Lecture 6.2: Flow Vis: Marker Techniques

• Motivation
  • Goals: see movement of fluid (direction/velocity) in instant or over time
  • Applications
    • aerodynamics
    • CAD -- airflow over surfaces such as cars
    • airflow through/around buildings
    • ocean currents
    • fluid flow through pumps/valves
    • electromagnetics
  • Traditionally
    • release markers (smoke, bubbles, dye) in flow
    • watch where it goes
    • many flow vis techniques mimic these techniques
  • Types of Flows
    • steady state: flow field static over visualization
    • unsteady: flow field changes over time
    • Implications
      • computational complexity
      • representational complexity
• Glyphs
  • Some techniques
    • arrows/hedgehog/tufts (VID: EPA 11/22/94)
    • arrows with range (Wittenbrink, Pang, & Lodha, TVCG Sept '96)
    • tensor glyphs (Post, Walsum, Post, & Silver, Vis '95)
  • Related Issues
    • glyph placement/density (Dovey, Vis '95)
    • interactive probing
    • analysis of topology
• Color
  • color as auxilliary: usually magnitude
  • color wheel (Johansen and Moorhead, CG&A '95) (VID: Vis '94)
          color angle shows orientation (convert u,v to polar coords)
          optionally, mag as saturation or lightness or both
  • interactive color (Boring and Pang, Vis '96) (VID: Vis '96)
          color to show relationship between vec and light
          interactive exploration
• Particle Advection
  • release weightless particles into flow, follow their paths (VID: NASA '91)
  • color by:
          release location
          age
          field characteristics: velocity, pressure, etc.
  • positional accuracy
          Euler's method: error O(dt^2)
          Runge-Kutta: error O(dt^3)
  • speed vs accuracy issues
          method
          stepsize
• Lines
  • Generally:
    • follow path of marker through flow
    • same speed/accuracy concerns as particles
  • Types of line markers
    • streamline: path of single marker (in steady or unsteady flow)
    • streakline: path of stream of markers from single release point in changing flow
  • Streamline placement
    • ideally:
            show off interesting features
            even spacing in merging/diverging fields
    • idea: use field to guide line placement incrementally (Turk & Banks, SIGGRAPH '96) (VID: SIG '96)
  • Tubes
    • 3D entities for improved perception
    • tube = surface at constant distance from streamline
    • auxillary information display
            can color to show value
            can vary radius to show value
  • Stream Polygon (Schroeder, Volpe, & Lorensen, Vis '91)
    • use polygon perpendicular to stream line
    • can be deformed by forces
  • Illuminated lines (Zockler, Stalling, and Hege, Vis '96) (VID: Vis '96)
    • idea: shade streamlines to give better shape/depth cues
    • catch: no hardware support for shaded lines
    • method:
      • use texture-mapping hardware to do line shading
      • diffuse: 1D texture indexed by L*T (light * tangent)
      • specular: 2D texture indexed by light and view angle
      • load texture maps to five correct illuminatio for those lighting/viewing conditions
• Stream Surface (Hultquist, Vis '92)
  • method
    • generate stream lines
    • join adjacent streamlines to form triangles
  • issue: diverging and merging flows