Clouds

Based on Sect 5.3.3 in Parent

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● Low-albedo (reflectance) techniques

– Assume scattering effects are negligible– For cirrus

● High-albedo techniques– Scattering is significant– For thick clouds

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

Early approaches

● Semitransparent surfaces– Fractal synthesis of plane textures– Ellipsoids with fourier-synthesized

transparency– Randomly placed, overlapping spheres with

solid texture, transparent near edges

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

Volumetric Cloud Examples

● From Nishita, et al, Siggraph 96

Volumetric Cloud Example

● Ebert’s method

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

Ebert Clouds (2)

● Implicit density function as summed implicit primitives

D_p_=∑ w i_ F __p_ q__,where q is closest point on element i from p

F _r_=_ _49

__r 6

R 6 __ _179

__r4

R 4 __ _229

__r2

R 2 __ 1

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 )

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 );

Animating Ebert Clouds

● Animate the particle system of implicit primitives – Only 100-1000 particles needed– Can use turbulence, vortex, etc to control

particle motion