Measurement, Inverse Rendering

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Measurement, Inverse Rendering

• COMS 6998-3, Lecture 4

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Motivations

• True knowledge of surface properties
• Accurate models for graphics
• Augmented reality, scene editing

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Photorealistic Rendering
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Flowchart
Photographs
Geometric model
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Flowchart
Forward RenderingAlgorithm
Photographs
BRDF
Rendering
Lighting
Geometric model
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Flowchart
Forward RenderingAlgorithm
Photographs
BRDF
Novel lighting
Rendering
Geometric model
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Next 3 slides courtesy George Drettakis
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Taxonomy 1
General function 12D
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Taxonomy 2
Single-wavelength Scattering function 8D
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Outline

• Motivation
• Taxonomy of measurements
• BRDF measurement
• Highlights from recent work
• Next week paper presentations

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Definition of BRDF
dA
Next several slides courtesy Szymon Rusinkiewicz
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Measuring BRDFs

• A full BRDF is 4-dimensional
• Simpler measurements (0D/1D/2D/3D) often useful
• Start with simplest, and get more complex

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Measuring Reflectance
0º/45º Diffuse Measurement
45º/45º Specular Measurement
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Integrating Spheres

• Sphere with diffuse material on inside
• Geometry ensures even illumination
• More accurate measure ofdiffuse reflectance

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Gloss Measurements

• Standardized for applications such as paint
  manufacturing
• Example contrast gloss is essentially ratio of
  specular to diffuse
• Sheen is specular measurement at 85

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Gloss Measuements

• Haze and distinctness of image are
  measurements of width of specular peak

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BRDF Measurements

• Next step up in complexity measure BRDF in plane
  of incidence (1- or 2-D)

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Gonioreflectometers

• Three degrees of freedom spread among light
  source, detector, and/or sample

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Gonioreflectometers

• Three degrees of freedom spread among light
  source, detector, and/or sample

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Gonioreflectometers

• Can add fourth degree of freedom to measure
  anisotropic BRDFs

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Image-Based BRDF Measurement

• Reduce acquisition time by obtaining larger (e.g.
  2-D) slices of BRDF at once
• Idea Camera can acquire 2D image
• Requires mapping of angles of light to camera
  pixels

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Marschners Image-Based BRDF Measurement

• For uniform BRDF, capture 2-D slice corresponding
  to variations in normals

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Marschners Image-BasedBRDF Measurement

• Any object with known geometry

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BRDF Measurement is Hard!
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Reflectance modeling (diff specular texture)
Input
Synthesized
Sato, Wheeler, Ikeuchi 97
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Image-based measurement of skin
Marschner et al. 2000
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Inverse Global Illumination
Yu et al. 99
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From a single image
Original Photo
Boivin and Gagalowicz 01
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Assignment (by tomorrow)

• Brief e-mail of proposed project, partners, plan
  of action (milestones)
• Iterate by e-mail or schedule appointments later
  in the week
• 1-2 page proposal due next Wed. including an
  intermediate milestone (Oct. 23)