Geometry Approximation on NURBS
NURBS curves and surfaces are approximated by straight segments to speed up processing and display. You can control the accuracy using geometry approximation.
Applying and Editing Geometry Approximation
By default, the scene root node has a Geometry Approximation property which is inherited by every object in the scene. You can apply a local Geometry Approximation property to selected objects, hierarchies in branch mode, groups, layers, and partitions. This allows you to control the display and rendering of different objects in different ways.
To apply a local Geometry Approximation property
1. Select an element: object, hierarchy (in branch mode), group, layer, or partition.
2. Choose Get > Property > Geometry Approximation from a toolbar.
To edit an existing Geometry Approximation property
1. Select an element.
2. Display its viewing or rendering properties, then click the Geometry Approximation tab.
Geometry Approximation on NURBS
Geometry Approximation is a multipurpose property that controls the display and rendering of polygon meshes, NURBS curves and surfaces, and objects with displacement maps. There are several tabs:
• Hardware Display controls how NURBS are displayed in the OpenGL views and is described in the following section.
• Surface, Surface Trim, and Displacement control how various elements are rendered. For more information, see Geometry Approximation in the Rendering guide.
• Polygon Mesh controls how polygons and subdivision surfaces are displayed and rendered. For more information, see Viewing and Rendering Polygon Meshes in the Polygon Modeling guide.
Hardware Display
The Hardware Display tab of the Geometric Approximations property editor lets you set the step value for approximating different elements:
• Surface objects along their U or V directions.
• Curve objects.
• Surface Curve elements. These include trims as well as surface curves — see Projecting and Trimming with Curves.
In all cases, the step value determines how many straight segments to use to approximate the exact shape of each NURBS segment. The higher the step value, the closer the approximation and you get smoother results. However, the trade-off is a longer processing time.
Note that this setting affects only the display; for example, objects on paths will follow the curve as mathematically defined, not as displayed by the path line.
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You can also select a curve or surface (or any of its components) and press the + and – keys on the numeric keypad to increase or decrease the steps. These keys automatically apply a local geometry approximation property on the object if none exists yet. |
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