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Measurement and editing of metallic car paint BRDF

Measurement and editing of metallic car paint BRDF. Martin Rump Computer Graphics Group University of Bonn, Germany. Outline. Motivation Car paint composition and resulting visual effects Previous work Our approach Results. Motivation. Measurement costly

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Measurement and editing of metallic car paint BRDF

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

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

  3. Motivation • Measurement costly • Sometimes only nice images interesting • Design process: want to try different paints Measured New Editing

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

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

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

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

  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. Outline • Motivation • Car paint composition and resulting visual effects • Previous work • Our approach • Results

  13. Hybrid approach • Image-based representation of sparkles = + BTF BRDF Flake BTF • Model-based reflection with direction dependent colors = Direction dependent color BRDF Intensity BRDF

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

  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. 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. Editing (1) • Exchange color table • Exchange flake BTF

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

  19. Editing (3) DEMO

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

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

  22. Result: Color edited

  23. Result: Color edited

  24. Result: Color+flakes changed

  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?

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