Supporting Animation and Interaction

1
Supporting Animationand Interaction

• Ingo Wald
• SCI Institute, University of Utah
• wald_at_openrt.de

2
Supporting Animation and Interaction

• Motivation
• Types of Animations
• Alternative Approaches
• Animated Scenes in OpenRT
• Practical Examples

3
The Need for Dynamic Scenes

• Part I How to achieve maximum performance ?
• Fast ray-surface intersection
• CPU-Friendly implementation
• AND High-quality acceleration data structures
  (ADS).
• Problem Building ADS takes time
• Up to minutes for realistically complex models
  (1MTri)
• For Walkthrough applications Not a problem
• Build once (preprocess), reuse in following
  frames
• BUT Cant change geometry

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The Need for Dynamic Scenes

• Depend on Precomputed, high-quality ADS ? Cant
  change geometry
• Probably (one of) the most important problems in
  ray tracing today !

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The Need for Dynamic Scenes

• Static Geometry Still many practical
  applications
• Architecture Global illumination walkthroughs
• Engineering Visualizing massively complex
  models(usually not animated, anyway)
• Industry Interactive design reviews (only
  inspect/review existing static model)
• Simulation of reflection/refraction in
  glass/headlights/
• Visualizing complete cars/planes/ with HQ shading

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The Need for Dynamic Scenes

• Static Geometry Already many practical
  applications
• Design reviews, massive engineering models,
  architecture, lighting/reflection simulation
• But Not applicable to todays typical
  low-end/mainstream use of graphics
• Games (the main driving force of 3D graphics!)
• Cant do even simple editing (switching variants,
  moving parts,)
• Future scenario RTRT as mainstream 3D graphics ?
• MUST offer ability to interact with model
• If only for games

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Kinds of Dynamic Scenes

• Need to differentiate between different kinds of
  dynamics
• Hierarchical Animation
• Typical application Scene graphs (VRML)
• Deformable Objects
• Relatively small, but non-hierarchical
  deformations of a mesh
• Typical application Skinning
• Mostly hierarchical Skeleton skinning
• Unstructured Motion
• Typical application Brownian motion, explosions,
  
• Timestepped Animation
• One out of k discrete poses per frame (game
  characters)

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Ray Tracing Dynamic ScenesFirst approaches

• Alternative 1 Handle dynamic objects separately
• Already used in 98/99 Parker
• Keep dynamic objects out of ADS, intersect
  separately
• Only works for very few dynamic objects
• Alternative 2 Rebuild ADS every frame
• (Super-)linear in objects
• Only applicable to tiny scenes
• Note This might change for future HW

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Ray Tracing Dynamic ScenesFirst approaches

• Alternative 3 Progressively build ADS
• Motivation Often only parts of scene visible
• Might only need small part of ADS
• Build hierarchical ADS lazily and progressively
• E.g., split node only once ray reaches that node
• Problem
• Even initial split (for kd-tree) already O(N) ?
  Too costly already
• Not really used in practice

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Ray Tracing Dynamic ScenesFirst approaches

• Alternative 4 Update ADS for dynamic objects
• Problem 1 How to avoid degradation of ADS ?
• Problem 2 How to efficiently update ADS ?
• Kd-trees Quite problematic
• Original cost estimates (SAH) wrong/useless after
  geom. changes
• Updating often not (easily) possible
• E.g., cant move root split would need to
  rebuild all sibling nodes
• Similar for other hierarchical ADS
• But works reasonably well for grids Reinhard01

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Reinhards Approach Dynamically update virtual
Grid

• Basic Idea Use (regular) grid, update
  dynamically
• Efficient updates Maintain different grid
  resolutions
• Large primitives on coarse levels, small
  primitives on fine levels
• Each prim. Covers few cells ? remove/add/move
  primitive in O(1)
• Avoiding degeneracy Grid wraps around
• Objects moving out of right side re-enter on left
  side
• No rebuild required even if scenes bounds
  changes
• Same for rays (slightly different traversal)
• Still Rebuild from scratch every few frames...

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Reinhards Approach Dynamically update virtual
Grid

• Results Works well for many scenes
• But Only works for grid-like ADS
• Traversal performance relatively low
• Problematic for large scenes with varying
  primitive distribution
• Does not easily generalize to more efficient ADS
• Plus quite problematic for hierarchical
  animation
• E.g., slight rotation of scene costs O(N) update

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Dynamic Scenes in OpenRT

• Main observations
• Prefer kd-trees for high performance and complex
  scenes
• Problematic for update/rebuild operations
• But interactive rebuild possible for few dynamic
  objects
• Update/rebuild approaches problematic for
  hierarchical animation (small change triggers
  complete rebuild)
• Most practical scenes use (mostly) hierarchical
  animation
• Scene changes often localized
• In particular Unstructured motion often very
  localized(e.g., explosion in complete game
  level...

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Dynamic Scenes in OpenRTHierarchical Scene
Organization

• OpenRT approach PGV 2003
• Application groups geometry into objects
• Wrt same properties concerning dynamic updates
• Similar to building display lists
• Each object has its own acceleration structure
• Objects can be re-used in subsequent frames
• No rebuild necessary
• Application can redefine object(s) any time

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Dynamic Scenes in OpenRTHierarchical Scene
Organization

• Objects are instantiated into the scene
• Each instance has a transformation attached to it
• Hierarchical Animation Inversely transform rays
  instead of objects
• Can keep object itself static
• Only need to update instance transformation
• Instances are organized in additional hierarchy
  level
• With its own acceleration structure (kd-tree of
  instances)
• Only this has to be rebuilt every frame
• Rebuild complete from scratch

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Dynamic Scenes in OpenRTHierarchical Scene
Organization

• Special Case Timestepped Animation
• Can be realized quite simply
• Build one object for every pose
• All kd-trees built in advance
• Hide all but current poses instance
• Simply switch instance per frame, no rebuild at
  all.
• Sufficient for many game-like applications
• Extension to (simple) skinning possible
• No details here (not yet published)

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Dynamic Scenes in OpenRTUnstructured Motion

• Support for unstructured motion
• Build special object for dynamic triangles
• Rebuild (only) this object ever frame
• Rebuild tolerable for few (1k-10k) objects
• Use cheap, low-quality kd-trees for these objects
• Trade objects traversal performance for
  construction time
• Can have multiple dynamic objects
• Problem Parallelization framework
• Need to send each dynamic triangle to each client
• Huge bandwidth required for many clients and dyn.
  triangles

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Dynamic Scenes in OpenRTSummary

• Two-level object/instance approach
• Transform rays, not objects, plus top-level
  kd-tree
• Status
• Limited support for unstructured motion
• Perfect for hierarchical animation
• (up to few 1000 instances, tris less important)
• Timestepped animation works as well

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Results Hierarchical Animation

• Hierarchical Animation Perfect
• Rebuild for lt10,000 instances Not a problem at
  all
• Standard OpenRT feature Used in almost any
  application
• Example BART Benchmark (images from 2002)

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Results Multiple Instantiation

• Side Effect Multiple Instantiation is for free !
• Different instances simply share same geometry
• Example Forest (150,000 instances, 1.5
  billion triangles)
• Dietrich, EGWNP05

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Results Unstructured Motion

• Unstructured Motion Possible, but costly
• Rebuild for lt10,000 instances Tolerable, but
  costly
• Main bottleneck Network bandwidth
• Need to transfer each updated triangle to each
  client in turn
• Not worked on since 02/03 Few practical
  applications

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Results Practical Applications

• Totally sufficient for typical VR
  applications(simple editing and variant
  switching)
• Note Cost only depends on instance, not on
  triangles
• UNC Powerplant (12.5 MTri), single PC

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Game Example 1 Quake3/RT(Joerg Schmittler,
Daniel Pohl)

• Quake3 bots/scenes, self-written game engine
• Fully ray traced (using OpenRT API)
• Dynamic bots, monsters timestepped animation
• Plus Lots of small moving objects, missiles,
  cartridges,

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Game Example 2 Oasen(Tim Dahmen et al.)

• Completely ray traced magic carpet clone
• Special emphasis on image quality
• Many lights, underwater caustics, atmospheric
  effects, water,
• Highly complex geometry (huge terrain, real
  trees, )
• With multiple instantiation
• Animated carpet timestepped animation plus
  hierarchical transf.

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Handling Dynamic ScenesSummary and Conclusions

• What to take home from this talk
• OpenRTs two-level approach can already do a lot
• and its easy to implement as well ? Try it !
• Research on dynamic RT important for tomorrows
  graphics
• Still too few attention to that problem