3.2.3. Contour Plot

This plot, shown in Figure 3.14, displays the location of values for scalar variables like density or pressure using lines for 2D plots and surfaces for 3D plots. In visualization terms, these plots are isosurfaces. VisIt’s Contour plot allows you to specify the number of contours to display as well as the colors and opacities of the contours.

Fig. 3.14 Example of Contour plot

Fig. 3.15 Contour plot attributes window

3.2.3.1. Setting the number of contours

By default, VisIt constructs 10 levels into which the data fall. These levels are linearly interpolated values between the data minimum and data maximum. However, you can set your own number of levels, specify the levels you want to see or indicate the percentages for the levels.

To choose how levels are specified, make a selection from the Select by menu. The available options are: N levels, Levels, and Percent. N levels, the default method, allows you to specify the number of levels which will be generated, with 10 being the default. Levels requires you to specify floating point numbers for the levels you want to see. Percent takes a list of percentages like 50.5, 60, and 40.0. Using the numbers just mentioned, the first contour would be placed at the value which is 50.5% of the way between the minimum and maximum data values. The next contour would be placed at the value which is 60% of the way between the minimum and maximum data values, and so forth. You specify all values for setting the number of contours by typing into the text field to the right of the Select by menu.

3.2.3.2. Setting Limits

The Contour plot attributes window provides controls that allow you to specify artificial minima and maxima for the data in the plot. This is useful when you have a small range of values that are of interest and you only want the contours to be generated through that range. To set the minimum value, click the Min check box to enable the Min text field and then type a new minimum value into the text field. To set the maximum value, click the Max check box to enable the Max text field and then type a new maximum value into the text field. Note that either the min, max or both can be specified. If neither minimum nor maximum values are specified, VisIt uses the minimum and maximum values in the database.

3.2.3.3. Scaling

The Contour plot typically creates contours through a range of values by linearly interpolating to the next value. You can also change the scale to a logarithmic function to get the list of contour values through the specified range. To change the scale, click either the Linear or Log radio buttons in the Contour plot attributes window.

3.2.3.4. Setting contour colors

The main portion of the Contour plot attributes window, also known as the Contour colors area, is devoted to setting contour colors. Contour plot colors can be assigned three different ways, the first of which uses a color table. A color table is a named palette of colors that you can customize to suite your needs. When the Contour plot uses a color table to color the levels, it selects colors that are evenly spaced through the color table based on the number of levels. For example, if you have five levels and you are coloring them using the “rainbow” color table, the Contour plot picks five colors out of the color table so your levels are colored magenta, blue, cyan, green, yellow, and red. The colors change when increasing or decreasing the number of levels when you use a color table because VisIt uses the new number of levels to sample different locations in the color table. As a rule, increasing the number of levels results in coloration that is closer to the color table because more colors from the color table are represented. To color a Contour plot with a color table, click on the Color table radio button and choose a color table from the Color table menu to right of the Color table radio button.

If you want all levels to be the same color, click the Single radio button at the top of the Contour plot attributes window and select a new color from the Popup color menu that is activated by clicking on the Single color button. The opacity slider next to the Single **color button sets the opacity for the single color.

Clicking the Multiple radio button causes each level to be a different, user-specified color. By default, multiple colors are set using the colors of the discrete color table that is active when the Contour plot is created. To change the color for any of the levels, click on the level’s Color button and select a new color from the Popup color menu. To change the opacity for a level, move its opacity slider to the left to make the level more transparent or move the slider to the right to make the level more opaque.

3.2.3.5. Wireframe view

The Contour plot attributes window provides a Wireframe toggle button used to draw only the lines along the edges of the contour. This option only has an effect on 3D Contour plots.