Clouds

1
Clouds

• Based on Sect 5.3.3 in Parent

2
Cloud Types

• Cirrus - wispy
• gt20,000 ft
• Ice crystals
• Altocumulus - puffy
• 6500-20000ft
• Water droplets
• Stratus, stratocumulus
• lt6500 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

• Eberts 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) uD(perturb(p)) (1-u)turbulence (
  p )
• Setting u1 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 (fminfreq fltmaxfreq f 2)
• Val fabs ( noise (xf, yf, zf) / 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