To trim a surface is to use a curve on the surface to cut away part of the surface, or to cut a hole in the surface.
Before you trim a surface, you must create a curve on that surface. These are the kinds of curves that can trim surfaces:
- U iso and V iso curves
- Surface-surface intersection curve
- Normal projected curve
- Vector projected curve
- CV curve on surface
- Point curve on surface
Once you've created the curve, you trim the surface by turning on Trim in the curve sub-object's parameters. A Flip Trim control inverts the trim direction.
The direction of the curve determines the initial direction of the trim. For example, a closed curve on surface created in a clockwise direction trims inward, creating a hole in the surface; while a closed curve on surface created in a counterclockwise direction trims outward, creating a curve-shaped portion of the surface.
When a surface is trimmed, its untrimmed version is still present in the 3ds Max scene. You can select it for the purposes of editing it, or replacing it as a parent to a dependent sub-object. For details, see Sub-Object Selection.
Procedures
Example: To cut a hole in a CV surface:
- Create a CV surface in the Top viewport.
- Create a closed CV curve sub-object that lies on top of (or above) the surface.
- In the toolbox, turn on
(Normal Projected Curve), then in the Top viewport click first the CV curve, then the surface. This creates a projection of the CV curve that lies on the surface, and can trim it.
- In the normal projected curve's parameters, click to turn on Trim.
A hole appears in the surface. Depending on the orientation of the Normal Projected curve, you might see everything but the hole.
- Use the Flip Trim toggle to invert the trim. Note: Trims aren't displayed in viewports if the NURBS surface's Surface Trims toggle is turned off on the General rollout's Display group box.
Above: CV curve on surface
Below left: Using the curve to trim the surface
Below right: Using Flip Trim to change the trimming direction
To select an untrimmed surface:
- Make sure the Keyboard Shortcut Override toggle is on.
- At the appropriate sub-object level or during a replace parent operation, press H. This opens the Select Sub-Objects dialog, which is a subset of the Selection Floater that you can use during sub-object creation as well as sub-object selection.
- If the untrimmed version is selectable at this level, the trimmed version appears as a "tree," with a plus sign next to it. Click the plus sign to expand the tree. The child is the untrimmed version. Highlight its name to select it.