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Clouds

Based on Sect 5.3.3 in Parent. Clouds. Cloud Types. Cirrus - wispy >20,000 ft Ice crystals Altocumulus - puffy 6500-20000ft Water droplets Stratus, stratocumulus <6500 ft, water droplets, layered. Rendering issues. Amorphous, volumetric structure Swirling, bubbling motion

kyle-jensen
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Clouds

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  1. Based on Sect 5.3.3 in Parent Clouds

  2. Cloud Types • Cirrus - wispy • >20,000 ft • Ice crystals • Altocumulus - puffy • 6500-20000ft • Water droplets • Stratus, stratocumulus • <6500 ft, water droplets, layered

  3. Rendering issues • Amorphous, volumetric structure • Swirling, bubbling motion • Low-albedo (reflectance) techniques • Assume scattering effects are negligible • For cirrus • High-albedo techniques • Scattering is significant • For thick clouds

  4. More rendering issues • Wavelength-dependent scattering • Self-shadowing, shadows on landscape • Volumetric shadowing – expensive • Trace rays from eye, accumulating density • For each point along ray, trace another ray towards light source to determine illumination

  5. Early approaches • Semitransparent surfaces • Fractal synthesis of plane textures • Ellipsoids with fourier-synthesized transparency • Randomly placed, overlapping spheres with solid texture, transparent near edges

  6. Volumetric Clouds • Surface-defined clouds don't allow flythroughs • Volumetric, density-based models do • Particle systems can model motion well, but need large numbers of particles • Volume-rendered implicit functions • Can control implicit function with a particle system

  7. Volumetric Cloud Examples • From Nishita, et al, Siggraph 96

  8. Volumetric Cloud Example • Ebert’s method

  9. Ebert's Volumetric Cloud Model • Two-level hierarchy • High-level control for animator • Implicit functions • Procedural low-level details • Turbulent volume densities • Benefits of approach • Abstraction of detail • Data amplification

  10. Ebert Clouds (2) • Implicit density function as summed implicit primitives

  11. Ebert Clouds (3) • Implicit primitives are non-solid • Before evaluating blending functions, the point's position is perturbed using noise and turbulence • Final density is a blend of perturbed implicit density and a turbulence function • Density(p) = u*D(perturb(p)) + (1-u)*turbulence ( p ) • Setting u=1 gives “cotton balls” • Finally, density is modified by an exponential - density = pow ( density, power )

  12. Noise and Turbulence • Noise(x,y,z) created by • Assign random values to grid vertices • Interpolate between vertices near (x,y,z) with splines • Turbulence(x,y,z) is bandlimited noise with a “fractal” spectrum (1/f) • For (f=minfreq; f<maxfreq; f *= 2) • Val += fabs ( noise (x*f, y*f, z*f) / f );

  13. Animating Ebert Clouds • Animate the particle system of implicit primitives • Only 100-1000 particles needed • Can use turbulence, vortex, etc to control particle motion

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