BRDF Measurement

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# BRDF Measurement - PowerPoint PPT Presentation

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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## BRDF Measurement

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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

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

Source

src

src

Detector

det

det

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

Source

src

src

Detector

det

det

Definition of BRDF
• Thus,

Source

src

src

Detector

det

det

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

dA

Source

src

src

Detector

det

det

Definition of BRDF

dA

Measuring BRDFs
• A full BRDF is 4-dimensional
• Simpler measurements often useful
Measuring Reflectance

0º/45º

Diffuse Measurement

45º/45º

Specular Measurement

Integrating Spheres
• Sphere with diffuse material on inside
• Geometry ensures even illumination
• More accurate measure ofdiffuse reflectance
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
• “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
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
• 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
• Collect reflected light with hemispherical (should be ellipsoidal) mirror [SIGGRAPH 92]
Ward’s BRDF Measurement Setup
• Result: each image captures light at all exitant angles
Marschner’s Image-BasedBRDF 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!
• Result of fitting model to Ward’s measurements