Measurement, Inverse Rendering

1. Measurement, Inverse Rendering COMS 6998-3, Lecture 4

2. Motivations • True knowledge of surface properties • Accurate models for graphics • Augmented reality, scene editing

3. Geometry 70’s, 80’s: Splines 90’s: Range Data Rendering Algorithm 80’s,90’s: Physically based Materials/Lighting (Texture Reflectance[BRDF] Lighting) Realistic input models required Arnold Renderer: Marcos Fajardo Photorealistic Rendering

4. Inverse RenderingAlgorithm BRDF Lighting Flowchart Photographs Geometric model

5. Flowchart Forward RenderingAlgorithm Photographs BRDF Rendering Lighting Geometric model

6. Flowchart Forward RenderingAlgorithm Photographs BRDF Novel lighting Rendering Geometric model

7. Next 3 slides courtesy George Drettakis

10. Assume time doesn’t matter (no phosphorescence) Assume wavelengths are equal (no fluorescence) Scattering function = 9D Assume wavelength is discretized or integrated into RGB (This is a common assumption for computer graphics) Single-wavelength Scattering function = 8D Taxonomy 1 General function = 12D

11. 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 Ignore direction of incident light Ignore dependence on position Ignore subsurface scattering Light Fields, Surface LFs = 4D BRDF = 4D Assume isotropy Low-parameter BRDF model Measure plane of incidence Assume Lambertian 0D 2D 3D Texture Maps = 2D Taxonomy 2 Single-wavelength Scattering function = 8D

12. Outline • Motivation • Taxonomy of measurements • BRDF measurement • Highlights from recent work • Next week: paper presentations

13. Source src src Detector det det Definition of BRDF dA Next several slides courtesy Szymon Rusinkiewicz

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

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

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

17. 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°

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

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

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

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

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

23. 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

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

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

26. BRDF Measurement is Hard!

27. Reflectance modeling (diff +specular texture) Input Synthesized Sato, Wheeler, Ikeuchi 97

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

29. Inverse Global Illumination Yu et al. 99

30. From a single image Original Photo Boivin and Gagalowicz 01

31. Assignment (by tomorrow) • Brief e-mail of proposed project, partners, plan of action (milestones) • Iterate by e-mail or schedule appointments later in the week • 1-2 page proposal due next Wed. including an intermediate milestone (Oct. 23)