1. Measurement and editing of metallic car paint BRDF Martin Rump Computer Graphics Group University of Bonn, Germany

2. 24.04.2008 CESCG 2 Outline Motivation Car paint composition and resulting visual effects Previous work Our approach Results

3. 24.04.2008 CESCG 3 Motivation Measurement costly Sometimes only nice images interesting Design process: want to try different paints MeasuredNew Editing

4. 24.04.2008 CESCG 4 Outline Motivation Car paint composition and resulting visual effects Previous work Our approach Results

5. 24.04.2008 CESCG 5 Pearlescent/flip-flop paint Simple paintMetallic paint Car Paint Composition Substrat Electro- coat Primer surfacer Finish Base paint Color pigments (30-100 nm) Al-Flakes (10-50 μm) Binder Car Coating Direction dependent color shift Glittering Base color

6. 24.04.2008 CESCG 6 Outline Motivation Car paint composition and resulting visual effects Previous work Our approach Results

7. 24.04.2008 CESCG 7 Previous Work (Simulation) Ershov et al. „Rendering Pearlescent Appearance Based On Paint-Composition Modelling“ (2001) –Multilayer model –Calculate operators for sublayers –Combine operators → BRDF –Choose colors for components Limitations: many parameters no guarantee for physical correctness

8. 24.04.2008 CESCG 8 Previous Work (Measurement) Günther et al. „Efficient Acquisition and Realistic Rendering of Car Paint“ (2005) –BRDF measurement –Cook-Torrance model –Realtime raytracing –Editing by changing CT- parameters Limitations: - cannot model all colors flops - no flakes

9. 24.04.2008 CESCG 9 Previous Work (Measurement) Matusik et al. „Efficient Isotropic BRDF Measurement“ (2003) Matusik et al. „A Data-Driven Reflectance Model“ (2003) Limitations: - no physical correctness - no flakes

10. 24.04.2008 CESCG 10 Previous Work (Measurement) Bidirectional Texture Function (BTF) Limitation: no straightforward editing possible

11. 24.04.2008 CESCG 11 Summary Simulation of layers: –Too many (unknown) parameters –No flakes BRDF measurement and fitting: –„simple“ Cook-Torrance cannot model all effects –No flakes BTF rendering: –Representation not suitable for editing

12. 24.04.2008 CESCG 12 Outline Motivation Car paint composition and resulting visual effects Previous work Our approach Results

13. 24.04.2008 CESCG 13 Hybrid approach BTFFlake BTFBRDF += = Image-based representation of sparkles Model-based reflection with direction dependent colors Intensity BRDF Direction dependent color

14. 24.04.2008 CESCG 14 Split color + intensity Store color in direction dependent table –Low-frequent → coarse sampling sufficient –200KB table –No restriction to mechanism of color shift Our BRDF model Basic model: Cook-Torrance model for intensity Direction dependent color table Colorless reflection of finish Sphere painted with color table

15. 24.04.2008 CESCG 15 Separation of BTF Split BRDF and BTF part Take light/view direction for every single pixel Subtract fitted BRDF model Synthesize BTF part - =

16. 24.04.2008 CESCG 16 Rendering Add together BRDF + synthesised BTF Path Tracing (mentalray) –High quality –Tonemapping: Use responsecurve from camera GPU shader (cgFX, nVidia 8800 GTX) –Only simple and few light sources –Environment mapping –Tonemapping: Reinhard et al. (2005)

17. 24.04.2008 CESCG 17 Exchange color table Exchange flake BTF Editing (1)

18. 24.04.2008 CESCG 18 Affine mapping of color plane –Drag, scale, rotate points projected onto plane –Preview color flop on sphere Editing (2) Mx+t

19. 24.04.2008 CESCG 19 Editing (3) DEMO

20. 24.04.2008 CESCG 20 Outline Motivation Car paint composition and resulting visual effects Previous work Our approach Results

21. 24.04.2008 CESCG 21 Results (path tracing) Painted sphere (BRDF only)Painted sphere (BRDF+BTF)

22. 24.04.2008 CESCG 22 Result: Color edited

23. 24.04.2008 CESCG 23 Result: Color edited

24. 24.04.2008 CESCG 24 Result: Color+flakes changed

25. 24.04.2008 CESCG 25 Conclusion Hybrid rendering approach combining BRDF + BTF Direction dependent color table for arbitrary color shift Editing by affine transformation of color table –Transformation in shader possible Editing by exchange of flake BTF Questions?