Computer Graphics Soft Body Animation Skinning

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Computer GraphicsSoft Body Animation - Skinning

• CO2409 Computer Graphics
• Week 22

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Lecture Contents

• Rigid Body vs Soft Body
• Skeletons
• Bone Influences / Vertex Weights
• Vertex Blending
• Technical Considerations

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Rigid Body Limitations

• Rigid body models are suitable for models made of
  distinct rigid parts
• We have looked at the example of a mechanical arm
• However, consider human joints
• When they bend, the body shape bends as well

• No distinct parts
• We cannot represent this with rigid bodies
• Or the pieces would separate, where there should
  be stretching or compression

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Soft Body Animation

• Such models are called soft-bodied
• Implying that the pieces of the model can stretch
  and flex
• Examples of soft body models
• Humans, animals, aliens and other creatures
• As well as other living organisms plants, trees
• Also clothing and material, rubber-like objects
  etc.
• This flexibility means there may not be a clear
  distinction between different pieces the geometry
• In the previous picture, the leg is a single
  piece of geometry, though it clearly has two
  parts
• How can we handle this?

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Skeletons (Bones)

• Note that these soft-bodied models appear to have
  hierarchies, much like rigid bodies
• So we can define an independent hierarchy of
  bones assumed to lie within the geometry

• This is called a skeleton
• Analogous to a human skeleton
• The movement of the bones drives the overlaid
  geometry
• Parts of the geometry bend and flex depending on
  the nearby bones

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Skeletons within Geometry

• The geometry itself is often a single part
• But it can have a hierarchy (different from the
  skeleton)

• So how to position the vertices as the bones
  move?
• Most vertices do follow the bones, in the
  example
• Dark blue follow the lower leg
• Purple remain with the upper
• But vertices at the joints are blended between
  positions
• Cyan area stretched between upper lower leg
  position
• Yellow area compressed

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Bone Influences / Vertex Weights

• More precisely, we consider the influence of each
  bone on each vertex
• The strength of the influence is a value from 0-1
• Varies for each vertex and is called the vertex
  weight
• Consider the influence of the lower leg bone

• The purple vertices get a vertex weight of 0
• Not affected by the lower leg
• The dark blue vertices get 1
• They follow the lower leg bone
• The shaded areas have weights ranging from 0 to 1
• Increasing closer to the lower leg

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Bone Influences / Vertex Weights

• For each vertex, the sum of the weights from all
  the influencing bones 1.0
• E.g. A kneecap vertex may have two influences, an
  0.2 weight from the upper leg, 0.8 from the lower
  leg
• In practice, most vertices are influenced by a
  small number of bones (around 1 to 4)

• Here, most of the leg vertices have one
  influence, the joints areas two
• Areas like the hips or shoulders may have more
  influences
• The bone influences and vertex weights are set up
  by the artists

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Vertex Blending

• Each bone in the skeleton hierarchy has a world
  matrix, just as with rigid bodies
• Again stored relative to the parent
• We can use a bones world matrix to transform the
  vertices it influences into world space
• But vertices are affected by multiple bones
• So calculate all the possible world space
  positions of a vertex, one for each bone that
  influences it
• Then linearly blend these world positions using
  the vertex weights from each bone
• Giving a final blended world position
• The maths follows

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Vertex Blending - Maths

• If a vertex V is influenced by N bones with world
  matrices M1 to MN and vertex weights W1 to WN,
  then the blended world position P is given by
• P VM1W1 VM2W2 VMNWN
• E.g. A kneecap vertex (N 2, W1 0.2, W2 0.8)
• Upper leg world pos VM1 (0.5, -1.5, 1.0)
  example
• Lower leg world pos VM2 (1.0, -1.0, 0.5)
• Then the final position is
• P (0.5, -1.5, 1.0) 0.2 (1.0, -1.0, 0.5)
  0.8
• (0.1, -0.3, 0.2) (0.8, -0.8, 0.5)
• (0.9, -1.1, 0.7)

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Overall Process

• To implement vertex blending for soft-bodies we
  need to
• Store a skeleton hierarchy with model geometry
• Store additional data for every vertex
• A list of influencing bones usually indices
  into the depth-sorted skeleton hierarchy
• An associated list of vertex weights (floats)
• Perform additional per-vertex processing
• Calculate a set of world positions using the
  world matrices from all influencing bones
• Blend these using the vertex weights for a final
  result

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Technical Considerations

• Using many matrices in a vertex shader is a
  burden
• Need to keep the number of bones per-vertex to a
  minimum
• Add this as a modelling constraint an upper
  limit
• But also need to keep the number of bones
  per-primitive down
• Possibly cut the geometry up into pieces that use
  a common set of bones

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Technical Considerations

• Soft body models require considerable extra
  detail per-vertex from the modellers
• Scope for modelling problems
• Often hidden until model animates
• Also soft-body objects may form part of a rigid
  body hierarchy
• In this case there will be two loosely related
  hierarchies - potentially overlapping tricky
• Can be tricky to import / export / pre-process
  such models
• Much scope for errors, technical and/or modelling
• Need very well tested conversion processes and
  tools