Picture Perception for Humans

1
Introduction to Global Illumination
Jack Tumblin CS 395 Advanced Computer
Graphics Winter 2003
2
Global Illumination

• Physical Simulation of Light Transport
• Accuracyaccount for ALL light pathsconservation
  of energy
• Predictionforward renderingcalculate light
  meter readings
• Analysisinverse rendering! find surface
  properties !
• Realism?perceptually necessary?

3
Local Illumination

• Everything is lit by Light Sources
• Screen color light source surface reflectance
• Refinements reflectance specular, diffuse,
  ambient, texture, light directshadow
  ambientenvironment maps,

4
Local Illumination

• Everything is lit by Light Sources
• Refine point light source ? Area light source
• Result? hard shadows ? soft shadows

5
Global Illumination

• Everything is lit by Everything Else
• Screen color entire scene surface reflectance
• Refinements Models of area light sources,
  caustics, soft-shadowing, fog/smoke,
  photometric calibration,

H. Rushmeier et al., SIGGRAPH98 Course 05 A
Basic Guide to Global Illumination
6
Global Illumination

• Idea ALL POSSIBLE PATHS of light source to eye

From Jensen et al., SIGGRAPH2000 Course 20 A
Practical Guide To Global Illumination Using
Photon Maps
7
Global Illumination

• Idea ALL POSSIBLE PATHS of light source to eye

From Jensen et al., SIGGRAPH2000 Course 20 A
Practical Guide To Global Illumination Using
Photon Maps
8
Limitations

• Geometric Optics Only
• All objects, apertures gtgt ? (wavelength)
• YES Reflection, Refraction, Scattering
• No fringes, diffraction, dispersion (see
  movie)
• Point-Based BRDF (see Wann-Jensen et
  al.SIGGRAPH2001

9
Summary I

• Big Ideas
• Measure Light Radiance
• Measure Light Attenuation BRDF
• Light will bounce around endlessly, decaying on
  each bounce The Rendering Equation
  (intractable must
  approximate)

10
Review Surface Properties

• Perfectly Specular
• Mirror
• infinite gloss
• Phong Specular Model
• L R cos?(?)

Incident LightRay
SurfaceNormal
ReflectedLight
?
Andrew Glassner et al.. SIGGRAPH94 Course
18 Fundamentals and Overview of Computer
Graphics
11
Review Surface Properties

• Slightly scattered Specular
• high gloss
• Phong Specular Model
• L R cos15(?)

Incident LightRay
SurfaceNormal
ReflectedLight
Andrew Glassner et al.. SIGGRAPH94 Course
18 Fundamentals and Overview of Computer
Graphics
12
Review Surface Properties

• More Scattered Specular
• medium gloss
• Phong Specular Model
• L R cos5(?)

Incident LightRay
SurfaceNormal
Andrew Glassner et al.. SIGGRAPH94 Course
18 Fundamentals and Overview of Computer
Graphics
13
Review Surface Properties

• Perfectly Diffuse
• flat, chalky,

Incident LightRay
SurfaceNormal
Andrew Glassner et al.. SIGGRAPH94 Course
18 Fundamentals and Overview of Computer
Graphics
14
Review Surface Properties

• Most Materials
• Combination of
• Diffuse and Specular

Incident LightRay
SurfaceNormal
Andrew Glassner et al.. SIGGRAPH94 Course
18 Fundamentals and Overview of Computer
Graphics
15
Point-wise Reflectance BRDF

• Bidirectional Reflectance Distribution Function
• ?(?i , ?i , ?r , ?r , ?i , ?r , ) (Lr
  / Li) a scalar

Illuminant Li
Reflected Lr
Infinitesimal Solid Angle
?
?
16
Point-wise Light Radiance L

• Radiance The Pointwise Measure of Light
• Free-space light power L (energy/time)
• At least a 5D scalar function L(x, y, z, ?, ?,
  )
• Position (x,y,z), Angle (?,?) and more (t, ?, )
• Power density units, but tricky

17
Radiance Units

• Tricky think Hemispheres
• with a floor

Solid Angle (steradians) dS fraction of
a hemispheres area (4?)
Projected Area
cos ? dA
?
dA
dA
18
Rendering Equation
(Kajiya 1986)

• .

Radiance from point
Radiance emitted from point
Radiance reflected from point (from all inward
directions)
19
Rendering Equation

• Opportunities
• Scalar operations only ?() and L(), indep. of ?,
  x,y,z, ?,?
• Linearity
• Solution weighted sum of one-light solns.
• Many BRDFs ? weighted sum of diffuse, specular,
  gloss terms
• SIGGRAPH2001 Result reflected light
  convolution(Lin, ?)
• Difficulties
• Almost no notrivial analytic solutions exist
  MUST use approximate methods to solve
• Verification tough to measure real-world ?() and
  L() well
• Notable wavelength-dependent surfaces exist
  (iridescent insect wings casing, CD grooves)
• BRDF doesnt capture important subsurface
  scattering

20
Implementation I

• Practical Approximations
• Diffuse-only reflectance Radiosity Solution
• Book presents old, slow, exact Gauss-Seidel
• Bounce-by-Bounce Progressive Refinement, Path
  Tracing
• Object-space Storage Adaptive Meshing

21
Remeshing Example
22
Progressive Radiosity
23
Implementation II

• Practical Approximations
• From Both Ends Bi-directional Tracing,
• Trace from light to surfaces store result, then
• Trace from eye to surfaces
• Scattering Rays where needed
• Monte-Carlo Methods,
• Distributed Ray Tracing
• Hybrids
• Numerical Methods (Galerkin, etc.),
• Photon Maps,
• Metropolis Transport,
• Particles, Illumination caching,
• 4D light volume sampling

24
Example Photon Maps

• Ideal Trace Photon Paths
• Trouble high compute costs (exponential)
• Photon Maps A Hybrid Solution
• big, sticky, aggregate photons
• Russian Roulette (reflect, transmit, absorb?)
• Trace photons outwards from light sources
• Store photons only at diffuse surfaces
• Scattered data interp.,
• Cache photons/illum. at each step.

25
Example Photon Maps

• Forward-traced Reverse-TracedPhoton
  Map Result

26
Photon Map Result

• .

27
Conclusion

• Physically accurate (geometric optics only)
  simulation of light transport.
• Ultimate Realism? perceptual, not physical
• Languished as tweak-hungry lab curiosity
• Gradual adoption for multitexturing source, for
  mixing real/synthetic images, Ph.Ds,
  theatre/architectural lighting, archaeology,
• Growing interest for use in inverse rendering
  tasks image-based rendering modeling