Character Animation 1

Transcription

1 Character Animation 1

2 Overview Animation is a big topic We will concentrate on character animation as is used in many games today humans, animals, monsters, robots, etc.

3 Character Representation A character is represented as a hierarchy of transforms (the skeleton) 4x4 transform matrices with parent/child relationships Called a bone, joint or node A particular configuration of a skeleton is called a pose the state of a skeleton at a particular time of animation

4 Regular layout 2 (no arcade box) Info Here (To switch between regular

5 Regular layout 2 (no arcade box) Info Here (To switch between regular

6 Motion Data Procedural Algorithmically generated E.g. Inverse kinematics Keyframed Animator lays down important keyframes & timing between frames Motion is reproduced through interpolation (tweening) of key frames Hybrid Combines both procedural and keyframed elements E.g. Key framed character with head tracking, Blending between last keyframe and Ragdoll

7 Key framing Ways to generate key frames Created by an animator with an animation tool (e.g. Maya, SoftImage, Max) Motion capture Acquired through sampling motion of actor(s) wearing sensors Requires extensive clean up before use Procedural E.g. sampling a complex cloth simulation generated offline

8 Interpolation A frame of animation data is usually generated via interpolation of key frames. Two commonly used methods Linear Cubic

9 Interpolation Cont d Linear interpolation is simple to implement and efficient But needs many key frames to record complex motion Motion can look robotic Cubic interpolation More natural-looking motion Can reproduce more complex motion with fewer key frames Memory savings More expense to evaluate Probably the best overall choice with modern hardware

10 Interpolation Cont d Interpolating orientation poses special problems Which way to interpolate? Usually we choose the shortest arc Problems with rotations near 180o Euler angle representation Orientations are not unique Gimbal lock Quaternions Interpolate nicely Larger to store

11 Motion Root Placement At Center of Mass At Pelvis for Bi-peds Less typically at Base of character (e.g. between feet)

12 Importance of the Motion Root Motion Root is Handle for Physics & AI to manipulate position, orientation & velocity of character A collision representation is attached to the Motion Root. Motion Root is an attachment point for camera Used for computing camera placement relative to character and environment Hence path traced by the motion root needs to be continuous

13 Control vs Animation: Conflicting Goals Gameplay designer wants responsiveness, player control Want player to have analog control of speed character Want to have fine-grained control for aligning player to objects in the world Want player to execute multiple actions simultaneously e.g. shoot while running Animators & Art directors want realistic motion that looks good Want artistic control Perfect transitions no sliding, no awkward joint positions Natural timing no snapping or odd pauses But we may not have manpower or memory to animate every transition

14 Organizing Animation to Maximize Reuse Layering Separate Layers eg. Face, Hand, Hair, Full Body Multiple animations within a layer Transition by blending: Cyclic Animations have same no. of frames and are frame sync Partial Sets: E.g. animate only arm for weapon reloads Base Layer Full Body Animation move Animation move-lean-l Animation move-lean-r Animation run-incline Animation run-spine-down Animation run-stairs-down Animation run-stairs-up Flipping GDC 2010: Animation & Player Control in Uncharted: Drake s Fortune Mirroring an animation symmetrically

15 Layering Example GDC 2010: Animation & Player Control in Uncharted: Among Thieves

16 Approach I : Sphere Man in Box World GDC 2010: Animation & Player Control in Uncharted: Drake s Fortune

17 Approach I : Sphere Man 100% Controller-driven approach Player modeled as 2 Spheres or 1 Capsule Controls drive the velocity of SphereMan Animation is played as dressing Works but not great for realistic human characters Lots of Sliding E.g. Uncharted 1, Jax & Daxter

18 Approach 2: Pure Animation Root node displacement is purely driven by animator AI reads root node position from motion data Character position, orientation updated from this displacement Advantages: Feel of motion is driven artistically Better footstep registration Cons: Memory intensive Labour intensive Not as responsive e.g. cannot handle intermediate speeds well

19 Approach 3: Blending between Animation & Player control More commonly used Based on Player Intention: Select the closest matching animation Play animation & blend towards Control direction 1.00 Blend Towards Player Control Blend GDC 2010: Animation & Player Control in Uncharted: Drake s Fortune

20 Approach 3: Cont d Warp animation extend the time between key frames to create sense of anticipation e.g. wind ups To create sense of release e.g. charged jump Cons Give up some controller responsiveness Some foot slippage Pros But surprisingly, feels rights (when properly tuned) Momentum -based design approaches are gaining popularity

21 Approach 4: Procedural Animation driven by procedural model Simpler for non-articulated objects: Cars, planes, crates, pucks, clothing, terrain Biomechanics integrates Forces & collisions by modeling character as a system of muscles, springs E.g. Horses in Red Dead Redemption Open research area for characters

22 Motion Sequencing For each character, the AI maintains the current behavioural state Motion Tree is a animation state machine: Conditions Nodes containing animation & playback parameters AI generates an intention structure which is evaluated by the Motion Tree Motion Tree generates a list of animations to be played in sequence and blend parameters Lower-level animation controllers- manage playback, transitions & frame syncing

23 Transitions Several ways to splice animations together Cut Immediately jump to beginning of new animation Instant visual feedback Features pop if animations aren t aligned Phase align and cut Jump to phase-aligned frame of new animation Instant visual feedback Less popping Assumes animations have single, compatible phase value Wait for alignment and cut Play current animation until phase aligns then cut Introduces lag Some animation may not have same alignment points

24 Transitions Several ways to splice animations together Glue animations Play a special artist-built animation between the current and new animation Looks good But needs a lot of them

25 Transitions Blending Cross-fade current and new animation Looks good for short transitions, Long transitions look robotic Animations should be similar Computationally expensive Can blend animation with procedural techniques e.g. IK, physics ragdolls Additive Blending add rotation delta

26 In Closing Animation systems are a lively research area We ve only covered High Level ideas Character animation is tough Complex trade-off between aesthetics and control responsiveness