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Viewing a Response Value as an Isosurface

An isosurface produces a three-dimensional surface by connecting all the data points with one response value. That is, the isosurface lets you see all locations in the volume grid when the response values are equal. An isosurface is particularly useful in viewing scientific data and physical properties such as temperature, density, and stress.

You must specify the response value to be displayed by the isosurface using the response histogram. The value shown corresponds to the lower histogram level.

Isolating a Response Value

With the response histogram, you specify the response value for an isosurface. The response histogram, which appears at the bottom left of the interface when you request a point cloud or an isosurface, displays the frequency distribution of data points that share the same response value. The histogram may also include software-interpolated values between the actual response values, which can be used to generate an isosurface.

Response Histogram to Specify Isosurface Value

[IMAGE]

There are actually two histograms:

The upper one represents all the response values in the data set. The positions of the lines are the minimum and maximum response values.
The lower one magnifies the portion of the data that lies between the two lines. The histogram stacks are larger or smaller relative to all the data between the lines. As you move the lines, you alter the span of values being compared to each other, so the stacks in the lower histogram change.
Flat histogram stacks do not necessarily indicate an absence of response data at that value. When you have a complete grid, the flat places may simply indicate much fewer values compared to the frequency of values represented by adjacent stacks. When you have only a few responses and they are widely separated, the values interpolated between them by the software may produce a flat stretch in the histogram.

Displaying an Isosurface

Select [Tools], [Planes], then [Isosurface].
To specify the response value, use the left mouse button and drag the histogram's left line to the desired value. The default position of the left line is the minimum response value, which cannot be used to generate an isosurface. (The right line has no effect on an isosurface specification.)

Render the isosurface by selecting:

[W/ depth sort]	displays the isosurface with depth sorting by sorting the planes front-to-back so that the image appears more realistically three-dimensional, for example, when moving planes through the volume grid. This method takes more time.
[W/o depth sort]	displays the isosurface without depth sorting, which is a faster, but less realistic method. It is recommended that you use this method when animating an isosurface with a BY variable to produce faster animation.
[Off]	turns off the displayed isosurface.

By default, the isosurface is drawn in the color shown in the histogram for the selected response value.

To assign a different color to the isosurface, select [Palette], [Isosurface], then [User-defined]. Adjust the RGB sliders. As you move the sliders, the current color appears in a square at the left, and the isosurface changes color in the Volume window. You can also select a color chip, located below the RGB sliders. (To return to the response value color, select [Response level].)

Isosurface

[IMAGE]

Note that you can specify a different response value when the isosurface is turned on or when it is turned off. However, if you have a large amount of response data, it may be faster to turn off the isosurface, then adjust the histogram.

Effect of a BY Variable on an Isosurface

For an isosurface with a BY variable specification, you can request a specific response value but not see the isosurface display. This is because the response value you selected is not contained in the data associated with the current BY value. (The histogram shows all responses, in ascending order, regardless of the value of the BY variable.)

To see the isosurface that was generated, use [Auto BY] to search for the appropriate BY value. Because the response value may be valid for more than one x,y,z location over time, an isosurface may be generated for more than one BY value. When the applicable BY value(s) is current, the isosurface is clearly visible.

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Copyright © 1999 by SAS Institute Inc., Cary, NC, USA. All rights reserved.