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 24. 04. 2008 CESCG 2

Motivation • Measurement costly • Sometimes only nice images interesting • Design process: want to try different paints Measured New Editing 24. 04. 2008 CESCG 3

Outline • • • Motivation Car paint composition and resulting visual effects Previous work Our approach Results 24. 04. 2008 CESCG 4

Car Paint Composition Simple paint Metallic paint Pearlescent/flip-flop paint Finish Binder Coating Direction dependent color shift Base paint Primer surfacer Electrocoat Substrat Car Color pigments (30 -100 nm) Base color 24. 04. 2008 CESCG Al-Flakes (10 -50 μm) Glittering 5

Outline • • • Motivation Car paint composition and resulting visual effects Previous work Our approach Results 24. 04. 2008 CESCG 6

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 24. 04. 2008 CESCG 7

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 CTparameters Limitations: - cannot model all colors flops - no flakes 24. 04. 2008 CESCG 8

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 24. 04. 2008 CESCG 9

Previous Work (Measurement) • Bidirectional Texture Function (BTF) Limitation: no straightforward editing possible 24. 04. 2008 CESCG 10

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 24. 04. 2008 CESCG 11

Outline • • • Motivation Car paint composition and resulting visual effects Previous work Our approach Results 24. 04. 2008 CESCG 12

Hybrid approach • Image-based representation of sparkles = BTF + BRDF Flake BTF • Model-based reflection with direction dependent colors = BRDF 24. 04. 2008 CESCG Intensity BRDF Direction dependent color 13

Our BRDF model • Split color + intensity • Store color in direction dependent table – Low-frequent → coarse sampling sufficient – 200 KB table – No restriction to mechanism of color shift • Basic model: Cook-Torrance model Direction dependent for intensity color table 24. 04. 2008 CESCG Sphere painted with color table Colorless reflection of finish 14

Separation of BTF • • Split BRDF and BTF part Take light/view direction for every single pixel Subtract fitted BRDF model Synthesize BTF part - 24. 04. 2008 CESCG = 15

Rendering • Add together BRDF + synthesised BTF • Path Tracing (mentalray) – High quality – Tonemapping: Use responsecurve from camera • GPU shader (cg. FX, n. Vidia 8800 GTX) – Only simple and few light sources – Environment mapping – Tonemapping: Reinhard et al. (2005) 24. 04. 2008 CESCG 16

Editing (1) • Exchange color table • Exchange flake BTF 24. 04. 2008 CESCG 17

Editing (2) • Affine mapping of color plane – Drag, scale, rotate points projected onto plane – Preview color flop on sphere Mx+t 24. 04. 2008 CESCG 18

Editing (3) DEMO 24. 04. 2008 CESCG 19

Outline • • • Motivation Car paint composition and resulting visual effects Previous work Our approach Results 24. 04. 2008 CESCG 20

Results (path tracing) • Painted sphere (BRDF+BTF) (BRDF only) 24. 04. 2008 CESCG 21

Result: Color edited 24. 04. 2008 CESCG 22

Result: Color edited 24. 04. 2008 CESCG 23

Result: Color+flakes changed 24. 04. 2008 CESCG 24

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? 24. 04. 2008 CESCG 25