29.6.2 Changing an Object's Display Properties

To enhance the scene in the graphics window, you can change the color, visibility, and other display properties of each geometric object in the scene. You can specify different colors for displaying the edges and faces of a mesh object to show the underlying mesh (edges) when the faces of the mesh are filled and shaded. You can also make a selected object temporarily invisible. If, for example, you are displaying the entire mesh for a complicated problem, you can make objects visible or invisible to display only certain boundary zones of the mesh without regenerating the mesh display using the Mesh Display dialog box. You can also use the visibility controls to manipulate geometric objects for efficient graphics display or for the creation of animations. These features, plus several others, are available in the Display Properties dialog box (Figure  29.6.2).

Figure 29.6.2: The Display Properties Dialog Box

To set the display properties described above, select one or more objects in the Names list in the Scene Description dialog box and then click the Display... button to open the Display Properties dialog box for that object or group of objects.

Controlling Visibility

There are several ways for you to control the visibility of an object. All visibility options are listed under the Visibility heading in the Display Properties dialog box.

• To make the selected object(s) invisible, turn off the Visible option. To "undo'' invisibility, simply turn the Visible option on again.

• To turn the effect of lighting for the selected object(s) on or off, use the Lighting check button. You can choose to have lighting affect only certain objects instead of all of them. Note that if Lighting is turned on for an object such as a contour or vector plot, the colors in the plot will not be exactly the same as those in the colormap at the left of the display.

• To toggle the filled display of faces for the selected mesh or surface object(s), use the Faces option. Turning Faces on here has the same effect as turning it on for the entire mesh in the Mesh Display dialog box.

• To turn the display of outer edges on or off, use the Outer Faces option. This option is useful for displaying both sides of a slit wall. By default, when you display a slit wall, one side will "bleed'' through to the other. When you turn off the Outer Faces option, the display of a slit wall will show each side distinctly as you rotate the display. This option can also be useful for displaying two-sided walls (i.e., walls with fluid or solid cells on both sides).

• To turn the display of interior and exterior edges of the geometric object(s) on or off, use the Edges option.

• To turn the display of the outline of the geometric object(s) on or off, use the Perimeter Edges check button.

• To toggle the display of feature lines (described in Section  29.1.1), if any, for the selected object(s), use the Feature Edges option.

• To toggle the display of the lines (if any) in the geometric object(s), use the Lines check button. Pathlines, line contours, and vectors are "lines''.

• To toggle the display of nodes (if any) in the geometric object(s), use the Nodes check button.

Once you have set the appropriate display parameters, click the Apply button to update the graphics display.

Controlling Object Color and Transparency

The Display Properties dialog box also lets you control an object's color and how transparent it is. All color and transparency options are listed under the Colors heading.

• To modify the color of faces, edges, or lines in the selected object(s), choose face-color, edge-color, line-color, or node-color in the Color drop-down list. The Red, Green, and Blue color scales will show the RGB components of the face, edge or line color, which you can modify by moving the sliders on the color scales. When you are satisfied with the color specification, click Apply to save it and update the display. The ability to set the colors for faces and edges can be useful when you wish to have a filled display for the mesh or surface, but you also want to be able to see the mesh lines. You can achieve this effect by specifying different colors for the faces and the edges.

• To set the relative transparency of an object, select face-color in the Color drop-down list. Move the slider on the Transparency scale and click the Apply button to update the graphics display. An object with a transparency of 0 is opaque, and an object with a transparency of 100 is transparent. By specifying a high transparency value for the walls of a pipe, for example, you will be able to see contours that you have displayed on cross-sections inside the pipe. (This feature is available on all platforms when the software $z$ buffer is used for hidden surface removal, but if your display hardware supports transparency, it will be more efficient to use the hardware $z$ buffer as the hidden surface method instead. You can select these methods in the Display Options dialog box, as described in Section  29.2.7.)

If you save a picture of a display with transparent surfaces, you should not set the File Type in the Save Picture dialog box to Vector.