brdf measurement. definition of brdf key point: brdf is a differential quantity, so limit must...
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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
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Detectordet
det
Definition of BRDFDefinition of BRDF
• First attempt:First attempt:
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rf
Definition of BRDFDefinition of BRDF
• Should Should ffrr vary with vary with srcsrc? No.? No.
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Definition of BRDFDefinition of BRDF
• Should Should ffrr vary with vary with detdet? ? Yes.Yes.
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Definition of BRDFDefinition of BRDF
• Thus,Thus,
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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
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dA
Definition of BRDFDefinition of BRDF
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E
L
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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!