Procedural Shading & Texturing

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Procedural Shading & Texturing Shinji Ogaki

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Procedural Shading • Procedural shading Procedural shading is a proven rendering technique in which a short user- written procedure, called a shader, determines the shading and color variations across each surface. - Marc Olano • Procedural texture A procedural texture is a computer-generated image created using an algorithm intended to create a realistic surface or volumetric representation of natural elements such as wood, marble, granite, metal, stone, and others, for use in texture mapping. - Wikipedia

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Advantages • Small memory footprint • Resolution independent

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Famous Algorithms • An Image Synthesizer – Ken Perlin (Siggraph 1985) • A Cellular Texture Basis Function – Steve Worley (Siggraph 1996)

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Today’s Topics • Voronoi cell • Fourier transform

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Voronoi Cell

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Voronoi Cell • Useful for mesoscale features (micro – meso - macro) • Inclusions, scratches, etc. Modeling Aventurescent Gems with Procedural Textures –Weidlich et al. (SCCG 2008) Multi-Scale Rendering of Scratched Materials using a Structured SV-BRDF Model – Raymond et al. (TOG 2016)

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Voronoi Cell • Grid based Algorithm • No precomputation • Zero memory cost

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Shading point Feature point 2D

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Shading point Closest feature point 2D

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Single Level

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Multiple Levels

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Coloration • Use exemplar • Histogram matching • Poor performance • Spatial information is not taken into account • Fractional Brownian motion in 2D texture space

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Voronoi Cell – Hash Function • 4D vector input – 4D vector output In (x,y,z,t): x,y,z:grid t:instance ID, etc. Out(x,y,z,t): x,y,z:feature point t:normal, color, density, etc. • Chaotic Hash Algorithms • Secure Hash Algorithms • MD5, SHA-0, SHA-1, SHA2, SHA-3, etc.

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Voronoi Cell – Hash Function • 4D vector input – 4D vector output In (x,y,z,t): x,y,z:grid t:instance ID, etc. Out(x,y,z,t): x,y,z:feature point t:normal, color, density, etc.

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w/o instance ID w/z instance ID

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Voronoi Cell – Hash Function • 4D vector input – 4D vector output In (x,y,z,t): x,y,z:grid t:instance ID, etc. Out(x,y,z,t): x,y,z:feature point t:normal, color, density, etc.

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Scratches • Remove if δ

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Voronoi Cell – Hash Function • 4D vector input – 4D vector output In (x,y,z,t): x,y,z:grid t:instance ID, etc. Out(x,y,z,t): x,y,z:feature point t:normal, color, density, etc.

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Inclusions • Ray marching • Normal vector • Sample warping δ>density, density’=density/δ • Spherical Fibonacci lattice id=(int)(N x density’)

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Deformation • Surface: Pref (Reference position) • Volume: Open problem (Tetrahedrarization? Curve interpolation?)

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Fourier Transform

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Fourier Transform • Convolution • Ocean waves • Useful tool to design or analyze procedural noise patterns

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Phase

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Wave

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Coil

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Fourier Transform Convolution

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Convolution Input Kernel Input (Frequency space) Kernel (Frequency space) FFT FFT Element-wise multiplication IFFT Output

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Convolution

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Convolution

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Convolution

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Convolution

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Convolution

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Convolution

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Convolution

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Convolution

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Convolution

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Convolution

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Convolution

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Convolution Sum

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Convolution Blurred signal

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Convolution Input Kernel Input (Frequency space) Kernel (Frequency space) FFT FFT Element-wise multiplication IFFT Output

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Convolution 2D

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Convolution 2D • Gabor noise

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Convolution 2D • Glare effect

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