Subdivide and Refining Form Topology

Overview#

Good Form modeling follows a rhythm: block out with few faces, then add resolution only where you need it. Subdivide is the main tool for adding resolution evenly. Together with Insert Edge, Thicken, Flatten, Make Uniform and Delete, it lets you refine a cage without ruining its flow.

Where to find it#

Form tab > Modify panel:

• Subdivide, Thicken, Flatten, Interpolate, Match, Pull, Insert Edge and Merge Edge are on Form > Modify.
• Make Uniform is on the Utilities panel (Autodesk's reference also lists it as Modify > Utilities > Make Uniform).
• Delete removes selected T-spline geometry (select it, then press Delete).

Step by step#

Subdivide#

1. Choose Modify > Subdivide and select the faces to split. Hold Ctrl (Cmd on macOS) to add or remove faces.
2. Set the number of divisions: for quadrilateral faces, Width Faces and Length Faces; for any other face, a single Faces value applied along each side. Each selected face is divided into four or more faces.
3. Choose the Insert Mode: Simple creates the minimum number of control points (the surface shape may change); Exact adds extra control points to maintain the shape.
4. Optionally enable Object Snap (with an Offset) to move the new vertices onto a solid, surface or mesh body.
5. Click OK. Subdivide only the faces that need detail; T-junctions are allowed in T-splines.

Compare with Insert Edge: Subdivide splits faces in both directions; Insert Edge adds edges from the selected edge or loop at an Insert Location (a decimal percentage), on a Single side or Both sides, with Simple or Exact Insertion Mode. Use Subdivide for broad detail, Insert Edge for a directional loop.

Thicken#

1. Choose Modify > Thicken and select the T-spline body (open or closed).
2. Drag the manipulator or enter the Thickness.
3. Choose a Thicken Type: Sharp (connects the surfaces with a straight face), Soft (rounded face, external or internal offset) or No Edge (surfaces are not connected).
4. Choose a Direction: Normal (perpendicular to the surface) or Axis (select an axis). Use Repeat Thicken to run it again, then click OK.

Thicken turns a surface-like form (a Plane-based leaf, an open Box shell) into a form with volume. Autodesk states it approximates a shell and does not give exact shelling; for exact results convert an open T-spline to a surface and use Surface > Thicken, or convert a closed one to a solid and use Solid > Shell.

Flatten#

Choose Modify > Flatten, pick a Direction type (Fit: best-fit plane, needs at least four vertices per surface; Select Plane; Select Parallel Plane), select the vertices, and choose a Flatten Type (Control Points or Surface Points). The more vertices you select, the flatter the result. Use it to create a flat mounting pad or flat bottom on an otherwise curved body.

Make Uniform#

Select one or more T-spline bodies. Make Uniform relaxes the surface, evenly distributes the angles at star points and makes faces as uniform as possible (it works on whole bodies, not a selection of faces). An optional Convert T-Points setting turns all T points into star points. Autodesk suggests it when the surface shows strange bunching after adding control points. It can shift your silhouette, so use sparingly.

Interpolate#

Select T-spline surfaces and choose Towards: Control Points (the surface fits through the existing control points) or Surface Points (the control points are moved to the surface). Autodesk recommends using it with Control Frame display. Use it only where you need accuracy; interpolated areas can ripple (general guidance).

Match#

Aligns T-Spline edges with sketch curves or with edges on solid or surface bodies. It has an Associative option so the match follows the target if the target moves. Continuity can be Connected (G0), Tangent (G1) or Curvature (G2); G1 and G2 are only available when matching to faces on a solid or surface body. Other options: Tangency Weight, Spacing (Uniform or Curvature), Tolerance and Falloff. Use it to fit the form to a mechanical interface, and combine it with Freeze to lock matched edges.

Pull#

Snaps selected vertices to a target face or surface. Target Select is Auto (nearest body) or Select Targets; Pull Type is Control Points or Surface Points. Useful for snapping edges to a solid body's surface.

Delete#

Removes selected vertices, edges, faces or whole bodies (double-click an edge for a loop; double-click a face or pick the body in the browser to select a whole body), then press Delete. Deleting faces can leave open edges that you may need to close with Fill Hole; the exact result per element type is not detailed in the help, so pick the element type deliberately.

Options reference#

Tips and pitfalls#

Tip: After a subdivide, re-run the global shape check. Smooth shape changes slightly at the boundary of subdivided areas; nudge a few vertices to restore the intended profile.

Tip: Avoid subdividing the whole model "just in case". Each level multiplies the faces (a quad becomes four or more), slows interaction, and makes broad shape changes harder.

Tip: If you want a hole through a form (for example a handle), connect two sets of faces or edges with Bridge rather than trying to cut with Delete.

Warning: Too many star points close together produce ripples and pinches. Triangles and five-edge poles are fine individually, but cluster them and the surface quality suffers. Aim for star points in flat or low-curvature areas, away from critical highlights.

Warning: Thicken does not check for self-intersection. If the thickness is greater than the minimum curvature radius of the object, the model will self-intersect, so inspect the result and move control points to fix it.

Tip: If a form behaves strangely, display it in Box mode and look for long thin faces, triangles, or tangled bunches of edges. Cleaning those with Delete, Insert Edge or Weld Vertices often helps (general guidance).

Warning: A Form body is smoothed by the T-spline algorithm; it does not meet exact dimensions. For anything that must be an exact size (a thickness that fits another part, a precise diameter), finish the form and use solid features to establish it.

Planning density: how many faces do you need?#

This is general modelling guidance, not Autodesk data. A workable method is to count features that need their own curvature and give each a minimum of two cage rows. A comfortable handgrip with a thumb rest, a finger groove and a rounded end needs about 4 rows along the grip length plus 2 extra for each feature, which is roughly 8 to 10 rows. If you begin with 3 rows, Subdivide once on the grip faces and you have 6, and Insert Edge adds the remaining detail loops one at a time.

Each Subdivide step splits a quad into at least four faces (more if you raise Width/Length Faces), so a 10 face area becomes at least 40 faces after one step and at least 160 after two. That is still light for a computer but heavy for a human. Rule of thumb: you should be able to name what every cage row is for.

Worked example: a grip on a 120 mm bean#

• Start: Quadball, 3 faces, scaled to 120 x 50 x 40 mm. Total faces: 54.
• Subdivide the 9 faces on the top surface once with 2 x 2 divisions: roughly +27 faces, now about 81 in total.
• Add two Insert Edge loops around the thumb area, 10 mm apart: the smooth dent now has enough rows to curve.
• Move the dent vertices down 4 mm with soft modification at a 15 mm radius.
• Flatten the bottom faces to a plane so the body rests flat. Check the base for flatness after Finish Form (for example with Inspect > Section Analysis).

Thicken in depth#

Thicken adds a second offset surface and joins the two. Key behaviours:

• It is the right way to turn open forms (planes, opened boxes, revolved profiles) into solid walls while still in Form, so later edits stay linked.
• Wall thickness must stay below the minimum curvature radius of the object, otherwise the result self-intersects (Autodesk's help). For a 3 mm wall, keep concave radii above 3 mm.
• For exact results, convert: an open T-spline to a surface then Surface > Thicken, a closed one to a solid then Solid > Shell. These accept parametric thickness values and are easier to change later. Choose Form Thicken when you still plan to move the cage; choose the Solid command when thickness is a design parameter.

Flatten, Straighten and Pull for mechanical interfaces#

Form is imprecise on its own. Three tools tie it to exact geometry.

• Flatten projects vertices to a best-fit or selected plane (Direction: Fit, Select Plane, Select Parallel Plane; Flatten Type: Control Points or Surface Points). Use it for a flat mounting pad or a base that must sit on a table.
• Straighten does the same for a line. Use it for a straight edge, such as the long side of a handle.
• Pull snaps vertices to a target face or surface, so the form meets a solid body exactly.

After any of these, check the smooth surface, not the cage, because the surface is what the BRep will follow. Interpolate can then fix residual deviation.

Cleaning up topology#

• Long thin faces come from heavy stretching. Fix them by Subdividing the neighbours or by using Insert Edge to even the proportions.
• Triangles and five-edge poles are acceptable singly; cluster them and you get ripples. Deleting an edge loop you no longer need can dissolve a cluster (Merge Edge is different: it joins two sets of boundary edges).
• Isolated dense areas in a coarse body create visible lumps at the density boundary. Grade density gradually over two or three rows.
• Inverted regions appear when thickened or pulled geometry passes through itself. Undo and reduce the move.

Common failures and fixes#

Practice exercise#

Refine a low-poly bean into a grip with a flat base and measure the result.

1. Create a Quadball of diameter 60 mm with 3 faces, then select all and scale non-uniformly to 120 x 50 x 40 mm.
2. Note the face count. Select the 9 faces on the top surface and Subdivide once. Note the new face count.
3. Use Insert Edge twice around the thumb area, placed 10 mm apart.
4. With Soft Modification at 15 mm radius, push the thumb dent down 4 mm.
5. Select the bottom faces and use Flatten to a plane at the base; use Freeze on them afterward.
6. Switch to Control Frame, look for long thin faces, and run Make Uniform on the body if the faces are uneven.
7. Finish Form. In Solid, add a 2 mm shell (Solid > Modify > Shell) with the bottom face removed. Measure the base flatness with Inspect > Section Analysis.

Expected result: a smooth bean-shaped body, high density only around the thumb area, a flat stable base, and a 2 mm hollow shell. Record the face counts you saw at steps 2 and 4: they show how density grows and why you add it locally.

Related#

• Creases, Insert Edge and topology tools
• Edit Form
• Finish Form and conversion