BRDF MeasurementBRDF Measurement

Definition of BRDFDefinition of BRDF

• Key point: BRDF is a differential Key point: BRDF is a differential quantity, so limit must existquantity, so limit must exist

Sourcesrc

src

Detectordet

det

Definition of BRDFDefinition of BRDF

• First attempt:First attempt:

Sourcesrc

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Detectordet

det

src

det

rf

Definition of BRDFDefinition of BRDF

• Should Should ffrr vary with vary with srcsrc? No.? No.

Sourcesrc

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Detectordet

det

Definition of BRDFDefinition of BRDF

• Should Should ffrr vary with vary with detdet? ? Yes.Yes.

Sourcesrc

src

Detectordet

det

Definition of BRDFDefinition of BRDF

• Thus,Thus,

Sourcesrc

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Detectordet

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src

det det

rf

Definition of BRDFDefinition of BRDF

• What about surface area?What about surface area?ffrr must be independent of surface must be independent of surface

areaarea

Sourcesrc

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Detectordet

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dA

Definition of BRDFDefinition of BRDF

Sourcesrc

src

Detectordet

det

E

L

dA

dA

src

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rf

dA

Measuring BRDFsMeasuring BRDFs

• A full BRDF is 4-dimensionalA full BRDF is 4-dimensional

• Simpler measurements often Simpler measurements often usefuluseful

• Start with the simplest, andStart with the simplest, andget more complexget more complex

Measuring ReflectanceMeasuring Reflectance

0º/45ºDiffuse Measurement

45º/45ºSpecular Measurement

Integrating SpheresIntegrating Spheres

• Sphere with diffuse material on Sphere with diffuse material on insideinside

• Geometry ensures even Geometry ensures even illuminationillumination

• More accurate measure ofMore accurate measure ofdiffuse reflectancediffuse reflectance

Gloss MeasurementsGloss Measurements

• Some additional aspects of appearance Some additional aspects of appearance can be captured with one, or a small can be captured with one, or a small number of measuementsnumber of measuements

• Standardized for applications such as Standardized for applications such as paint manufacturingpaint manufacturing

• Example: “contrast gloss” is essentially Example: “contrast gloss” is essentially ratio of specular to diffuseratio of specular to diffuse

• ““Sheen” is specular measurement at 85°Sheen” is specular measurement at 85°

Gloss MeasuementsGloss Measuements

• ““Haze” and “distinctness of image” Haze” and “distinctness of image” are measurements of width of are measurements of width of specular peakspecular peak

BRDF MeasurementsBRDF Measurements

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

GonioreflectometersGonioreflectometers

• Three degrees of freedom spread Three degrees of freedom spread among among light sourcelight source, , detectordetector, , and/or sampleand/or sample

GonioreflectometersGonioreflectometers

• Three degrees of freedom spread Three degrees of freedom spread among light source, among light source, detectordetector, , and/or and/or samplesample

GonioreflectometersGonioreflectometers

• Can add fourth Can add fourth degree of degree of freedom to freedom to measure measure anisotropic BRDFsanisotropic BRDFs

Issues in BRDF Issues in BRDF MeasurementMeasurement

• Light sourceLight source: angular size, : angular size, brightness, stability, specklebrightness, stability, speckle

• DetectorDetector: angular size, sensitivity, : angular size, sensitivity, noise, resolution (if spatially noise, resolution (if spatially varying)varying)

• PositioningPositioning: accuracy, drift, : accuracy, drift, hysteresishysteresis

• Acquisition timeAcquisition time

Image-Based BRDF Image-Based BRDF MeasurementMeasurement

• Reduce acquisition time by Reduce acquisition time by obtaining larger (e.g. 2-D) slices of obtaining larger (e.g. 2-D) slices of BRDF at onceBRDF at once

• Requires mapping of angles of Requires mapping of angles of light to camera pixelslight to camera pixels

Ward’s BRDF Measurement Ward’s BRDF Measurement SetupSetup

• Collect reflected light with Collect reflected light with hemispherical (should be ellipsoidal) hemispherical (should be ellipsoidal) mirror mirror [SIGGRAPH 92][SIGGRAPH 92]

Ward’s BRDF Measurement Ward’s BRDF Measurement SetupSetup

• Result: each image captures light Result: each image captures light at all exitant anglesat all exitant angles

Marschner’s Image-BasedMarschner’s Image-BasedBRDF MeasurementBRDF Measurement

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

Marschner’s Image-BasedMarschner’s Image-BasedBRDF MeasurementBRDF Measurement

• Any object with known geometryAny object with known geometry

BRDF Measurement is Hard!BRDF Measurement is Hard!

• Result of fitting Result of fitting model to Ward’s model to Ward’s measurementsmeasurements

BRDF Measurement is Hard!BRDF Measurement is Hard!