Using Interactive Ray Tracing for Interactive Global Illumination

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Using Interactive Ray Tracing forInteractive
Global Illumination
Computer Graphics Lab Saarland University,
Germany http//graphics.cs.uni-sb.de
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Interactive Global Illumination - Goals

• Support for all relevant features
• Complete direct indirect diffuse illumination
• Specular Effects Refractions, Reflections,
  Caustics
• Be independent of geometry (scalability) !
• Interactive gt 1 fps at video resolution
  (640x480)
• Full recomputation every frame
• Lights, Materials, Geometry, Camera,
• No approximative or image-based techniques
• Interactive, user-controllable speed/quality
  tradeoff
• E.g. trade some flickering or aliasing for speed
  
• Automatic and progressive convergence to
  high-quality solution when possible (e.g within
  2-5 sec)

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Interactive Global Illumination - Goals
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Interactive Global Illumination

• Previous solutions
• Image-Based approaches
• Artifacts
• Radiosity OpenGL
• Missing specularities, Meshing artifacts,
• High-quality Global Illumination uses ray tracing
  anyway
• Ray Tracing is now interactive

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Interactive Global Illumination

• Previous solutions
• Image-Based approaches
• Artifacts
• Radiosity OpenGL
• Missing specularities, Meshing artifacts,
• High-quality Global Illumination uses ray tracing
  anyway
• Ray Tracing is now interactive

Simplisitic Idea Interactive ray
tracingRay-traced global illumination
Interactive global illumination
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Interactive Global Illumination

• But Real World isnt simplistic
• Problem GlobIllum often doesnt scale
  (shared-memory)
• Radiosity
• Shared global data for radiosities,
  geometry-dependency
• Photon Map
• KD-Tree reconstruction every frame, global photon
  map data,
• Have to live with extremely low sample rates
• Even 16 MRays/sec is only 16 rays/pixel at
  640x480x3Hz
• Path-Tracing and Bidirectional Path-Tracing too
  noisy
• ? Need another approach

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Instant Global Illumination

• Our approach Instant Global Illumination
• Done in cooperation with Kaiserslautern
  University(Alex Keller and Thomas Kollig)
• Most of the algorithms are based on their work !
• Idea Combine advantages of several, different
  algorithms
• Instant Radiosity smooth diffuse lighting
• Ray Tracing reflections, refractions, visibility
  testing
• Interleaved Sampling (ILS) better quality, easy
  to parallelize
• Discontinuity Buffer removes ILS artifacts
• Caustic Photon Mapping the only way for sensible
  caustics

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Basic Ingredient Instant Radiosity Keller

• Trace few (10-20) particles from (area) light
  sources
• Use these to light the scene as virtual point
  lights (VPLs)
• Contains both direct and indirect diffuse
  illumination
• Shadows originally with OpenGL, now with Ray
  Tracing
• Raytracing gives reflections/refractions for free
• Inherently smooth, since radiosity-based
• Artifacts for few VPLs
• Converges, but at only 10-20 VPLs clearly
  visible
• Plus Add PhotonMapfor Caustics
• Hack for faster query

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Too few samples?Add Interleaved Sampling

• Use different sets of virtual point lights for
  diff. Pixels
• Every SxSth pixel uses same set of VPLs and
  caustic photons
• Recomputing VPLs and photons for every pixel too
  costly
• Better quality
• 9 times as many VPLsper image than without
• Same for photons !
• Still cheap pixels !
• Easily parallelizable
• Each CPU needs only 1 set? Scales with CPUs !!!
• Aliasing
• Can see SxS grid

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ILS Aliasing ? Add Filtering

• Discontinuity Buffer Keller, Kollig
• Filter irradiances of neighbouring pixels
• Smoothing/removal of ILS-artifacts
• Like irradiance caching, but more stable
• Only filter in smooth regions, detect
  discontinuities
• Criterion normal distance
• Problem Clients dont have neighboring pixels !
• Server has to filter
• ?High server load
• Server has to get required data
• Normal, irradiance, distance
• High network bandwidth !

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Finally, add QMC

• Use Randomized Quasi Monte Carlo Keller et al
• Much faster convergence, especially for such
  small sample rates !
• Can be combined easily with Interleaved sampling
• Plus Technical advantages of QMC
• Fast random number generation (table lookup
  bit-ops)
• Can reproduce any sequence of samples based on
  single seed value
• Can easily synchronize different clients on same
  data
• Each client can easily reproduce the sample set
  of any other client
• Avoid jumping of VPLs
• Just start with same seed every frame
• For progressive convergence, just advance the
  seed value
• QMC sequences perfectly combine into the future

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Summary

• Base ingredient
• Instant Radiosity Ray Tracing
• Plus fast caustic photon maps
• Combine with Interleaved Sampling
• Better quality
• Parallelizable
• Remove ILS artifacts with Disco-Buffer
• Faster convergence
• Better parallelizability
• Use randomized QMC
• Low sampling rates, parallelizability
• Result Definitely not perfect
• But not too bad for only 20 rays/pixel !

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Parallelization Scalability

• Adapt Scheduler to ILS
• Different clients work on different sets
• Same client preferably works on same set
• ? In theory, each client only has to compute one
  sample set
• In practice, its still about only 1.5-2 out of 9
• Very small overhead (few redundant operations)
• But Server Bottleneck due to Disco-Buffer
• Filtering cost and Network bandwidth limit max.
  framerate
• Currently On dual-Athlon 1800 max. framerate
  of 5 fps..
• But Can still scale in quality
• Twice CPUs Twice VPLs/pixel at same framerate
  !
• ? Limited only by max. framerate, not by number
  of clients !

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Results

• Up to 5 fps at video resolution (640x480)
• With 8 dual Athlon MP 1800
• All targeted effects
• Reflections, refractions and caustics
• Smooth direct and indirect diffuse illumination
• Everything recomputed every frame
• No limitation on interaction types
• All parameters can be changed interactively
• Sampling rate, photon query radius, filter size,
  
• Automatic convergence to high-quality solution
• high good quality in about 2-5 sec.

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Current Research

• Have implemented a new improved system from
  scratch
• Main Features
• 2-3 times faster through ray packet traversal and
  streaming architecture
• Removed photon shooting
• Support for accumulation on client side,
  necessary for providing high quality during
  interaction and animation
• Nearly linear scalability
• up to 22 fps at 640x480 with reduced quality
• Sub-linear costs in image resolution
• Fully programmable shading

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Example Images
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Remaining Problems Future Work

• Still to slow (its always too slow)
• Incorporate glossyness/arbitrary BRDFs
• Emission properties for light sources
• Caustics
• Main Limitation Inefficient in highly occluded
  scenes !
• Were working on it

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Questions ?
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Missing Caustics ?Add Caustic Photon Map

• Basically just like standard photon mapping
  Jensen
• Shoot directly at caustic generators
• But too slow for our purpose
  (Cost(Query)10Rays)
• Fix query radius to a fixed R
• Can be interactively changed by the user
• Store photons in grid with width 2R
• Have to touch only 8 voxels for each query !
• Can use hashing to keep reduce storage cost
• Fast, but not perfect
• Can afford only few photons(reconstruction cost
  too high)
• Looks bad if R is chosen bad