Measurement, Inverse Rendering COMS 6998-3, Lecture 4.
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Transcript of Measurement, Inverse Rendering COMS 6998-3, Lecture 4.
Measurement, Inverse Rendering
COMS 6998-3, Lecture 4
Motivations
• True knowledge of surface properties
• Accurate models for graphics
• Augmented reality, scene editing
Photorealistic Rendering
Materials/Lighting(Texture Reflectance[BRDF] Lighting)
Realistic input models required
Arnold Renderer: Marcos Fajardo
Rendering Algorithm
80’s,90’s: Physically based
Geometry
70’s, 80’s: Splines90’s: Range Data
Flowchart
Photographs
Geometric model
InverseRenderingAlgorithm
Lighting
BRDF
Flowchart
Photographs
Geometric model
ForwardRenderingAlgorithm
Lighting
BRDF
Rendering
Flowchart
Photographs
Geometric model
ForwardRenderingAlgorithm
BRDF
Novel lighting
Rendering
Next 3 slides courtesy George Drettakis
Taxonomy 1( , , , , , ) ( , , , , , )in outx y t x y t
General function = 12D
Scattering function = 9D
Assume time doesn’t matter (no phosphorescence)
Assume wavelengths are equal (no fluorescence)
Single-wavelength Scattering function = 8D
Assume wavelength is discretized or integrated into RGB(This is a common assumption for computer graphics)
( , , , ) ( , , , )in outx y x y
Taxonomy 2
Single-wavelength Scattering function = 8D
( , , , ) ( , , , )in outx y x y
Bidirectional Texture Function (BTF)Spatially-varying BRDF (SVBRDF) = 6D
Ignore subsurface scattering (x,y) in = (x,y) out
Bidirectional Subsurface ScatteringDistribution Function (BSSRDF) = 6D
Ignore dependence on position
Light Fields, Surface LFs = 4D
Ignore direction of incident light
( , , , )outx y
Texture Maps = 2D
Assume Lambertian
( , )outx y
3D
Assume isotropy
BRDF = 4D
Ignore subsurface scattering
( , ) ( , )in out
Ignore dependenceon position
2D
Measure plane of incidence
0D
Low-parameter BRDF model
Outline
• Motivation
• Taxonomy of measurements
• BRDF measurement
• Highlights from recent work
• Next week: paper presentations
Definition of BRDF
Sourcesrc
src
Detectordet
det
E
L
dA
dA
src
det det
rf
dA Next several slides courtesy Szymon Rusinkiewicz
Measuring BRDFs
• A full BRDF is 4-dimensional
• Simpler measurements (0D/1D/2D/3D) often useful
• Start with simplest, and get more complex
Measuring Reflectance
0º/45ºDiffuse Measurement
45º/45ºSpecular Measurement
Integrating Spheres
• Sphere with diffuse material on inside
• Geometry ensures even illumination
• More accurate measure ofdiffuse reflectance
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°
Gloss Measuements
• “Haze” and “distinctness of image” are measurements of width of specular peak
BRDF Measurements
• Next step up in complexity: measure BRDF in plane of incidence (1- or 2-D)
Gonioreflectometers
• Three degrees of freedom spread among light source, detector, and/or sample
Gonioreflectometers
• Three degrees of freedom spread among light source, detector, and/or sample
Gonioreflectometers
• Can add fourth degree of freedom to measure anisotropic BRDFs
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
Marschner’s Image-Based BRDF Measurement• For uniform BRDF, capture 2-D slice
corresponding to variations in normals
Marschner’s Image-BasedBRDF Measurement
• Any object with known geometry
BRDF Measurement is Hard!
Reflectance modeling (diff +specular texture)
Input Synthesized
Sato, Wheeler, Ikeuchi 97
Image-based measurement of skin
Marschner et al. 2000
Inverse Global Illumination
Yu et al. 99
From a single image
Boivin and Gagalowicz 01
Original Photo
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)