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Understanding BRDF Measurements for Accurate Surface Modeling in Computer Graphics

This overview discusses the importance of Bidirectional Reflectance Distribution Functions (BRDFs) in accurately modeling surface properties for graphics. It examines why measuring BRDFs provides true knowledge of surfaces, highlighting different measurement techniques like gonioreflectometers and integrating spheres. The document also delves into methods for obtaining spatially varying BRDFs and the potential for relighting and changing viewpoints in scenes. Understanding these concepts is crucial for creating realistic graphics simulations and improving material representations in visual computing.

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Understanding BRDF Measurements for Accurate Surface Modeling in Computer Graphics

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  1. Measuring BRDFs

  2. Why bother modeling BRDFs?Why not directly measure BRDFs? • True knowledge of surface properties • Accurate models for graphics

  3. Measure, measure, measure: BRDF, BTF, Light Fields Thanks to Steve Marschner, Shree Nayar, Ravi Ramamoorthi, Marc Levoy, Pat Hanrahan, Kristin Dana, Ken Perlin, Debevec, Matusik

  4. Measuring BRDFs • A full BRDF is 4-dimensional • Simpler measurements (0D/1D/2D/3D) often useful • Lets start with simplest and get more complex

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

  6. Integrating Spheres • Sphere with diffuse material on inside • Geometry ensures even illumination • More accurate measure oftotal integrated diffuse reflectance

  7. Gloss Measurements • Standardized for applications such as paint manufacturing • Example: “contrast gloss” is essentially ratio of specular to diffuse • “Sheen” is specular measurement at 85°

  8. Gloss Measurements • “Haze” and “distinctness of image” are measurements of width of specular peak

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

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

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

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

  13. Image-Based BRDF Measurement • Reduce acquisition time by obtaining larger (e.g. 2-D) slices of BRDF at once • Idea: Camera can acquire 2D image • Requires mapping of angles of light to camera pixels

  14. Ward’s BRDF Measurement Setup • Collect reflected light with hemispherical mirror

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

  16. Image-Based BRDF Measurement • For uniform BRDF, capture 2-D slice corresponding to variations in normals (Marschner et al.)

  17. Image-Based BRDF Measurement • Any object with known geometry Marschner et al.

  18. Image-based measurement of skin Marschner et al. 2000

  19. Next Step in the Appearance Chain Textures Spatially Varying BRDFs CURET Database – [Dana, Nayar 96]

  20. Next Step in the Appearance Food Chain • Why bother about measuring patches or spheres? • Why not measure the scenes themselves directly? • Change only lighting (for Relighting) • Change only viewpoint (Light Fields) • Change both lighting and viewpoint

  21. Debevec et al. Siggraph 2000

  22. VIDEO and DEMO for relighting

  23. Next Step in the Appearance Chain • Capture Light Fields directly by changing • viewpoint • Levoy &Hanrahan, Gortler et al., Siggraph 96

  24. Can we measure our way out of misery?

  25. Assume time doesn’t matter (no phosphorescence) Assume wavelengths are equal (no fluorescence, raman scattering) Scattering function = 9D Assume wavelength is discretized or integrated into RGB (This is a common assumption for computer graphics) Single-wavelength Scattering function = 8D Dimensionality of Appearance General function = 12D

  26. Ignore subsurface scattering (x,y) in = (x,y) out Ignore dependence on position Bidirectional Texture Function (BTF) Spatially-varying BRDF (SVBRDF) = 6D Bidirectional Subsurface Scattering Distribution Function (BSSRDF) = 6D Assume isotropy Low-parameter BRDF model 0D 3D Single-wavelength Scattering function = 8D Ignore dependence on position Ignore subsurface scattering Ignore direction of incident light BRDF = 4D Light Fields, Surface LFs = 4D Assume Lambertian Measure plane of incidence Texture Maps = 2D 2D

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