Haptic Rendering

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Haptic Rendering

• Max Smolens
• COMP 259
• March 26, 2003

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What is haptics?

• Using the sense of touch to interact with
  computers and virtual environments

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What is haptic rendering?

• The process of computing and generating forces in
  response to use interactions with virtual objects

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Why use haptics?

• Increases the information flow between the
  computer and the user
• Intrinsically bilateral
• When we push on an object, it pushes back on us

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Why use haptics? (2)

• Our sensing of forces is closely tied to our
  visual system and sense of three-dimensional
  space
• Information and intent can be conveyed in a
  physically direct and primal way

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Haptic Applications

• Medicine
• Surgical simulators for training
• Manipulating robots for minimally invasive
  surgery
• Telemedicine, remote diagnosis
• Accessibility for the disabled

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Haptic Applications (2)

• Entertainment
• Video games, simulators that enable the user to
  feel and manipulate objects in the environment
• Education
• Feel phenomena at a variety of spatial and
  temporal scales
• Studying complex data sets

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Haptic Applications (3)

• Industry
• CAD systems
• Virtual prototyping
• Assembly and disassembly can guide final design
• Shape sculpting
• Expressive, free-form shape generation and
  modification

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Haptic Applications (4)

• The arts
• Virtual painting, sculpting
• Virtual musical instruments

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Haptic interaction
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Human haptics

• Mechanical, sensory, motor and cognitive
  components
• Two classes of sensory information
• Tactile
• Kinesthetic

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Human haptics (2)

• Tactile information
• From skin in contact with an object
• Spatial and temporal variations of forces within
  the contact region
• Slipping, fine textures, small shapes, and
  softness

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Human haptics (3)

• Kinesthetic information
• Net forces along with position and motion of
  limbs
• Coarse properties of object
• Large shapes, spring-like compliances

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Human haptics (4)

• Kinesthetic resolution
• 2 degrees for fingers and wrist
• 1 degree for shoulder
• Force exerted by a finger
• 50 to 100 N maximum
• 5-15 N typically during exploration and
  manipulation

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Haptic interfaces
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What makes a good interface?

• Must work with human abilities and limitations
• Approximations of real-world haptic interactions
  determined by limits of human performance

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A good haptic interface

• Free motion must feel free
• Low back-drive inertia and friction
• No motion constraints
• Ergonomics and comfort
• Pain, discomfort and fatigue will detract from
  the experience

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A good haptic interface (2)

• Suitable range, resolution and bandwidth
• User should not be able to go through rigid
  objects by exceeding force range
• No unintended vibrations
• Solid objects must feel stiff

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Haptic rendering

• Two parts collision detection, response

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Two types of interactions

• Point-based haptic interactions
• Only end point of device, or haptic interface
  point (HIP), interacts with virtual object
• When moved, collision detection algorithm checks
  to see if the end point is inside the virtual
  object
• Depth calculated as distance between HIP and
  closest surface point

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Two types of interactions (2)

• Ray-based haptic interactions
• Probe of haptic device modeled as a line-segment
  whose orientation matters
• Can touch multiple objects simultaneously
• Torque interactions

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Collision detection

• Detect collisions between haptic probe and
  virtual objects
• Bounding volume hierarchies, spatial partitioning
• H-COLLIDE, hybrid technique
• Partition virtual workspace as uniform grid
• For each grid cell containing primitives,
  computes OBBTrees

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Simple collision response

• Haptic rendering of 3D sphere

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Simple collision response (2)

• Reaction force calculated using the linear spring
  law Fkx
• k stiffness of object
• x depth of penetration
• Direction of force along surface normal

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Penalty methods

• Subdivide object and associate each subvolume
  with a surface
• Determine feedback force directly from
  penetration
• Works well for simple geometric shapes

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Penalty methods (2)

• There are some problems
• Two possible paths to reach same location, which
  path was taken?

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Penalty methods (3)

• Force summation for multiple objects
• Compute net force by adding
• Correct for perpendicular surfaces
• For obtuse angle, force vector becomes too large
• When almost parallel, force vector too large by a
  factor of 2

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Penalty methods (4)

• Problems with thin objects
• If pushed halfway through an object, will be
  pulled through the rest of the way

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Solution? God-object

• Zilles, Salisbury (1995)
• Cannot stop HIP from penetrating virtual objects
• Define additional variables to represent the
  virtual location of the haptic interface
  (god-object, IHIP, proxy)

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God-object (2)

• In free space, HIP and IHIP are collocated
• When HIP moves into an object, the IHIP remains
  on the surface
• IHIP computed such that its distance from the HIP
  is minimized
• Correct force vector is unambiguous

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God-object (3)

• Infinite surface
• Active if the old IHIP is a positive distance
  from the surface and the HIP is a negative
  distance from the surface
• Finite extent
• If a line traced from the old IHIP to new HIP
  passes through the facet, then consider the facet
  active

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God-object (4)

• When touching convex portion of an object, only
  one surface should be active at a time

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God-object (5)

• When touching concave portion of an object,
  multiple surfaces can be active
• 2 surfaces constrain IHIP to a line
• 3 surfaces constrain IHIP to a point
• IHIP might cross another surface before HIP
• Solution iterate the process, until no new
  constraints found

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God-object (6)

• Location computation using Lagrange multipliers
• x, y, z coordinates of IHIP
• xp, yp, zp coordinates of HIP
• Constraints added as planes

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God-object (7)

• Minimize L by setting its six partial derivatives
  equal to 0, solvable with 65 multiplies and
  divides

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Rendering surface details

• Smoothing
• Friction
• Textures

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Force shading

• Render objects as smooth and continuous, even if
  underlying representation is not
• Compute force vector for each vertex, interpolate
  over polygonal surfaces (like Phong shading)

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Surface friction

• Without friction, virtual objects feel
  icy-smooth
• Coulomb friction sticking and sliding
• Forces tangential to surface, direction opposite
  of motion

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Haptic texturing

• Force perturbation
• Modify the direction and magnitude of the force
  vector
• Max and Becker (1994)

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Haptic texturing (2)

• Image-based
• Construct texture field from 2D image data
• Map heights onto the object surface
• Procedural
• Generate synthetic texture fields using
  mathematical functions

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Haptic texturing (3)
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Challenges

• Graphics update rate must be between 20-30 Hz
• Haptic update rate must be around 1kHz
• Decouple simulation and haptic loops using
  multiple processors or a dedicated machine

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6-DOF haptics challenges

• Detect all surface contact instead of just at a
  single point
• Calculate a reaction force and torque at every
  point or region of contact
• Maintain the 1kHz refresh rate

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Examples
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References

• Basdogan, C., Srinivasan, M.A. Haptic rendering
  in virtual environments. http//network.ku.edu.tr
  /cbasdogan/-Papers/VRbookChapter.pdf
• Chen, E. Six degree-of-freedom haptic system for
  desktop virtual prototyping applications. Proc.
  First International Workshop on Virtual Reality
  and Prototyping, p. 97-106, 1999.
• Gregory, A., Lin, M. , Gottschalk, S. and Taylor,
  R. A Framework for Fast and Accurate Collision
  Detection for Haptic Interaction. Proc. of the
  IEEE Virtual Reality (VR 99), p. 38-45, 1999.
• Mark, W. et al. Adding force feedback to
  graphics systems issues and solutions. Proc.
  ACM SIGGRAPH 1996.
• Massie, Thomas H. and Kenneth Salisbury. The
  PHANTOM haptic interface a device for probing
  virtual objects. Proc ASME Symposium on Haptic
  Interfaces for Virtual Environment and
  Teleoperator Systems, 1994.
• McNeely, W., Puterbaugh K., and Troy, J. Six
  degree-of-freedom haptic rendering using voxel
  sampling. Proc. ACM SIGGRAPH 1999.

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References (2)

• Ruspini, Kolarov and Khatib. The haptic display
  of complex graphical environments. Proc. ACM
  SIGGRAPH 1997.
• Salisbury, J.K. et al. Haptic rendering
  programming touch interaction with virtual
  objects. Proc. ACM SIGGRAPH 1995.
• Salisbury, J.K. and Srinivasan, M.A.
  Phantom-based haptic interaction with virtual
  objects. IEEE Computer Graphics and
  Applications, 17(5), p. 6-10.
• Salisbury, J.K. Making graphics physically
  tangible. Communications of the ACM, 42(8), p.
  74-81.
• Srinivasan, M.A. and Basdogan, C. Haptics in
  virtual environments taxonomy, research status,
  and challenges. Computers Graphics, 21(4), p.
  393-404.
• Zilles, C.B. and Salisbury, J.K. A
  constraint-based god-object method for haptic
  display. Proc. IEE/RSJ International Conference
  on IntelligentRobots and Systems, Human Robot
  Interaction, and Cooperative Robots, Vol. 3, p.
  146-151, 1995.