CS 39549525: Spring 2003

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CS 395/495-25 Spring 2003

• IBMR Week 9B
• Image-Based Physics
• Measuring Light Materials
• Jack Tumblin
• jet_at_cs.northwestern.edu

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Reminders

• ProjA graded Good Job! 90,95, 110
• ProjB graded Good! minor H confusions.
• MidTerm graded novel solutions encouraged.
• ProjC due Friday, May 16 grading done.
• ProjD posted, due Friday May 30
• Take-Home Final Exam Assign on Thurs June 5,
  due June 11

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IBMR Measure,Create, Modify Light

• How can we measure rays of light? Light
  Sources? Scattered rays? etc.

Cameras capture subset of these rays.
Shape, Position, Movement,
Emitted Light
Reflected, Scattered, Light
BRDF, Texture, Scattering
Digital light sources (Projectors) can produce a
subset of these rays.
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Full 8-D Light Field (10-D, actually time, ?)

• Cleaner Formulation
• Orthographic camera,
• positioned on sphere around object/scene
• Orthographic projector,
• positioned on spherearound object/scene
• (and wavelength and time)
• F(xc,yc,?c,?c,xl,yl ?l,?l, ?, t)

camera
projector
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Light Measurement Terms

• Flux W power, Watts, photons/sec
• Irradiance E Watts/area dW/dA
• Radiance L (Watts/area)/sr (dW/dA)/sr
• BRDF Measure EMITTED radiance that results from
  INCOMING irradiance from just one directionBRDF
  Fr Le / Ei (Watts/area) /
  (Watts/area?sr)

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IBMR Tools

• Digital Light Input
• Light meter measure visible irradiance E
• Camera pixels measure Radiance Li flux
  arriving at lens from one (narrow solid) angle
• Digital Light Output
• Luminaires point lights, extended(area) sources
• Emissive Surfaces CRT, LCD surface
• Projectors laser dot,stripe,scan video display
• Light Modifiers (Digital?)
• Calibration objects, shadow sources, etc.
• Lenses,diffusers, filters, reflectors,
  collimators...
• ?Where are the BRDF displays / printers?

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Whats wrong with Images?
What We Want
What We Get
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ProblemMap Scene to Display
Domain of Human Vision from 10-6 to 108 cd/m2
starlight
moonlight
office light
daylight
flashbulb
10-6
10-2
1
10
100
104
108
??
??
0 255
Range of Typical Displays from 1 to 100 cd/m2
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High-Contrast Image Capture?

• An open problem! (esp. for video...)
• Direct (expensive) solution
• Flying Spot Radiometer brute force instrument,
  costly, slow, delicate
• Novel Image Sensors line-scan cameras,
  logarithmic CMOS circuits, cooled detectors,
  rate-based detectors...
• Most widely used idea multiple exposures
• Elegant paper (Debevec1996) describes how

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Use Overlapped Exposure Values
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Use Overlapped Exposure Values
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Use Overlapped Exposure Values
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Use Overlapped Exposure Values
?
What is the camera response curve? What are the
pixel intensities?
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Use Overlapped Exposure Values

• Its a null-space problem!
• Input intensities unknown vector x
• Film response R(x) known film value vector b
• Multiple, Aligned, known Exposures
• R0(x20) b0
• R1(x21) b1
• R2(x22) b2
• R3(x23) b3...
• Rearrange, stack, solve with SVD. (see paper)

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Camera Abilities / Limitations

• Nonlinear Intensity Response S-shaped
• Low-Contrast Devices Noise limited
• Varied Spectral Response RGB1 ! RGB2...
• Color Sensing Strategies
• 3-chip cameras best, but expensive!
• Mosaic sensor trades resolution for color
• Nonuniform sensitivity geometry
• Lens limitations (vignetting, radial distortion,
  bloom/scatter, uneven focus, ...)
• CCD Sensor geometry VERY exact, repeatable

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Display Abilities / Limitations

• Nonlinear Intensity Response r-shaped
• Low-Contrast Devices
• scattering usually sets upper bounds
• Best Contrast laser projectors, some DLP
  devices, specialized devices...)
• Varied Spectral Response RGB1 ! RGB2...
• Color Reproducing Strategies varied...
• Nonuniform sensitivity geometry
• CRTs e-beam cos(?), distortion, focus,
  convergence...
• LCDs, DLPs VERY exact, (but pixels die, etc.)

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Light Modifiers? ? ? Discuss!

• Low-Contrast BRDF Devices to measure light?
• Light Probe mirror sphere BRDF ?
• Diffuse reflectances limited to about 0.02?0.95
• Diffractive materials complex BRDF may be
  useful...
• (Transmissive LCDs?) ?Can you name more?
• PRECISELY Linear Response to light... BRDFs
  are fixed ratios no intensity dependence!
• Smudge, nick may modify BRDF drastically
• Shadows? Precision? Inter-reflections?
• PRECISE input/output symmetry
• --BUT--
• Scattering WITHIN material can be trouble...

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What is the complete IBMR toolset?

• Camera(s) light probe, etc ? arbitrary
  Radiance meter.
• Sphere of Projectors/CRTs ? arbitrary
  Irradiance source.
• Some (as yet unknown) device ? arbitrary BRDF /
  light ray modifier
• Is our toolset complete complete?
• have we spanned the IBMR problem? ...

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Missing the most important tool

• Human Visual System.
• the receiver/user for MOST IBMR data.
• Eye is a very poor light meter, but very good at
  sensing BRDF and (some) shape.
• Eye senses change integration used to estimate
  the world
• Eye permits tradeoffs of geometry vs. surface
  appearance
• Eye permits selective radiance distortions,
  especially to illumination

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Picture Copy Appearance
Details Everywhere segmented partial -ordering
of intensities. Local changes matter. Absolute
intensities dont matter much, but boundaries,
shading, CHANGES do. ---WANTED--- visually
important information in machine-readable form.
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Two Big Missing Pieces

• Computer controlled BRDF.
• Can we really do without it?
• are cameras and projectors enough to import the
  visible world into our computers?
• BRDF is not enough
• Subsurface scattering is crucial aspect of
  photod images
• ? how can we model it? measure it? use it?

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Scattering Difficulties

• For many surfaces, single-point BRDFs do not
  exist

Angles Depend on refractive index, scattering,
cell wall structures,
Depends on total area of cell wall interfaces
Dicotyledon leaf structure
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Subsurface Scattering Models

• Classical Kubelka-Monk(1930s, for paint many
  proprietary variants),
• CG approach Hanrahan Krueger(1990s)
• Recent (2001, Jensen)

Marble BSSRDF
Marble BRDF
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Subsurface Scattering Models

• Classical Kubelka-Monk(1930s, for paint many
  proprietary variants),
• CG approach Hanrahan Krueger(1990s)
• Recent (2001, Jensen)

Skin BSSRDF (approximated)
Skin BRDF (measured)
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BSSRDF Model

• Approximates scattering result as embedded point
  sources below a BRDF surface
• BSSRDF A Practical Model for Subsurface Light
  Transport Henrik Wann Jensen, Steve Marschner,
  Marc Levoy, Pat Hanrahan, SIGGRAPH01 (online)

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BSSRDF Model

• Embedded point sources below a BRDF surface
• Ray-based, tested, Physically-Measurable Model
• ?Useful as a predictive model for IBMR data?

Wann Jensen et al., 2001
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