Empirical framework

The surface of an object consists of polygonal faces. The

orientation of these faces depends on the direction of the surface normals. Normals are imaginary vectors that are located

perpendicular to one side of each face. The normals should face toward the viewer (or camera) for the applied materials to display correctly. The imported geometry should automatically have its surface normal direction pointing towards the camera, but sometimes it needs to be corrected.

Checking the Surface Normals

It is important that you verify that all of the normals are facing towards the camera and correct the surfaces that have reversed normals before applying materials or editing the geometry. In the Menu Bar, select Visualization>Vertex/Face Normal

Rendering, or press <F5> to turn on the Vertex/Face Normal rendering mode. The surfaces can display in four different colors, which can indicate different problems with the geometry, as shown in Figure 2–20.

Learning Objective

• Check and fix the surface normals of the imported geometry.

Gold

Green

Violet

Green surfaces render correctly, but surfaces that display in blue, gold, or violet have issues that must to be corrected, as described below:

• Green: Indicates that the faces and the vertices are consistent, as well as their normals are facing towards the camera. Green surfaces render correctly.

• Blue: Indicates that the faces and vertices of the geometry are consistent, but that all of their normals are pointing away from the camera.

• Gold: Indicates that the faces and vertices of the geometry are inconsistent. The face normals are turned away from the camera, and the vertex normals are pointing towards the camera.

• Violet: Indicates that the faces and vertices of the geometry are inconsistent. the face normals pointing towards the camera, and the vertex normals turned away from the camera.

Correcting the Surface Normals

You can correct issues with the surface normals using the Geometry Editor module, as shown in Figure 2–21. To open the Geometry Editor module, in the Menu Bar, select Scene>

Geometry Editor. or use the Icons or Quick Access Bar’s shortcut menu. In the Normals tab, in the Normal Calculations rollout, the Flip Normals area provides you with the options for correcting the orientation of the surface normals.

The options in the Flip Normals area are as follows:

Correcting the Surface Normals for Portions of a Single Geometry

There are times when some geometry portions of an object are not green and requires correcting. Selecting the part selects the complete object, including the parts that are already green and do not need correcting. In situations like those, you cannot use the Flip Normal options available in the Geometry Editor. You can correct the surface normals of those geometry portions by holding <Alt> and then right-clicking on the portion that needs correcting. <Alt> and right-clicking cycles through all of the colors. While holding <Alt>, continue right-clicking on the portion of the object until it becomes green.

Option Description

Use this to repair surfaces that display as blue. This reverses the normals for all of the faces and vertices so that they point towards the camera.

Use this to repair surfaces that display as gold. This reverses the face normals so that they point towards the camera.

Use this to repair surfaces that display as violet.

This reverses the normals of the vertices so that they point towards the camera.

Hint: Using the Adjacency Selection>Crease Angle option to Correct Surface Normals

If the colors consistently change to other colors except green, even after using the <Alt> and right-click option many times, use the Crease Angle in the Adjacency Selection rollout of the Geometry Editor, as shown in Figure 2–22. Reduce the number in small increments (i.e., change it to 40, then 35, etc.) and with each reduction step try the <Alt> and right-click option on the faulty geometry. Continue to reduce the Crease Angle until all of the geometry is green, and then return Crease Angle to the default value of 45.00.

Figure 2–22

How To: Fix Surface Normals

1. Check the normals by selecting Visualization>Vertex/Face Normal Rendering, or press <F5>, to enter the Vertex/Face Normal rendering mode.

2. Select Scene>Geometry Editor to open the Geometry Editor module.

3. In the Render Window, select the geometry which is not green in color.

4. Depending on the color, click on the appropriate button in the Geometry Editor module, in the Flip Normals area. If a portion of a single object needs correction, use the <Alt> + right-click option.

5. Select Visualization>Realistic Rendering, or press <F3>, to return to the Realisitic Rendering mode.

Hint: Using the Normal Calculations>Crease Angle Option In the Geometry Editor module, in the Normals tab, in the Normal Calculations rollout, you can set the Crease Angle, as shown in Figure 2–23. This is an evaluation angle, which defines the smooth angle for the vertices. A higher Crease Angle value creates a smoother surface, while as a lower number makes the geometry more faceted. Changing this value overwrites the original vertex information, and should be changed with caution.

Figure 2–23

2.4 Transforming Geometry