Reflectance and Texture of Real-World Surfaces

1. Reflectance and Texture of Real-World Surfaces KRISTIN J. DANAColumbia UniversityBRAM VAN GINNEKENUtrecht UniversitySHREE K. NAYARColumbia UniversityJAN J. KOENDERINKUtrecht University ACM Transactions on Graphics, Vol. 18, No. 1, January 1999

2. Overview • Introduce BRDF and BTF • BTF Texture Gathering Technique • CUReT Database • BTF Applications • Future Work • Pretty Pictures

3. Bidirectional Reflectance Distribution Function (BRDF) • Nicodemus [1970] and Nicodemus et al. [1977] • Coarse scale level • local surface variations are subpixel • local intensity is uniform • Bidirectional: • Camera Angle • Light Angle • “Objects look differently when viewed from different angles, and when illuminated from different directions”

4. Bidirectional Texture Function (BTF) • Fine scale level • Surface variations give rise to local intensity variations • Bidirectional: • Camera Angle • Light Angle • “Objects look differently when viewed from different angles, and when illuminated from different directions”

5. BRDF vs. BTF

6. Why do we need BTFs? • Traditional 2-D texture synthesis and texture-mapping do not take into account the change in texture appearance as the viewing and illumination directions change • A single digital image of a rough surface is mapped onto a 3-D object and the appearance of roughness is usually lost or distorted • Bump-mapping [Blinn 1977, 1978] preserves some of the appearance of roughness • knowledge of the surface shape is required • shadows cast from the local surface relief are not rendered • ray tracing can be used • exact geometry of the surface must be known • high computational cost • solid texturing: combine a volumetric texture synthesis with volume rendering techniques • computationally intensive • applicable for a limited variety of textures. • BTF database • “potential exists for 3-D texturing algorithms using images, without the need for a volumetric texture model or surface synthesis procedure”

7. BTF: Where do we start? • Already BRDF databases • Employ new techniques to create BTF database • Pull Together: • Robot • Lamp • PC • Photometer • Video camera

8. Texture Gathering Technique • Fixed light source • Halogen bulb with a Fresnel lens (single- beam focusing) • Camera moves through 7 positions • 22.5°, 45°, 67.5°, 90°, 112.5°, 135°, 157.5° from light source • Texture sample moves through multiple orientations • Robot arm orients sample normal along vertices of quarter-sphere facing the light source

9. Texture Gathering Technique Quarter-Sphere Orientations: Camera Positions • At each camera position, texture is captured with its normal along quarter-sphere vertices • Not all vertices captured at each position • At position 7, only a few normals are actually visible to the camera

10. Texture Gathering Technique Quarter-Sphere Orientations: Illumination Directions • Sample lies in xs–ys plane with its global normal pointing in the direction of zs • Each circular marker represents a distinct illumination direction • For each of these illumination directions, the sample is imaged from seven viewing directions

11. Texture Gathering Technique Special Case: Anisotropic Textures • Textures that have grids or grains • Measurements are repeated rotating sample about zs by either 45° or 90° depending on the structure of the anisotropy • Examples: • Linen (square grid) rotated 45° • Corduroy (vertical lines) rotated 90 °

12. Texture Gathering Technique Control Considerations • Relate radiance to pixel values • Use Kodak standard card image for every sample measured. • Letting r denote the total radiance and p denote the average pixel value, a linear relationship was found • Data with significant pixel underflow (pixel values near 0) or overflow (pixel values near 255) were not used.

13. End Product • 205 images for each sample • 640 x 480 pixels • 24 bits per pixel (8 bits per RGB channel). • Database total: over 14,000 images (61 samples, 205 measurements per sample, plus 205 additional measurements for anisotropic samples) • CUReT Database: www.cs.columbia.edu/CAVE/curet/

14. Columbia-Utrecht Reflectance and Texture Database (CUReT)

15. BTF Applications • Top row • Two images of “plaster_b” with different illumination and viewing directions • Bottom row • Spatial spectrum of “plaster_b” with zero frequency at the center and brighter regions corresponding to higher magnitudes • Notice orientation change due to change of illumination direction causing change in shadow direction. • Computer vision: • Texture recognition algorithms often based on spectral content of image textures • BTF should be considered for recognition of real-world surfaces Sample 11: “plaster_b”

16. BTF Applications • BTF texture gathering technique allows easy gathering of BRDF data • Pros: • Simple system • Simultaneously gather BRDF and BTF measurements • Cons: • Not as accurate as traditional BRDF measurement systems

17. Future Work Synthesizing Bidirectional Texture Functions for Real-World Surfaces Xinguo Liu, Yizhou Yu,Heung-Yeung Shum • 3 Step approach to synthetically generate BTFs • Recovers approximate 3D geometry of surface details using a shape-from-shading approach • Generates a novel version of the geometric details with the same statistical properties as the sample surface • Uses an “appearance preserving procedure” to synthesize novel images under various viewing/lighting settings, defining a novel BTF

18. Show me some BTF pictures!!! • 13 images per sample used from database collection of 205 • 1 image of frontal view • 12 oblique views • Use averaging of three pixels at the section borders to reduce the appearance of seams

19. Pretty Pictures Sample 11 (plaster) Traditional 2-D texture-mapping BTF 3-D texture-mapping

20. Pretty Pictures Sample 8 (pebbles) Traditional 2-D texture-mapping BTF 3-D texture-mapping

21. Pretty Pictures Sample 45 (concrete) Traditional 2-D texture-mapping BTF 3-D texture-mapping

22. Pretty Pictures Sample 28 (crumpled paper) Traditional 2-D texture-mapping BTF 3-D texture-mapping

23. Pretty Pictures Sample 19 (plush rug) Traditional 2-D texture-mapping BTF 3-D texture-mapping

24. Pretty Pictures Sample 56 (wood) (anisotropic) Traditional 2-D texture-mapping BTF 3-D texture-mapping

25. fine