Creases, Insert Edge and Topology Tools

Overview#

A smooth T-spline never produces a sharp edge on its own. To get a hard corner, panel line, or razor edge, you either crease an edge, or bring cage edges very close together. You also need tools to add cage detail where you need it (Insert Edge, Insert Point) and to join or heal separate parts of a form (Bridge, Merge Edge, Fill Hole, Weld Vertices). Together these are the topology tools.

Where to find it#

Form tab > Modify panel:

• Crease and Uncrease
• Insert Edge and Insert Point
• Merge Edge
• Bridge
• Fill Hole
• Weld Vertices and Unweld Edges
• Bevel Edge, Slide Edge
• Delete (Delete key)

Tool grouping can differ between releases; use the command search if you cannot find one.

Step by step#

Crease an edge#

1. Choose Modify > Crease.
2. Select one or more cage edges (or a loop; double-click selects a loop).
3. Click OK. Crease has no weight or angle field in Autodesk's help for the tool: the edge becomes sharp. If the creased edge is on a flat area you may need to move it with Edit Form before you see the effect.
4. Yellow highlighted regions show where the crease fade-off will be short (around star/cap points). Vertices can only be creased or uncreased at outside corners.
5. To remove, choose Modify > UnCrease, select the creased edges and click OK.

A Crease is full sharpness. Autodesk's two routes to a smooth crease are to move adjacent edges closer together, or to add a sharp crease, convert the T-spline to a solid or surface, and use the Fillet tool to add a small radius.

Insert Edge#

1. Choose Modify > Insert Edge.
2. Select an edge or loop. A preview line shows where the new edge loop will go. The Insertion Mode is Simple (no other points move, but the surface shape is likely to change) or Exact (extra points are added to maintain the exact surface shape).
3. Choose Insert Side (Single or Both) and set the Insert Location (a decimal percentage) by dragging the manipulator or typing a value. Object Snap and Offset are also available.
4. Click OK.

Use Insert Edge to add resolution for a feature (such as a recess or a handle) or to put two loops either side of a crease to tighten it.

Insert Point#

Adds edges within a face by picking points on its edges: select a point on an edge, keep picking points on other edges, then double-click to finish. Insertion Mode is Simple or Exact. Pick the red midpoint snap points to minimise the change. Autodesk warns that Insert Point can have unexpected effects when it creates star points, triangles or n-sided faces, and that the usual error is a body that only displays in Box mode, which Repair Body can fix.

Merge Edge#

1. Choose Modify > Merge Edge.
2. Select the first group of edges (Edge Group One), then Edge Group Two (in the same body or a second body).
3. Click OK. The first group moves to meet the second, joining boundary edges and closing gaps. Use it to combine two bodies or to eliminate gaps between faces of one body. Maintain Crease Edges keeps the crease at the merged edges. Merge Edge and Weld Vertices perform similar operations; the difference is that you select edges rather than vertices.

Bridge#

1. Choose Modify > Bridge.
2. Select the Side One faces/edges and Side Two faces/edges (for example, two holes' edge loops).
3. Set the number of connecting Faces, the number of Twists (complete rotations between the sides), use Flip Direction1/2 if needed, optionally use Follow Curve to guide the bridge along a curve, and Maintain Crease Edges to keep creases. Click OK.

Bridge creates a handle or tunnel by connecting two openings, or joins two bodies. The number of edges on each side must be equal or a multiple of each other.

Fill Hole#

1. Choose Modify > Fill Hole.
2. Select one edge of the hole. Choose the fill mode: Reduced Star (minimum number of star points), Fill Star (a single face, creating star points at each vertex) or Collapse (all vertices collapse to a central point). Optionally check Keep Creases.
3. Click OK.

Fill Hole plugs open ends, such as the cut end of a Pipe or an opening you made temporarily while shaping.

Weld and Unweld#

• Weld Vertices has three Weld Mode settings: Vertex to Vertex (the first vertex moves onto the second), Vertex to Midpoint (both move to the midpoint) and Vertex to Tolerance (a window selection welds everything within the Weld Tolerance, including hidden vertices). Maintain Crease Edges keeps creases along modified edges.
• Unweld Edges disconnects an edge or loop (hold Ctrl for multiple edges); a darker line marks an unwelded edge. It is roughly the Form equivalent of cutting a seam.

Options reference#

Tips and pitfalls#

Tip: Autodesk notes that edges placed closer together produce sharper corners, so a crisp-but-not-razor edge can be made with two loops (Insert Edge or Slide Edge) close together on either side of the ridge, without a crease.

Tip: Creases that end in the middle of a surface create ugly pinches. Run a crease all the way to a boundary, to another crease, or to a star point.

Tip: Do not over-crease. If most edges are creased, you are fighting the tool and should consider starting from solid modeling instead.

Warning: Crease does not exist as a concept in the finished BRep. After Finish Form, the creased edge becomes a true sharp edge between two surfaces; the neighboring surfaces keep their curvature. You can fillet that edge afterwards with the Solid Fillet tool.

Warning: Insert Edge may continue through the cage (general behaviour) and add edges where you did not want them, such as on the back of the model. Check the far side after inserting.

Warning: When using Bridge, the edge counts on each side must be equal or multiples of each other, and the loops' directions can cause a twist. Adjust Twists or Flip Direction in the dialog rather than redoing the operation.

Note: Merge Edge joins two sets of edges; it is not a way to delete an edge loop.

How a crease really behaves#

A crease changes the subdivision rule along an edge, so the surface stops blending between the two faces and instead meets them at a corner. The faces on either side keep their own smooth curvature up to the crease. Three behaviours follow from that.

• Where the crease ends matters. Autodesk notes that regions around star/cap points cause creases to end abruptly (shown in yellow). A crease that ends at an ordinary vertex in the middle of a surface can leave a visible dimple (general experience), so end creases on boundaries, other creases or star points.
• Creased loops and the finished body. After Finish Form a creased edge becomes a true edge between two BRep faces. You can fillet it with the Solid Fillet command afterward, which is usually a better way to get a controlled radius than any Form technique. For a 2 mm radius on a 60 mm part, crease first and fillet in Solid.
• Creases and symmetry. If symmetry is active, creasing an edge creases the mirrored edge as well. Isolate Symmetry (on the Symmetry panel) excludes selected elements from the symmetry if you want an asymmetric crease. (Whether creases mirror under symmetry is general behaviour; the Crease help page does not state it.)

Choosing between crease, close loops and fillet#

Insert Edge in detail#

Insert Edge adds a loop of edges that runs through the quads of the selected span. Autodesk's reference lists an Insertion Mode (Simple or Exact), an Insert Side (Single or Both) and an Insert Location, and the insertion mode is the key decision.

• Simple does not move any other points, but the surface shape will likely change. It avoids adding extra cage faces.
• Exact adds points so the surface shape is maintained. Use it when you want to add detail to a form that already matches a reference. It increases face count more than Simple does.

Worked example: you have a 100 mm long box with 4 faces along its length and you want a ridge 10 mm from the end. The first face is 25 mm long. Select the edge at the end of the first row, start Insert Edge, and set the Insert Location so the new loop sits at 40 percent of the span (10 mm). The faces become 10 mm and 15 mm. The smooth surface will bulge slightly to follow, so check afterward with Measure.

Insert Point inserts an edge by selecting points on edges. Use it when you want a partial cut across a face or a localized detail, not a full loop. The result introduces T-junctions, which T-splines allow (Autodesk notes T points are curvature continuous).

Bridge, Fill Hole and Erase and Fill#

Bridge connects two sets of opposing edges or faces with new faces. The usual use is a handle: delete a face on each side of a body and bridge the holes. The edge counts on the two sides must be equal or multiples of each other. If counts differ, first use Subdivide or Insert Edge on the side with fewer edges. When the bridged path is curved, use Follow Curve.

Fill Hole closes an internal hole and offers three modes: Reduced Star (fewest star points), Fill Star (one face, a star point at each vertex) and Collapse (all vertices to a central point). Autodesk advises using as few star points as possible, using Fill Star only if minimal shape distortion is required, and using Reduced Star to maintain symmetry.

Erase and Fill deletes connected geometry you select and fills the gap with faces, with Reduced Star and Fill Star modes. It can remove struts or bridges. Alt+H / Ctrl+H toggles Feature Selection there.

Common failures and fixes#

Practice exercise#

Make a rounded tray with a sharp rim and a controlled inner transition.

1. Create a Box 100 x 70 x 20 mm with 3 x 3 x 2 faces on a horizontal plane. Do not enable symmetry yet.
2. Select the top face and press Delete to open the box, and confirm the top shows an opening.
3. Select the top rim edge loop (double-click an edge of the rim) and apply Crease. Confirm the rim line looks different in the cage.
4. Select the vertical edge loop around the walls and use Insert Edge in Simple mode, placing it at an Insert Location about 15 percent from the top. Notice how the wall tightens near the rim.
5. Repeat Insert Edge in Exact mode near the base at 15 percent from the bottom. Compare how many cage faces each mode added.
6. Move the corner vertices outward with Edit Form to round the plan, a few millimetres at a time.
7. Click Modify > Thicken, set a 2 mm Thickness, and finish the form.
8. Select the rim edge on the finished body and add a 1 mm Solid Fillet to compare with the sharp rim.

Expected result: an open tray with smooth walls and a crisp rim that you can fillet. A common mistake is selecting only part of the loop for Crease, which leaves a half-sharp rim; if you see that, Uncrease and reselect with a double-click.

Related#

• Edit Form
• Subdivide and refine
• Finish Form and conversion