1. BRDF Measurement

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

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

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

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

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

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

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

9. Measuring BRDFs A full BRDF is 4-dimensional
Simpler measurements often useful
Start with the simplest, and get 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 of diffuse 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-Based BRDF Measurement
For uniform BRDF, capture 2-D slice corresponding to variations in normals

23. Marschner’s Image-Based BRDF Measurement
Any object with known geometry

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

25. BRDF Measurement is Hard!