4. Assigning Material Properties and Raytracing

To recall the primitive shapes made in the previous lesson, start MGED and go to the File menu and select Open. A control panel will appear with a list of folders and files. Select shapes.g and Open. A new box will appear, and you should select OK.

You should now have two windows prominently displayed on your screen. At the MGED prompt in the Command Window, type:

draw sph2.s rcc2.s Enter

Before you can raytrace your design, you have to make a region of the two shapes. A region is an object that has uniform material properties. Most applications that use BRL-CAD models consider regions as the basic components of the model. Regions are constructed using the basic Boolean operations of union, intersection, and subtraction, which are discussed in the next chapter.

At the MGED prompt, type:

r shapes2.r u sph2.s u rcc2.s Enter

This command tells MGED to:

Now type in:

mater shapes2.r Enter

MGED will respond with:

Type in:

plastic Enter

MGED will ask:

Type in:

0 255 0 Enter

This will assign a light green color to the region. MGED will now ask:

Type:

0 Enter

When you have completed this set of commands, your Command Window should look like the following example:

The one-line version of this set of commands would be:

mater shapes2.r plastic 0 255 0 0 Enter

Diagrammed, this command says to:

An easy way to clear the Graphics Window of the old design and draw the new region is to type at the MGED prompt:

B shapes2.r Enter

This command tells MGED to:

The Blast command is shorthand for the combination of the Z and draw commands.

Now go to the File menu and select Raytrace. A dialog box called the Raytrace Control Panel will appear. At the top are menus for Framebuffer and Objects. Select Framebuffer. A drop-down menu will appear with six choices: Active, All, Rectangle Area, Overlay, Interlay, and Underlay. Make sure the Active, All, and Underlay options are activated (as shown by the presence of a red indicator to the left of each choice). Select OK.

Change the background color produced by the raytracer by selecting Background Color in the Raytrace Control Panel. A drop-down menu will appear with some predefined color choices plus a color tool. Select the white option. The select button should now appear white, in accordance with your selection.

Next select Raytrace from the four options along the bottom of the box. The Graphics Window should start changing, and you will soon see your design in shades of green with the wireframe superimposed on the design, as shown in the following example:

As we have seen, you can specify the background color for the raytraced image.

You can also fill the entire framebuffer with the background color. To do this, select the desired color and then click the fbclear (framebuffer clear) button at the bottom of the Raytrace Control Panel.

The Graphics Window of MGED is used to display different types of graphical information: 3D wireframes and 2D pixels (or images). Conceptually, each type of data occupies a separate layer in the display. The 3D wireframes occupy the wireframe layer, while the 2D pixels (images) occupy the framebuffer layer. These layers can be thought of as transparencies, and the order in which they are stacked and displayed can be changed.

As mentioned previously, there is a Framebuffer menu within the Raytrace Control Panel. At the top of this menu is a toggle button labeled Active. This turns the display of the framebuffer layer on and off. Near the bottom of the same menu are three radio buttons: Overlay, Interlay, and Underlay. When the underlay mode is selected, the pixel data are displayed under or behind the vector data. Conversely, when the overlay mode is selected, the pixel data are in front of the vector data. The interlay option is similar to the overlay mode. The subtle difference is an advanced topic not covered here.

To see how this works, go to the framebuffer menu and select Overlay. Notice that the wireframe representation disappears. Where does it go? If you answered "behind the framebuffer," you would be correct. To view the model’s geometry, you would have to make the framebuffer inactive or select underlay mode.

The wireframe layer has a yellow dot in the center that marks the center of the view talked about in Lesson 2. This allows you to determine whether the framebuffer is in overlay or underlay mode. If you can see the yellow dot, the framebuffer is in underlay mode. If you’ve told MGED to draw some geometry and the Graphics Window seems to remain blank, you are probably seeing a blank framebuffer masking the wireframe layer.

Note that you can change the view in the wireframe, but the view in the framebuffer does not automatically update to match. It is not possible to directly manipulate the view in the framebuffer. You must raytrace again in order to update the framebuffer image.

To completely clear the Graphics Window, you have to handle both the wireframe and framebuffer layers. Recall that you can clear the wireframe layer with the Z command. For the framebuffer layer, there is the fbclear button on the Raytrace Control Panel.

In some instances, you may prefer to turn off the framebuffer instead of clearing it. When the framebuffer is turned off, MGED runs faster because it doesn’t have to redraw the framebuffer each time it updates the display. You can turn the framebuffer on and off by toggling the Active item in the Raytrace Control Panel’s framebuffer menu.

In this lesson you: