Outline • Visualizing scalar data • A number of the most popular scalar visualization techniques • Color mapping • Contouring • Height plots 5.1 Color mapping 5.2 Design of effective colormaps 5.3 Contouring in 2D and 3D 5.4 Height plots
5.1 Color Mapping • Most widespread visualization method for scalar data • Associate color with scalar values • Methods for defining scalar-to-color function • Color look-up tables (colormap) • Transfer functions (more detail in Chp 9 &10)
5.2 Designing effective colormaps • Attract user to certain value ranges or individual values • Colormap uses particularly salient colors • Colormap can be influenced by: • Application • Domain-specific convention &Traditions
5.2 Designing effective colormaps Fig 5.1. Construction of rainbow colormap (for weather applications: temp.; heat; height fields etc.)
5.2 Designing effective colormaps Listing 5.1. Rainbow colormap construction
5.2 Designing effective colormaps • Some application, emphasize the variation of the data • Colormap containing two or more alternating colors • Many other colormap designs • Geographical application • Classical medical imaging
5.2 Designing effective colormaps Fig 5.2. Visualizing the scalar function with (a) a luminance colormap and (b) a zebra colormap. The luminance colormap shows absolute values, whereas the zebra colormap emphasizes rapid value variations.
5.2 Designing effective colormaps Fig 5.3. Medical visualization with luminance and rainbow colormaps
5.2 Designing effective colormaps • Requirement for colormaps: (1) invertibility; (2) linearity. (3) scheme of color mapping: • Texture-based color mapping • Vertex-based color mapping [The relationship among • the colormap design • scalar data variation speed • domain sampling frequency.]
5.2 Designing effective colormaps Fig 5.4.Texture-based color mapping. The sphere geometry is sampled with (a) 64 x 64 points, (b) 32 x 32 points, and (c) 16 x 16 points. Advantage: produce reasonable results even for a sparsely sampled dataset
5.2 Designing effective colormaps Fig 5.5. Vertex-based color mapping. The sphere geometry is sampled with (a) 64 x 64 points, (b) 32 x 32 points, and (c) 16 x 16 points. Advantage: simplicity and direct support by even low-end graphics hardware
5.2 Designing effective colormaps • The choice of the number of Color N • A small N: color banding effect, artifact • Typical scalar visualization applications use 64 to 256 different colors • Other factors for Colormap • Geometric factors • User group • The medium used to present the visualization
5.2 Designing effective colormaps Fig 5.6. Color banding caused by a small number of colors in a look-up table.
5.2 Designing effective colormaps • Conclusion: • Color mapping : generate color values from scalar values by • Colormap • Color transfer function • Design issues for effective colormaps: • Knowledge of the application domain conventions • Typical data distribution • Visualization goals • General perception theory • Intended output devices • User preference