Rendering Caustics Using Photon Mapping and Ray Tracing

1
Rendering Caustics Using Photon Mapping and Ray
Tracing

• D. Narayan Brooks
• nbrooks_at_cse.ucsc.edu
• Computer Science Department,
• UC Santa Cruz
• CS 260 Professor Alex Pang
• May 14, 2002

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Introduction

• Approximating realistic lighting phenomenon in
  computer graphics scenes creates a more
  compelling and engrossing viewing experience
• The human eye is very adept at pinpointing visual
  discrepancies, especially involving lighting
• Lighting discrepancies can cause the viewer to
  lose their current suspension of disbelief when
  viewing computer generated scenes

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Introduction Continued

• The intricate play between lights and the 3D
  surfaces in a scene is a computationally
  intensive process
• The lighting process needs to be efficient while
  maintaining an acceptable depiction of the
  interaction between light and the 3D surfaces
• Caustics are an indirect lighting phenomenon that
  add realism to the scene
• Photon mapping attempts to efficiently capture
  lighting in general, and caustics in particular

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Background and Previous Work

• There has been a lot of research in global
  illumination
• The goal is to create realistic lighting
  depictions in complex scenes in the most
  efficient manner
• There have been many techniques and modifications
  of techniques to optimize and capture all
  different types of lighting

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Background and Previous Work

• Whitted ray tracing was introduced in the 80s as
  a more realistic approximation of light
  interacting with surfaces
• Backwards ray tracing was introduced later by
  Arvo to deal with shadows and indirect
  illumination
• Radiosity was developed as an alternative to the
  sharp images created by ray tracing
• Many hybrids of the two techniques have been
  developed to compensate for the failings of each
  technique
• Photon mapping is used by Jensen in 96 to
  achieve global illumination while capturing
  caustics

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Caustics

• What are caustics?
• Distinct areas of high light intensity much
  brighter than the neighboring area
• A result of indirect lighting, adding more
  realism to the scene
• Created by light refraction and specular
  reflection
• Refraction light bends when it encounters
  different mediums, focusing the light
  transmitted
• Specular reflection light reflects form
  specular surfaces and illuminates diffuse
  surfaces
• Caustics are generally encountered in scenes with
  liquids and glass objects

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Caustics
Image from Jensen98
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Caustics

• Image from Jensen96

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Global Illumination

• There are many techniques that have been used for
  global illumination
• Radiosity
• Ray tracing
• Hybrid methods
• Photon mapping

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Global Illumination

• Radiosity
• Spread color energy throughout the scene until
  a stasis point is reached
• Only handles diffuse reflection, no specular
  reflections
• Large memory consumption
• Very realistic soft illumination images can be
  produced

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Global Illumination

• Ray tracing
• Handles specular reflections well
• Recursive, which can be very time consuming
• Captures shadows, reflections, and refractions
• Creates images that have a distinct synthetic
  signature
• Two methods
• Forward from the eye (view dependent)
• Backwards - from the light source to the objects

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Global Illumination

• Hybrid methods
• Incorporate diffuse and specular components into
  lighting models as well as capture indirect
  illumination
• Suffer from large storage requirements and
  computation time
• Photon mapping
• An efficient hybrid method
• Combines photons (energy shooting) with ray
  tracing
• Achieves the different aspects of global
  illumination caustics shadows, reflections,
  refraction, and diffuse color
• Accounts for direct and indirect illumination

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Photon Mapping

• What are Photon maps?
• An efficient way to store light intensity on
  surfaces
• Created by emitting packets of energy (photons)
  Jensen96 from a light source using backwards
  ray tracing
• Store the photon when it hits a surface
• High resolution map used for caustics, lower
  resolution map for diffuse and secondary
  reflection approximation

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Photon Mapping - Photons

• What is a photon?
• Packet of energy emitted from a light source
• Travels in a ray from the light source
• Can be absorbed, reflected, or transmitted by a
  surface (using the Russian roulette method)
• Stored in the photon map when an intersection
  occurs
• Incoming flux is the density of photons stored on
  a surface
• Photon structure has position, energy, incoming
  direction and caustic/regular flag
• Photons are classified into direct and indirect
  illumination photons as well as shadow photons

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Photon Mapping Photon Classification

• Image courtesy of Jensen/Cooper98

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Photon Mapping Photon Maps

• Store photon density where photon intersects
  surface
• High resolution caustic map
• Use for direct visualization of caustics
• Stores photons that were originally shot at
  specular surfaces and then reflected/refracted to
  hit diffuse surfaces
• Lower resolution global photon map
• Approximates light/flux
• Stores photons emitted towards all objects
• Not visualized directly, used for optimizing
  shadow and recursive reflection rays

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Photon Mapping - Geometry

• Photon maps are not coupled with the underlying
  geometry
• If you can ray trace it, you can map it
  Jensen/Cooper98
• Very practical for storing information on
  implicit and other procedural surfaces (i.e.
  fractals)
• Can be extended to visualize volumes for
  participating media light transport simulation
  (clouds, god rays, smoke, etc.)
• Kept in 3D spatial data structure based on hit
  location

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Description

• Photon mapping is a two pass process
• Create the photon maps by light path tracing
• Render the scene using ray tracing optimized by
  the photon maps information

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Description

• Light Path Tracing (LPT)-
• Shoot photons from different positions on light,
  along rays to objects, and trace reflections and
  transmissions
• Each ray carries a fraction of the photon energy
• Deposit photon in map when it intersects a
  surface
• Determine if the photon is absorbed or reflected
  using Russian roulette
• If reflected, use the BRDF of the surface to find
  reflected direction

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Description

• Create a global and a caustic photon map by using
  LPT
• Only shoot to specular objects for caustic map
• Get shadow photons by only storing regular photon
  on first hit, storing shadow photons on
  subsequent hits along the ray
• Store photon maps in a balanced multi-dimensional
  search tree, the kd tree, based on hit location

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Description

• Find irradiance (color energy) of the surface by
  extending a sphere to radius r, where it will
  contain the N nearest photons
• Sum the energy from the N photons in the sphere
  and divide it by the approximated area
• Only use caustic photons for the caustic photon
  density estimate
• E(position x) SNei /pr2

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Description

• Finding N nearest photons in a sphere of radius
  r. Image from Jensen/Cooper98

r
x
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Description

• Use caustic map to visualize caustics directly
• Use global map to optimize shadow tracing by only
  tracing shadow rays from locations with some
  regular photons and some shadow photons
• Optimize reflections by using irradiance
  approximation in global photon map for secondary
  recursive reflections

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Rendering

• Render the scene using a two pass method
• Create global and caustic photon maps using
  backwards ray tracing
• Use the information stored in the photon maps for
  caustic visualization and to optimize the forward
  ray tracing
• Use the photon densities for irradiance (color)
  information

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Rendering First Pass

• Create photon maps
• Use backwards ray tracing to create caustic and
  global photon maps
• Emit photons from light to objects
• Store regular photons in photon map at first hit,
  and shadow photons at subsequent hits along the
  ray
• If photon is reflected or transmitted trace
  photon along new ray, recording hits in photon
  map
• View independent

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Rendering Second Pass

• Ray trace scene from the eye
• Use forward ray tracing methods such as Monte
  Carlo or other distributed ray tracers
• Use information stored in photon map to
• visualize caustics
• optimize shadow casting
• approximate secondary recursive reflections
• Importance sampling can be based on any
  bi-directional reflectance distribution function
  (BRDF)

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Conclusion

• Photon mapping is an efficient way to perform
  global illumination
• Photon maps deal with depicting direct and
  indirect illumination as well as diffuse and
  specular reflections
• Caustics are rendered using a specific caustic
  photon map
• Can create visually compelling scenes due to the
  lighting realism

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References

• Jensen95 H. Jensen, N. Christensen. Photon
  Maps in Bidirectional Monte Carlo Ray Tracing of
  Complex Objects. Computing and Graphics. Vol.
  19, No. 2, pp. 215-224, 1995, Elsevier Science
  Ltd., Great Britain.
• Jensen96 Henrik Wann Jensen. Global
  Illumination using Photon Maps. Rendering
  Techniques 96 Proceedings of the Seventh
  Eurographics Workshop on Rendering. pp. 21-30,
  1996, Springer-Verlag.
• Jensenr98 Henrik Wann Jensen and Per.
  Christensen, "Efficient Simulation of Light
  Transport in Scenes with Participating Media
  using Photon Maps" , Siggraph'98, pp. 311-320.
• Jensen/Cooper98 Presentation by S. Cooper based
  on "Efficient Simulation of Light Transport in
  Scenes with Participating Media using Photon
  Maps", 2000. http//www.cs.unc.edu/scooper/
  comp238/photonmaps.ppt