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BRDF Measurement

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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  1. BRDF Measurement

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

  3. Source src src Detector det det Definition of BRDF • First attempt:

  4. Source src src Detector det det Definition of BRDF • Should fr vary with src? No.

  5. Source src src Detector det det Definition of BRDF • Should fr vary with det? Yes.

  6. Source src src Detector det det Definition of BRDF • Thus,

  7. Source src src Detector det det Definition of BRDF • What about surface area?fr must be independent of surface area dA

  8. Source src src Detector det det Definition of BRDF dA

  9. Measuring BRDFs • A full BRDF is 4-dimensional • Simpler measurements often useful • Start with the simplest, andget more complex

  10. Measuring Reflectance 0º/45º Diffuse Measurement 45º/45º Specular Measurement

  11. Integrating Spheres • Sphere with diffuse material on inside • Geometry ensures even illumination • More accurate measure ofdiffuse reflectance

  12. 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°

  13. Gloss Measuements • “Haze” and “distinctness of image” are measurements of width of specular peak

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

  15. Gonioreflectometers • Three degrees of freedom spread among light source, detector, and/or sample

  16. Gonioreflectometers • Three degrees of freedom spread among light source, detector, and/or sample

  17. Gonioreflectometers • Can add fourth degree of freedom to measure anisotropic BRDFs

  18. 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

  19. 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

  20. Ward’s BRDF Measurement Setup • Collect reflected light with hemispherical (should be ellipsoidal) mirror [SIGGRAPH 92]

  21. Ward’s BRDF Measurement Setup • Result: each image captures light at all exitant angles

  22. Marschner’s Image-BasedBRDF Measurement • For uniform BRDF, capture 2-D slice corresponding to variations in normals

  23. Marschner’s Image-BasedBRDF Measurement • Any object with known geometry

  24. BRDF Measurement is Hard! • Result of fitting model to Ward’s measurements

  25. BRDF Measurement is Hard!

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