position keyposition[numkeyframestrans]; // local animation matrices }; 20

21 How to represent joints? struct MS3D_Runtime_Joint { std::string name; // name of joint std::string parentname; // name of parent joint int parentid; // index of parent joint float startrotation[3]; // initial local reference matrix float startposition[3]; // initial local reference matrix int numkeyframesrot; // number of rotation keyframes int numkeyframestrans; // number of translation keyframes MS3D_KeyFrame_Rotation keyrotation[numkeyframesrot]; // local animation parameters MS3D_KeyFrame_Position keyposition[numkeyframestrans]; // local animation parameters matrix4x4 skeletonlocalmatrix, skeletonglobalmatrix; // matrices for initial pose matrix4x4 localmatrix, globalmatrix; // matrices for new pose }; 21

22 Tying It Together Need the following information stored at runtime Initial vertex positions and their corresponding normals Texture coordinates and materials for vertices Animation data Initial position and orientation of joints Position and rotation data for keyframes 22

23 How to produce animation? Use linear transformations (matrix multiplication) stored in joints Joints move or rotate bones Vertices move with the bones A series of matrix multiplications are used for handling the parent-child relationship among joints 23

24 What about tag points? Also known as attachment points Applies translation and rotation transformations to a secondary mesh such that it appears to be attached to the original mesh 24

25 What about tag points? We will use existing joints within the skeleton to attach other models Simply store a pointer to the joint where other models will be attached 25

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27 Algorithm (Initialization) Load model information from file Store model information in a format more optimal for runtime use Build bone/joint hierarchy for initial pose 27

28 Algorithm (Initialization) Algorithm : InitializeJoints Input : MESH (a model loaded from disk, stored in memory) Output : MESH, with an initialized bone/joint hierarchy for the initial pose Return : void // Use integers to store reference to parent joint (joints stored in array) for all joints in MESH joint[x].parentid = IndexOfJointWithName(joint[X].parentName); // -1 if no parent // Compute transformation matrices for each joint for all joints in MESH joint[x].skeletonlocalmatrix.initwithrotation(joint[x].startrotation); joint[x].skeletonlocalmatrix.initwithposition(joint[x].startposition); int PID = joint[x].parentid; if PID = -1 joint[x].skeletonglobalmatrix = joint[x].skeletonlocalmatrix; else joint[x].skeletonglobalmatrix = joint[pid].skeletonglobalmatrix * joint[x].skeletonlocalmatrix; 28

29 Algorithm (Initialization) initwithrotation and initwithposition only affect specific matrix entries initwithrotation initwithposition 29

30 Algorithm (Tag Points) Each model maintains a hash table of pointers to joints (using string-based IDs) for fast retrieval Creating a tag point only involves retrieving a pointer to the intended joint Pointer will be used to access joint s global matrix (stores joint s position and rotation with respect to the origin) The joint s matrix will be passed as a parameter into the animation algorithm 30

31 Algorithm (Animation) Manipulate the skeleton into new pose according to the current frame Transform vertices and normals using new pose Render the model in new pose 31

32 Algorithm (Animation) Algorithm : SetFrame Input : MESH (a model loaded from disk, stored in memory) FRAME (floating point number representing animation frame) BASE_MATRIX (initial translation / rotation matrix (for tagging)) Output : MESH, with an initialized bone/joint hierarchy for the specified pose Return : void if FRAME < 0.0f for all joints in MESH joint[x].localmatrix = joint[x].skeletonlocalmatrix; joint[x].globalmatrix = joint[x].skeletonglobalmatrix; else for all joints in MESH TransformJoint(MESH, joint[x], FRAME, BASE_MATRIX); 32

33 Animation (Animation) Determine the keyframes that the current frame lies between Get position and rotation information from keyframes and store in matrices (initwithposition, initwithrotation) Interpolate between the resultant matrices; store in TransformMatrix 33

34 Algorithm (Animation) Algorithm : TransformJoint Input : MESH (a model loaded from disk, stored in memory) JOINT (joint currently being processed) FRAME (floating point number representing animation frame) BASE_MATRIX (initial translation / rotation matrix (for tagging)) Output : JOINT, with initialized transformation matrices for the specified pose Return : void 34

35 Algorithm (Animation) float pos[3]; // Stores transformed position if JOINT.numKeyPositions > 0 int i1 = -1, i2 = -1; for all keyframes (except LAST) in JOINT if((frame >= JOINT.keyPosition[X].time) and (FRAME < JOINT.keyPosition[X+1].time)) i1 = X; i2 = X + 1; break; if((i1 = -1) or (i2 = -1)) if FRAME >= JOINT.keyPosition[LAST].time { pos = JOINT.keyPosition[LAST]._3D; } else if FRAME < JOINT.keyPosition[0].time { pos = JOINT.keyPosition[0]._3D; } else MS3D_KeyFramePosition *PrevFrame = &JOINT.keyPosition[i1], *CurFrame = &JOINT.keyPosition[i2]; float DELTA = ((FRAME PrevFrame.time) / (CurFrame.time PrevFrame.time)); pos = (PrevFrame._3D + (CurFrame._3D PrevFrame._3D)) * DELTA; 35

36 Algorithm (Animation) matrix4x4 transformmatrix; // Final transformation matrix if JOINT.numKeyRotations > 0 int i1 = -1, i2 = -1; for all keyframes (except LAST) in JOINT if((frame >= JOINT.keyRotation[X].time) and (FRAME < JOINT.keyRotation[X+1].time)) i1 = X; i2 = X + 1; break; if((i1 = -1) or (i2 = -1)) if FRAME >= JOINT.keyRotation[LAST].time transformmatrix.initwithrotation(joint.keyrotation[last]._3d); else if FRAME < JOINT.keyRotation[0].time transformmatrix.initwithrotation(joint.keyrotation[0]._3d); else MS3D_KeyFrameRotation *PrevFrame = &JOINT.keyPosition[i1], *CurFrame = &JOINT.keyPosition[i2]; float DELTA = ((FRAME PrevFrame.time) / (CurFrame.time PrevFrame.time)); matrix4x4 prevmatrix, curmatrix; prevmatrix.initwithrotation(prevframe._3d); curmatrix.initwithrotation(curframe._3d); transformmatrix = prevmatrix + ((curmatrix prevmatrix) * DELTA); 36

37 Algorithm (Animation) transformmatrix.initwithposition(pos); JOINT.localMatrix = JOINT.skeletonLocalMatrix * transformmatrix; if JOINT.parentID = -1 JOINT.globalMatrix = JOINT.localMatrix * BASE_MATRIX; else JOINT.globalMatrix = MESH.joints[JOINT.parentID].globalMatrix * JOINT.localMatrix; 37

38 Link 38

39 Considerations The MS3D model file format does not support tag points, but formats exist that do support this feature Linear interpolation (LERP) can cause distortions during animation; fix by using spherical linear interpolation (SLERP) or by increasing the number of keyframes 39

40 Future Research GPU-based skeletal animation GPU-based particle systems Normal mapping 40

41 Do you have any questions? 41