Normal Handling

Video captions: This tutorial will give you an overview of how to correct surface normals and will provide an example to explain the steps involved. To get an illustration of the direction of face normals in VRED, we need the Vertex/Face Normal Rendering mode.

To activate this, select Vertex/Face Normal Rendering from the Visualization menu in the main menu bar. All objects in the scene will be visualized with the colors green, blue, gold, magenta, red, or pink, which give us an overview over the normal directions of the geometry.

Each polygon of the geometry contains surface normal directions at every vertex point defining the surface. Every color in our vertex face mode describes the respective condition of the geometry’s normal direction.

The normals of the geometry must always be turned outwards. This doesn't necessarily mean towards the camera. Imagine the following example, we are looking directly at a double-walled glass panel. The panel is made up of two surfaces, each rotated outwards, so always green, pointing at the camera. If the panel was only one sided, it would be blue on one side of the hemisphere and green on the other. For this principle to function, all objects should be modelled with volume, accordingly, even a double-walled glass panel.

A green coloration of the geometry indicates the face and vertices are consistent and point in the direction of the camera. This state illustrates the correct normal direction in a workflow.

A blue coloration signals that face and vertices are consistent but are pointing away from the camera.

The golden coloration warns us that face and vertices are inconsistent, with the face normals pointing away from the camera and the vertex normals pointing at it.

Magenta addresses inconsistent face and vertex normals, indicating the face normals pointing towards the camera and the vertex normals pointing away.

Red normals are NaN (not a number, but an undefined or unrepresentable value) or have a zero length, which creates NaNs when normalized.

Pink normals have an absolute angle between the face and vertex normal above a threshold, such as 85°.

In our example, the normals of the surface hood, hood air outtake, and wings are each colored differently, as the normals don't have a corrected direction. To correct them, we need the Geometry Editor.

In the main menu, select Scene > Geometry Editor. In the Normal Calculation section, we find all the functionality for modifying and correcting vertex or face normals of the geometry.

The Crease Angle provides the evaluation angle. The value, over which, surrounding vertices are viewed. Vertices that lie within this specific crease angle will be smoothed within this range. If you define a high angle, more vertices are detected, which results in a softer surface. With a small angle, the surfaces are consequently harder and more faceted. Attention is required when using this function, as the original vertex information will be overwritten.

We can use the option, Flip Normals, to influence the direction of our normals and correct them for further use in our workflow. In the toolbar, click Boundings to enable the bounding box.

First, we will select the hood (UV_body), which is colored blue. To do so, hold down the Shift key and left-click the surface in our render view. As mentioned before, a blue tint indicates the face and vertex normals are consistent but are facing away from the camera.

This means, that in this case, the face and vertex normals need to be turned. To do that, click the Face + Vertex button next to Flip Normals. As you see, the surface is now colorized green, as all normals have been correctly rotated.

Next, we will select the wings (Wings) and the Vertex button, as the magenta coloration of the surface indicates the face and vertex normals are inconsistent, though the face normal is pointing towards the camera. This means, for this case, that only the vertex normals need to be correctly rotated.

Finally, we must correct the hood air outtake (Airouttake_hood) normals. We will select the air outtake, then click the Face/Surface button. We know that a golden coloration of the geometry indicates that face and vertex normals are inconsistent. However, only vertex normals are pointing away from the camera; therefore, only these must be turned.