KIPA Game Engine Seminars

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KIPA Game Engine Seminars
Day 11

• Jonathan Blow
• Seoul, Korea
• December 7, 2002

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Character Animation

• The goal let artists make animation, and get it
  into the game
• Usually animation is made in a separate tool
• Maya / MAX
• Made out of hierarchical transforms
• Probably we need to write an exporter plugin and
  some game-engine import code
• Unless we use a licensed character animation
  system (like Granny)

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Goals for in-enginereproduced animation

• As much like the art packages version of the
  animation as possible
• But small (low memory) and fast

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How animations are authored

• The artist sets up a bunch of keyframes for
  transform data
• Usually separated into translation, rotation,
  scale
• The art tool interpolates between these keyframes
  at runtime
• Maybe we should export the keyframes and
  interpolate them in the engine

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Why you dont want toexport the keyframes (1)

• They can be in weird formats that require
  confusing code to interpolate
• 3DS max TCB curves
• They can be in formats that are slow to
  interpolate
• Euler angles
• They are not necessarily well-compressed
• Keyframes get fragmented over time
• What happens when you import an animation from
  another package

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Why you dont want toexport the keyframes (2)

• Artists use IK tools to help create animations
• You would have to reconstruct all this IK
  functionality exactly for the animation to look
  right
• Footstep controller
• Guy grabbing something from a table

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A better solutionSample the transforms

• Dont worry about the original keyframe data,
  just evaluate the transforms at some sampling
  interval
• Save those transforms out, interpolate them at
  runtime
• Sample at perhaps 30Hz

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Interpolating transforms?

• We dont want to interpolate a 4x3 matrix
• It will denormalize, introduce shear
• Break it into intuitive pieces (translation,
  rotation, scale)
• By matrix decomposition
• But which decomposition you choose matters!
• Shear easily confused with rotation
• Use the polar decomposition

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Interpolating transforms? (2)

• If samples are close together, our interpolation
  will not diverge much from the art tools
• Euler vs slerp for small angles
• But those samples take a lot of memory!
• Example of how much

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Want to compress those samples

• Hopefully at export time (small file size)
• How to compress?
• Fourier / Wavelet compression
• Curve fitting
• Performance ramifications of each method

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Compression

• Probably, curve fitting is the best choice
• Each curve stored as an array of unevenly spaced
  knots
• At runtime we just evaluate a simple polynomial
  to reconstruct values
• For linear values like translation and scale
• What about rotation?

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Rotation interpolation

• For interpolation between two rotations, slerp is
  the right answer
• This may be too slow
• Review of slerp code
• Can we do something else

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Different kinds of rotation interpolation

• slerp
• exponential map
• lerp
• lerp approximates slerp for small angles
• sin(x) x as x - 0
• Its not that bad for large angles either

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Re-derivation of slerpin 2D

• Did this once before, but will do it again
  quickly, as an introduction to the lerp
  derivation

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What happens when welerp two quaternions?

• Assuming they dont differ by more than 90
  degrees
• double cover!
• We can analytically quantify the distoritions in
  angle and length

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Using lerp as asubstitution for slerp

• Probably we have to renormalize
• Maybe we have to compensate for the angle
• Discussion of fast unit vector normalization
• Newton-raphson approximation to 1/sqrt(x) in the
  neighborhood near 1
• Walk-through of code

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How to compensate forthe angle

• Discussion of approximating the inverse of the
  angular distortion

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Maybe we dont have to compensate for the angle

• When curve-fitting, we measure our error with
  respect to the lerped/normalized reconstruction
  of a spline (not the ideal slerp solution)
• This causes us to insert more knots in the curve
  when we might need better angle accuracy
• So maybe there is a trade-off here, memory for
  runtime speed

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Fitting algorithm

• (discussion on whiteboard, including quirks)

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Data structures

• An animation structure contains
• Number of nodes, names of the nodes
• Keyframe data for compressed curves
• Information about how to blend in/out of
  animation, when to start/stop
• Explanation of some different ways you might want
  to arrange things

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Data structures

• A mesh contains
• Number of nodes, names of the nodes
• Vertex data
• Vertex weight information

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Binding data structures

• A lot of systems work by building a table that
  cross-references between node names in the
  animations, and node names in the mesh
• Animations might not be stored in the same order
  node-wise
• Some might have nodes that the others dont
• This is called the bind step

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I dont like the bind step

• It means that you cant have animation playback
  without a mesh attached
• I want animations to be meaningful without meshes
  yet, i.e. I want to be able to play them back and
  mix them
• Though there are bone length assumptions built
  into the animations
• Lately I rewrote my animation system to not use a
  bind step

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What I do

• An animation player class handles animation
  playing
• One channel per animation on the same figure
  (so, 2 channels for upper/lower body)
• The player has a string hash table that keeps
  track of values, and maps each channels nodes to
  a central node array
• To animate an object, we map that objects node
  names through the hash table (every frame,
  potentially)

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Structuring node orderby dependency

• So you can compose a bunch of relative transforms
  into object-space transforms in one pass
• Only constraint is that any nodes parent has to
  have an index less than its own
• If you mix two arrays like this, it is easy to
  maintain this constraint
• So the players central animation array will be
  correctly ordered

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Mixing Animations

• Sometimes you want to play back two different
  animations on various nodes of the body
• Description of how this reduces data size and
  artist time
• This will produce inappropriate results for some
  actions
• Because the meanings of various rotations depend
  on whats higher up in the hierarchy
• Example Guy swinging a sword

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Mixing Animations

• Interpolate transforms relative to a common root
  between the two animations
• Put these into the relative transform slots
  during animation playback
• Or else interpolate in object space, but that
  might be inconvenient

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Brief discussion quaternion splines

• Very hard to precompute, not cheap to evaluate
  either
• Though Casey says you can do a de Casteljau-like
  formulation, using slerp instead of lerp
• I am not sure if this is the right answer but
  it seems to work in many cases
• So if lerp is not good enough, think about using
  this layered slerp technique

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Mixing IK withplaying animations

• Evaluate the animations first
• Solve IK given those object-space coordinates
• Mix the IK back in as though it were an animation
  channel

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Putting sound effectsin animation data

• Only robust way to generate sound cues at
  appropriate times!
• Put tag names in the animation these tags are
  later converted to sound effect names
• based on material contacted, etc
• (example of footsteps)

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Putting other auxiliary datain animations

• Example of attack strength in swordfighting
  game
• Advantage Easy for animator to change this
  without programmers, easy to keep in sync with
  animation
• Disadvantage Maybe animator should not be in
  charge of this might be better to have a
  separate specification system

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Instead of IKfor aiming things

• Blending between pre-generated animations
• (Example of aiming a gun)
• Can do an iterated search through the animation
  space until you find something close enough

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Reasons you still mightwant IK

• Example of two-handed weapon
• Shows why requiring a tree hierarchy is bad

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Facial Animation

• Probably should use morph targets, not a bone
  system
• Each facial expression is probably a short
  animation (not a frozen mask)

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Code Inspection
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Questions?