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BRDF Measurement - PowerPoint PPT Presentation


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BRDF Measurement. Source  src  src. Detector  det  det. Definition of BRDF. Key point: BRDF is a differential quantity, so limit must exist. Source  src  src. Detector  det  det. Definition of BRDF. First attempt:. Source  src  src. Detector  det  det.

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definition of brdf

Source

src

src

Detector

det

det

Definition of BRDF
  • Key point: BRDF is a differential quantity, so limit must exist
definition of brdf3

Source

src

src

Detector

det

det

Definition of BRDF
  • First attempt:
definition of brdf4

Source

src

src

Detector

det

det

Definition of BRDF
  • Should fr vary with src? No.
definition of brdf5

Source

src

src

Detector

det

det

Definition of BRDF
  • Should fr vary with det? Yes.
definition of brdf6

Source

src

src

Detector

det

det

Definition of BRDF
  • Thus,
definition of brdf7

Source

src

src

Detector

det

det

Definition of BRDF
  • What about surface area?fr must be independent of surface area

dA

definition of brdf8

Source

src

src

Detector

det

det

Definition of BRDF

dA

measuring brdfs
Measuring BRDFs
  • A full BRDF is 4-dimensional
  • Simpler measurements often useful
  • Start with the simplest, andget more complex
measuring reflectance
Measuring Reflectance

0º/45º

Diffuse Measurement

45º/45º

Specular Measurement

integrating spheres
Integrating Spheres
  • Sphere with diffuse material on inside
  • Geometry ensures even illumination
  • More accurate measure ofdiffuse reflectance
gloss measurements
Gloss Measurements
  • Some additional aspects of appearance can be captured with one, or a small number of measuements
  • 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
Gloss Measuements
  • “Haze” and “distinctness of image” are measurements of width of specular peak
brdf measurements
BRDF Measurements
  • Next step up in complexity: measure BRDF in plane of incidence (1- or 2-D)
gonioreflectometers
Gonioreflectometers
  • Three degrees of freedom spread among light source, detector, and/or sample
gonioreflectometers16
Gonioreflectometers
  • Three degrees of freedom spread among light source, detector, and/or sample
gonioreflectometers17
Gonioreflectometers
  • Can add fourth degree of freedom to measure anisotropic BRDFs
issues in brdf measurement
Issues in BRDF Measurement
  • Light source: angular size, brightness, stability, speckle
  • Detector: angular size, sensitivity, noise, resolution (if spatially varying)
  • Positioning: accuracy, drift, hysteresis
  • Acquisition time
image based brdf measurement
Image-Based BRDF Measurement
  • Reduce acquisition time by obtaining larger (e.g. 2-D) slices of BRDF at once
  • Requires mapping of angles of light to camera pixels
ward s brdf measurement setup
Ward’s BRDF Measurement Setup
  • Collect reflected light with hemispherical (should be ellipsoidal) mirror [SIGGRAPH 92]
ward s brdf measurement setup21
Ward’s BRDF Measurement Setup
  • Result: each image captures light at all exitant angles
marschner s image based brdf measurement
Marschner’s Image-BasedBRDF Measurement
  • For uniform BRDF, capture 2-D slice corresponding to variations in normals
marschner s image based brdf measurement23
Marschner’s Image-BasedBRDF Measurement
  • Any object with known geometry
brdf measurement is hard
BRDF Measurement is Hard!
  • Result of fitting model to Ward’s measurements