Volume visualization of data sets defined on 3D grids. help matlab

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Volume visualization is the creation of graphical representations of data sets that are defined on three-dimensional grids. Volume data sets are characterized by multidimensional arrays of scalar or vector data. These data are typically defined on lattice structures representing values sampled in 3-D space. There are two basic types of volume data:

• Scalar volume data contains single values for each point.
• Vector volume data contains two or three values for each point, defining the components of a vector.

An example of scalar volume data is that produced by the flow M-file. The flow data represents the speed profile of a submerged jet within an infinite tank. Typing

[x,y,z,v] = flow;

produces four 3-D arrays. The x, y, and z arrays specify the coordinates of the scalar values in the array v.

The wind data set is an example of vector volume data that represents air currents over North America. You can load this data in the MATLAB^® workspace with the command

load wind

This data set comprises six 3-D arrays: x, y, and z are the coordinate data for the arrays u, v, and w, which are the vector components for each point in the volume.

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Selecting Visualization Techniques

The techniques you select to visualize volume data depend on what type of data you have and what you want to learn. In general,

• Scalar data is best viewed with isosurfaces, slice planes, and contour slices.
• Vector data represents both a magnitude and direction at each point, which is best displayed by stream lines (particles, ribbons, and tubes), cone plots, and arrow plots. Most visualizations, however, employ a combination of techniques to best reveal the content of the data.

The material in these sections describes how to apply a variety of techniques to typical volume data.

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Steps to Create a Volume Visualization

Creating an effective visualization requires a number of steps to compose the final scene. These steps fall into four basic categories:

1. Determine the characteristics of your data. Graphing volume data usually requires knowledge of the range of both the coordinates and the data values.
2. Select an appropriate plotting routine. The information in this section helps you select the right methods.
3. Define the view. The information conveyed by a complex three-dimensional graph can be greatly enhanced through careful composition of the scene. Viewing techniques include adjusting camera position, specifying aspect ratio and project type, zooming in or out, and so on.
4. Add lighting and specify coloring. Lighting is an effective means to enhance the visibility of surface shape and to provide a three-dimensional perspective to volume graphs. Color can convey data values, both constant and varying.

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Volume Visualization Functions

MATLAB functions enable you to apply a variety of volume visualization techniques. The following tables group these functions into two categories based on the type of data (scalar or vector) that each is designed to work with. The reference page for each function provides examples of the intended use.

Functions for Scalar Data

Function	Purpose
contourslice	Draw contours in volume slice planes
isocaps	Compute isosurface end-cap geometry
isocolors	Compute the colors of isosurface vertices
isonormals	Compute normals of isosurface vertices
isosurface	Extract isosurface data from volume data
patch	Create a patch (multipolygon) graphics object
reducepatch	Reduce the number of patch faces
reducevolume	Reduce the number of elements in a volume data set
shrinkfaces	Reduce the size of each patch face
slice	Draw slice planes in volume
smooth3	Smooth 3-D data
surf2patch	Convert surface data to patch data
subvolume	Extract subset of volume data set

Functions for Vector Data

Function	Purpose
coneplot	Plot velocity vectors as cones in 3-D vector fields
curl	Compute the curl and angular velocity of a 3-D vector field
divergence	Compute the divergence of a 3-D vector field
interpstreamspeed	Interpolate streamline vertices from vector-field magnitudes
streamline	Draw stream lines from 2-D or 3-D vector data
streamparticles	Draw stream particles from vector volume data
streamribbon	Draw stream ribbons from vector volume data
streamslice	Draw well-spaced stream lines from vector volume data
streamtube	Draw stream tubes from vector volume data
stream2	Compute 2-D stream line data
stream3	Compute 3-D stream line data
volumebounds	Return coordinate and color limits for volume (scalar and vector)