The Trim/Extend modifier is primarily used to clean up overlapping or open splines in a multi-spline shape so that lines meet at a single point. As with the Fillet/Chamfer modifier, this modifier operates on the splines at the sub-object level in the shape. It does not work between two or more independent shape objects.
To trim you need intersecting splines. You click the portion of the spline you want to remove. The spline is searched along its length until it hits an intersecting spline, and deleted up to the intersection. If the section intersects at both ends, the entire section is deleted up to the two intersections. If the section is open on one end and intersects at the other, the entire section is deleted up to the intersection and the open end. If the section is not intersected, nothing happens.
To extend you need an open spline. The nearest end of the picked point is extended until it reaches an intersecting spline. If there is not intersecting spline, nothing happens. Curved splines extend in a direction tangent to the end of the spline. If the end of a spline lies directly on a boundary (an intersecting spline), the it looks for an intersection further along.
Pick Locations: Click this to turn on pick mode. While in this mode, click any spline to either trim or extend it, based on the settings below.
The items in this area specify the type of operation that’s performed on the picked spline.
Auto: When this is chosen, a Trim is first looked for and, if not found, an Extend is attempted. In most cases, a Trim will occur when Auto is chosen. An Extend can occur, however, in cases where an open spline exists without intersecting other splines.
Trim Only: Choose this to perform only trims. Turn on Pick Locations, and then click the spline section you want to trim.
Extend Only : Choose this to perform only extends. Turn on Pick Locations, and then click the open spline section you want to extend.
Infinite Boundaries : For the purposes of calculating intersections, check this to treat open splines as infinite in length. For example, this lets you trim one linear spline against the extended length of another line that it doesn't actually intersect.
Note: As the number of open splines in the shape increases, the chance of finding an intersection, when using Infinite Boundaries, increases as well. This can produce results you might not have expected because of projected spline intersections you hadn’t consideredparticularly if you’re in Auto mode. For predictable results, avoid using Auto mode when using Infinite Boundaries.
The options in this area specify how the Trim and Extend determine a valid intersection.
View : Projects the lines onto the active viewport, and judges the intersections accordingly. These are the intersections as you see them in the active viewport.
Construction Plane : Projects the lines onto the current construction plane.
None (3D) : Considers only true intersections as the splines exist in 3D space. They must physically intersect to be considered.