Validation of Color Managed 3D Appearance Acquisition

1. Validation of Color Managed 3D Appearance Acquisition Michael Goesele Max-Planck-Institut für Informatik(MPI Informatik) Vortrag im Rahmen des V3D2 Workshops 29.+30. November 2004 in Berlin

2. Acquired BRDF Model rendered BRDF model of a carafe Villeroy & Boch (Mettlach, 19th century) Michael Goesele

3. Acquired BRDF Model Which model is (more) correct? Michael Goesele

4. Background: BRDF • bidirectional reflectance distribution function (BRDF) • ratio of reflected to incident radiance at one pointfor any pair of directions Michael Goesele

5. Background: BRDF Acquisition • based on acquisition system for spatially varying BRDFs[Lensch et al. 2003] • determine local reflection properties for each surface point • uses Lafortune BRDF model [Lafortune 1997] • shown at previous V3D2 Workshops … Michael Goesele

6. Background: BRDF Acquisition Michael Goesele

7. Validation Questions • How exact can this method capture describe the behavior of a real object? • How exact can we reproduce an objects appearance? • How can this be achieved? will be discussed in this talk • How good is the Lafortune model? will not be discussed in this talk see e.g. [Ngan et al., SIGGRAPH Sketch 2004] Michael Goesele

8. Color and Computer Graphics • yes, we have color … • it works (somehow) • RGB is always the same (?) • looks nice after some tuning … • more seriously … • not the main concern, other technologies more important • eye can adjust to bad color reproduction • comparison to ground truth often not possible/required • an important issue for digitization! Michael Goesele

9. Background: Color Management • goal: ensure correct color reproduction across devices • used in graphical arts and printing industry • defined profile connection space (PCS) with CIE XYZ or CIE Lab color space • profiles describe conversion to PCS for all devices • take limitations of devices into account Michael Goesele

10. ICC Profile Generation • example: input profile • capture known test target with camera • lighting conditions (spectrum) identical to finally used conditions • profile generated by(commercial) software • captures properties of • camera • lighting • test target Michael Goesele

11. Key Idea • introduce color management into BRDF acquisition and reproduction workflow • convert input images into defined color space(during HDR image generation) • convert rendered images into output device color space • allows for objective assessment of quality of acquired models • important for libraries, conservation, … • color management ensures best possible color reproduction • takes limitations of devices into account Michael Goesele

12. move to calibrated move to output color space color space BRDF Acquisition Workflow Michael Goesele

13. Accuracy of the Acquired Model • compare spectrophotometer measurements with BRDF model evaluated under same conditions • illumination at 45°, observation at 0° (along surface normal) • performs measurement in spectral domain • can be converted to other color representations BRDF spectrophotometer Michael Goesele

14. Accuracy of the Acquired Model • quality metric: E • distance in CIELab color space • 1 E just noticable color difference under perfect conditions Michael Goesele

15. 50 E 0 E Minerva of Arezzo (Florence, 3rd century B.C. or 1st century A.C.) Accuracy of the Acquired Model • compare renderings with photographs captured under identical conditions • quality metric: E Michael Goesele

16. Accuracy of the Acquired Model • until now: only comparison of acquired model to ground truth data • important for conservation, long term storage • further goal: include output devices in validation • can we (physically) reproduce the object correctly? Michael Goesele

17. printout (color laser printer) Accuracy of Renderings • visual comparison between rendering (on screen, printout) and real object under identical conditions • all devices are calibrated • no manual color adjustment was performed! screen real object Michael Goesele

18. Accuracy of Renderings • visual comparison between rendering (on screen, printout) and real object under identical conditions Michael Goesele

19. Accuracy of Renderings • visual comparison between rendering (on screen, printout) and real object under identical conditions Michael Goesele

20. Conclusion • color management integrated into BRDF acquisition and rendering pipeline • enables quantitative and visual validation of results • correct color acquisition and rendering is important! • approach: • acquire best possible model • use best possible reproduction • important for long term storage • reproduction technology improves (displays, printers, …) • model should support these as far as possible Michael Goesele

21. Future Work • improve quality of color management • some colors are still quite problematic • color management for HDR images? • ongoing work in the community • handling of high contrast, out-of-gamut colors • tone mapping problem • new display technologies (e.g., HDR display) Michael Goesele

22. Thanks to … • Hendrik Lensch • DFG Schwerpunktprogramm V3D2 “Verteilte Vermittlung und Verarbeitung digitaler Dokumente” • More information … • Michael Goesele, Hendrik P. A. Lensch, Hans-Peter Seidel:Validation of Color Managed 3D Appearance Acquisition.Proc. IS&T’s 12th Color Imaging Conference, pp. 265-270, 2004. • Hendrik P. A. Lensch, Jan Kautz, Michael Goesele, Wolfgang Heidrich, Hans-Peter Seidel:Image-Based Reconstruction of Spatial Appearance and Geometric Detail.ACM Transactions on Graphics, vol. 22, 2, pp. 234-257, 2003. Michael Goesele