Measurement and editing of metallic car paint BRDF

Martin Rump

Computer Graphics GroupUniversity of Bonn, Germany

224.04.2008 CESCG

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

324.04.2008 CESCG

Motivation• Measurement costly• Sometimes only nice images interesting• Design process: want to try different paints

Measured New

Editing

424.04.2008 CESCG

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

524.04.2008 CESCG

Pearlescent/flip-flop paintSimple paintMetallic paint

Car Paint Composition

Substrat

Electro-coat

Primersurfacer

Finish

Base paint

Color pigments (30-100 nm)Al-Flakes (10-50 μm)

Binder

Car

CoatingDirection dependentcolor shift

GlitteringBase color

624.04.2008 CESCG

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

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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 componentsLimitations:

• many parameters• no guarantee for physical

correctness

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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

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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

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Previous Work (Measurement)• Bidirectional Texture Function (BTF)

Limitation: no straightforward editing possible

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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

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

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Hybrid approach

BTF Flake BTFBRDF

+=

BRDF

=

• Image-based representation of sparkles

• Model-based reflection with direction dependent colors

Intensity BRDFDirection dependent color

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• 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:

( , ) , ( , ) ( , )base glossV L I V L V L I V L

Cook-Torrance modelfor intensity

Direction dependent color

table

Colorless reflectionof finish

Sphere painted with color table

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Separation of BTF• Split BRDF and BTF part• Take light/view direction for every single pixel• Subtract fitted BRDF model• Synthesize BTF part

- =

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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)

),),((,,, LVxsynthBTFLVLVxtotal

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• Exchange color table

• Exchange flake BTF

Editing (1)

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• Affine mapping of color plane– Drag, scale, rotate points projected onto

plane– Preview color flop on sphere

Editing (2)

Mx+t

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Editing (3)

DEMO

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

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

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Result: Color edited

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Result: Color edited

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Result: Color+flakes changed

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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?