Normal Handling

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

To activate the Vertex/Face Normal Rendering, please 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, orange, magenta, red, or pink, which give us an overview over the normal directions of our geometry. Each polygon of a geometry contains the normal directions of the surface at every vertex point that defines the surface. Every color in our vertex face mode describes the respective condition of the normal's direction of our geometry.

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. Would the panel only be single 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, also, a double-walled glass panel.

A green coloration of the geometry indicates that 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, passenger door, and fender are each colored differently, as the normals don't have a corrected direction. To correct them, we need the Geometry Editor. Please select it from the main menu, under Scene > Geometry Editor. Under the Normal Calculations tab, we find all the functionality to modify and correct vertex or face normals of a geometry.

The Crease Angle is 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, when using this function, the original vertex information is overwritten.

In the sub-item, Flip Normals, we can influence the direction of our normals and correct them for further use in our workflow.

Please activate the bounding box in your toolbar. First, we select the hood, which is colored blue. To do so, hold down the Shift key and select, with the use of the left mouse button, the surface in our render view. As mentioned before, a blue tint means 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, please select Face and Vertex under Flip Normals. As you see, the surface is colorized green, as all normals have been correctly rotated.

For the next step, we select the fender and Vertex from the sub-menu, 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.

As a last thing, we have to correct the normals of the passenger door. Therefore, we select the passenger door and proceed to select Face/Surface in the sub-menu. We know that a golden coloration of the geometry describes that face and vertex normals are inconsistent. Yet, only vertex normals are pointing away from the camera; therefore, only these have to be turned.