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:
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
sub-object. For details, see
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
NoteTrims 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
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
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.