Creating Curves from Other Objects

 
 
 

You can use other objects in your scene to create curves. A modeling relation exists between the new curve and the input objects used to define it (unless, of course, you are modeling in immediate mode). Modifying the originals in any way also modifies the created curve.

In addition, you can select the created curve and open the property editor of the corresponding operator to adjust its parameters. The modeling relation exists until you freeze the generator operator (the bottom node in the created curve's geometry stack).

Extracting Curve Segments

To extract an arbitrary segment of an existing curve

  1. Select an isopoint on a curve.

  2. Choose Create Curve Extract Segment from the Model toolbar.

  3. Pick another isopoint on the curve.

    This creates a new curve and the Extract Curve Segment property editor opens. If desired, use the controls on the Curve Point Adaptor pages to adjust the start and end points.

    The original curve and the extracted segment.

Extracting Curves from Edges

You can extract a linear curve from edges on a polygon mesh object.

To extract curves from edges

  1. Select some edges on polygon mesh objects.

    • Disconnected edges on the same object become separate subcurves of a single curve object.

    • Edges on separate polygon meshes become separate curve objects.

  2. Choose Create Curve Extract from Edges.

    New curves are created and the Extract Edge Loop property editor opens. By default, there is a single linear span for each edge in the original polygon mesh.

  3. Adjust the Mesh Subdivision Level. Higher values produce smoother, denser curves, as if the original polygon mesh was subdivided using the Catmull-Clarke rule.

Extracting Curves from Surfaces

To create a curve object based on a curve component of a surface

  1. Select one or more isoline, knot curve, boundary, surface (projected), or trim curve on a surface.

  2. Choose Create Curve Extract from Surface from the Model toolbar.

    If you did not select any curves to extract in step 1, you are prompted to pick a curve now.

    By default, the picking session selects a U isoline with the Rectangle tool, but you can change the filter using the buttons on the Select panel and you can change the tool using the commands on the Select Tools menu or the keyboard equivalent (for example, F10 for the Raycast tool).

    A new object is created for each selected curve and the Extract Curve from Surface property editor opens. The curves are automatically closed if the surface is closed in the corresponding direction; otherwise the curves are open.

  3. If desired, use the controls on the Surface Curve Adaptor page to adjust the extracted curve.

    In the case of assembled surface meshes, the Extract on Whole Mesh option lets you extract the curve across multiple subsurfaces. For more information about surface meshes in general, see Surface Meshes.

Extracting Curves from Subcurves

Some curve objects, particularly text and imported EPS curves, contain multiple subcurves. You can extract subcurves to create new curve objects.

To extract subcurves

  1. Select one or more subcurves, or a cluster of subcurves, on a curve object.

  2. Choose Create Curve Extract from Subcurves from the Model toolbar.

Fitting Curves onto Curves

You can fit one curve onto another. This is useful for cleaning curves that were drawn with the Sketch tool and for increasing or decreasing the number of points on a curve. You can specify the number of points and the degree of the new curve. The parameterization of the new curve is uniform.

The original sketched curve and a new curve fitted onto it.

To fit one curve onto another

  1. Select a curve.

  2. Choose Create Curve Fit on Curve from the Model toolbar.

    If you didn't select a curve in step 1, you can pick one now.

  3. A new curve is created and the Fit Curve property editor opens. Set values as desired:

    • Subdivisions controls the number of control points on the created curve and (indirectly) how accurately the new curve follows the original.

    • Degree controls whether the curve is a Cubic or Linear NURBS.

    • Close does not create a true periodic curve ; instead, it superimposes the last point on the first.

Creating Curves from Intersecting Surfaces

You can create a curve that represents the intersection of two surfaces. For best results, first make sure that the surfaces have the same parameterization. For more information, see Reparameterizing Surfaces.

   

Two surface spheres and the curve formed by their intersection.

To intersect surfaces

  1. Select two intersecting surfaces.

  2. Choose Create Curve Intersect Surfaces from the Model toolbar.

    If you didn't select both surfaces in step 1, you can pick them when prompted.

  3. This creates a curve and the Intersect Surfaces property editor opens.

    • If the two surfaces intersect in multiple places, the new curve will be composed of multiple disjoint segments. However, the curve is one single 3D object as shown in the explorer.

    • If the curve cannot be calculated, for example if the two surfaces do not intersect, the result is a unit circle at the global origin.

  4. You may need to adjust the tolerance parameters for the curve.

Note

If the intersection curve does not seem right, try increasing the subdivisions of the surfaces.

If you intersect assembled surface meshes with multiple subsurfaces, a single NURBS Curve List object is created with separate subcurves that start and end at the junctions of the subsurfaces.

Blending Curves

Blending two curves creates a third curve that joins the two originals. Unlike Merging, this feature simply joins two existing curves without creating a new curve.

The orginal curves and the new blend curve between them.

To blend curves

  1. Select two curve boundaries.

  2. Choose Create Curve Blend from the Model toolbar.

    • If you didn't pick both curve boundaries in step 1, you can pick them now.

    • To blend between the two boundaries of a single curve, you must choose the command before picking one or both curve boundaries.

  3. This creates a third curve and the Blend Curves property editor opens.

    If necessary, adjust the parameters of the blend curve. For more information, see Blend Curves Property Editor [Properties Reference].

    If you selected curve objects instead of their boundaries, the blend curve may not be connected at the desired ends. Use the controls in the Curve Point Adaptor pages to adjust the start and end points.

As an alternative to blending curves, you can merge them as described on Merging Curves.

Filleting Curves

You can create a curve that is a fillet, or smooth blending arc, between two curves. Depending on how the curves are drawn and the options you set, the two original curves do not need to intersect.

Two intersecting curves and the fillet curve between them.

To fillet curves

  1. Select two intersecting curves.

  2. Choose Create Curve Fillet Intersection from the Model toolbar.

    If you didn't pick both curves in step 1, you can pick them now.

  3. An arc is drawn from the beginning of the first curve selected to the end of the second and the Fillet Curves property editor appears.

    NoteIf the two curves intersect in multiple places, the new curve will be composed of multiple disjoint segments. However, the curve is one single 3D object as shown in the explorer.
  4. Adjust the Radius and the Tolerance. If no fillet can be created with the specified settings, a unit circle is created at the origin.

Tip

To change where the fillet is created on the crossing curves, invert one or both curves using Modify Curve Inverse.

For a non-circular fillet, you can move points or otherwise deform the created curve. The modeling relation with the original curve remains, so that if you modify the original curves the deformed fillet changes accordingly.

As an alternative to filleting curves, you can blend curves as described in the previous section or merge them as described in the next section.

Merging Curves

Merging two curves creates a third curve that spans the original two. Unlike Blending, this feature actually creates an extra curve.

Two separate curves and a single merged curve.

To merge curves

  1. Select two curve boundaries.

  2. Choose Create Curve Merge from the Model toolbar.

    • If you didn't pick both curve boundaries in step 1, you can pick them now.

    • To merge across the two boundaries of a single curve, you must choose the command before picking one or both curve boundaries.

  3. This creates a third curve spanning the originals and the Merge Curves property editor opens.

    If necessary, adjust the parameters of the merged curve. For more information, see Merge Curves Property Editor [Properties Reference].

    If you selected curve objects instead of their boundaries, the merged curve may not span the desired ends. Use the controls in the Curve Point Adaptor pages to adjust the start and end points.

As an alternative to merging curves, you can also stitch them as described on Stitching Curves or blend them as described on Blending Curves.

Creating Curves from Animation

If you have animated the translation of an object, you can plot the motion of its center to generate a curve. For example, this can be used to create a trajectory curve. You can also plot the movement of a selected point or cluster.

To create a curve from an animation

  1. Select an object, point, or cluster.

  2. Choose Tools Plot Curve from the Animate toolbar. The Plot Curve dialog box opens.

  3. Set the options as desired:

    • The Start Frame and End Frame specify the frames at which to begin and end plotting.

    • The Step Value controls the sampling of the generated curve. If this value is 1, a point on the curve is created for every frame; if this value is two, a point is created for every other frame; and so on.

    • Curve Name is the name to use for the generated curve object.

  4. Click OK. The scene is played back and a curve is created. The original animation remains.