GMS:Isosurfaces

Iso-surface rendering is a powerful tool for visualizing 3D data sets. Iso-surfaces can be generated for 3D grids and 3D meshes. An iso-surface is the 3D equivalent of a contour line. While a contour line is a line of constant value extracted from a surface, an iso-surface is a surface of constant value extracted from a 3D data set.

Defining Iso-values

Iso-surfaces are computed using the active scalar data set for the grid or mesh. The Iso-surface Options dialog is accessed through the Iso-surface Options command in the Data menu or through a button in the 3D Grid Display Options or 3D Mesh Display Options dialogs. The items in the Iso-surface Options dialog are as follows:

Active Data Set

At the top of the dialog the active scalar data set and active time step is listed. The maximum and minimum data set values are also listed.

Iso-Values

In the next section of the dialog the number of iso-surfaces and the iso-values are defined. A maximum of 12 iso-surfaces may be created. The Default button can be used to automatically set up a number of iso-values. For example, if the number of iso-surfaces is three and the button is selected, three iso-values, equally spaced between the maximum and minimum data set values are generated.

Fill Between

The Fill Between boxes to the right of the iso-values are used to generate surfaces on the exterior of the mesh or grid between two iso-values. For example, in part a of the figure shown below, two iso-surfaces have been generated using two iso-values. The image shown in part b was computed using the same iso-values and with the Fill Between box checked similar to figure d below. This causes the region of the mesh or grid boundary between the two iso-values to be defined as surfaces. The image shown in part c was generated using a single iso-value with the Fill Between box checked on the line after the iso-value like figure e. This causes the boundary with data set values greater than the specified value to be defined as surfaces.

File:Isosurface.gif	File:Iso ss.gif	(d)
	File:Iso cap.gif	(e)

Iso-Surfaces (a) No Fill (b) Fill Between Two Iso-Values (c) Fill Above or Below an Iso-Value (d) Filling between 2 iso-surfaces (e) Filling above an iso-surface.

Define as Cross-Section

Like contour lines, iso-surfaces are temporary in nature. In other words, if the active data set is changed, the current iso-surfaces are deleted and new iso-surfaces are computed using the new data set values. In some cases, it is useful to create an iso-surface as a permanent object. This can be accomplished by selecting the Define as cross section option in the Iso-surface Options dialog. This causes the computed iso-surfaces to be treated as cross sections. As cross sections, these iso-surfaces can be saved to a file, hidden, or deleted. In addition, if a new data set is selected, the iso-surfaces are not deleted. In fact, the values associated with the new data set are interpolated to the cross section iso-surfaces and can be displayed on the iso-surfaces as color fringes or contours. This makes it possible to effectively display two data sets at once.

Interior Edge Removal

By default, whenever an iso-surface is computed from a mesh or a grid, the lines corresponding to the intersection of the iso-surface with the cell or element boundaries are displayed on the iso-surfaces. If the Interior edge removal option is selected, only the edges on the iso-surface corresponding to a feature angle break greater than the specified value are displayed on the iso-surface. For example, if the feature angle were set at 30.00 degrees, the angle formed by the two polygonal faces adjacent to each edge in the iso-surface would be checked and only those edges where the computed angle is less than 180 - 30 = 150 degrees would be displayed. Typically, a small value (e.g., 0.001) is used so that only the edges adjacent to two coplanar faces are removed (made invisible).

Visible Region Only Option

If the Compute for visible region only option is chosen, the iso-surfaces are not computed in regions where the cells or elements are not visible. Otherwise, iso-surfaces are computed for all regions of the mesh or grid.

Iso-Surface Edges

If this option is on, the lines that make up the iso-surface are displayed.

Iso-Surface Faces

If this option is on, the iso-surface is displayed a color-filled surface.

Specify Range

The specified range command allows the user to control the minimum and maximum value used to define the colors assigned to the iso-surfaces.

Isosurface Volumes

The volume within an iso-surface or the volume between two iso-surfaces is computed using the Iso-Surface Volume command in the Data menu. This command brings up a dialog with a list of iso-values and volumes. The listed iso-values correspond to the iso-values defined in the Iso-Surface Options dialog. The listed volumes represent the volumes between each of the iso-values. For example, the first volume represents the volume below (on the "low" side of) the lowest iso-value, the second volume represents the volume between the first and second iso-values, etc. The total volume listed at the bottom of the dialog should correspond to the total volume of the grid or mesh.

Beginning with GMS 7.0 the Isosurface Volumes dialog has changed to display a single volume associated with each isosurface. In addition to showing a single volume, the user can also view the volume of the cells (or elements) that are less than the isovalue, the volume of the cells that are greater than the isovalue, and the volume of the cells intersected by the isosurface by turning on the Display additional volume information toggle. In some cases GMS is unable to compute the volume of the isosurface. In those cases the user can still have an estimate of the isosurface volume by looking at the volume of the cells. The volume of the cells should be close to the volume of the isosurface when the grid (or mesh) is composed of very "small" cells. The difference between the isosurface volume and the volume of the cells decreases as the size of the cells decreases.

See also

• Data Sets