Form Primitives and Creating Forms

Overview#

Every Form model starts from a primitive or from a profile. The choice matters more than beginners expect: the primitive determines the topology of your cage, and good topology is what makes later edits easy. A Box with 3x3x3 faces edits very differently from a Quadball with the same face count, even though both can become a sphere.

This page covers the commands in the Create panel of the Form tab.

Where to find it#

Open Form with Design > Solid > Create > Create Form, then use the Create panel in the Form tab. Autodesk lists the primitives as Box, Plane, Cylinder, Sphere, Torus, Quadball, Pipe and Face; the sketch-driven tools are Extrude, Revolve, Sweep and Loft (all in Form > Create). Panel order can differ between releases.

Step by step#

Box#

1. Choose Create > Box.
2. Pick a Rectangle Type (Center or 2-Point), select a plane or planar face, and draw the rectangle.
3. Set Length / Width / Height (drag the manipulators or type values) and the Length Faces / Width Faces / Height Faces counts.
4. Choose a Direction (One Side or Symmetric relative to the starting plane).
5. Choose a Symmetry option: None or Mirror (Length, Width and Height Symmetry mirror along the X, Z and Y axes respectively), so later edits apply to both sides.
6. Choose an Operation (New Body; New Component is available in Direct Modeling mode only) and click OK.

A Box is the workhorse for blocky organic shapes such as ergonomic grips, shoes and bodies with flat-ish sides.

Plane#

A flat sheet of faces. It is the usual start for surface-like shapes (fins, leaves, terrain, sails) and for building out a form by extruding edges. It is open, so the result finishes as a surface unless you thicken it.

Cylinder#

Select a plane, click a centre point, then set the Diameter and Height. The face counts are Diameter Faces (around) and Height Faces (along the length). Direction (One Side or Symmetric) and Symmetry (None, Mirror, or Circular with Symmetric Faces) are also offered. Use it for bottles, handles, limbs and capped tubes.

Sphere#

Select a plane, click a centre point, then set the Diameter and the Longitude Faces and Latitude Faces counts. Symmetry offers None, Mirror or Circular (with Symmetric Faces). Use it for round bodies. For shapes you intend to deform heavily, many modelers prefer a Quadball (general guidance); try both.

Torus#

Creates a donut. Select a plane, click the centre, set Diameter 1 (the inner diameter), Diameter 2 (the size of the ring section), and the Diameter 1 Faces and Diameter 2 Faces counts. Symmetry offers None, Mirror or Circular. Useful for handles, rings, tires, and anything that loops.

Quadball#

Creates a sphere-like primitive built from quad faces. In the dialog you specify Diameter and the Span Faces count; Symmetry offers None or Mirror only (no Circular option). Quadballs are a good starting point for heads, joints, bulbs and any round object that you intend to deform into something else.

Pipe#

Creates a tube along a path. The dialog has three tabs (Sections, End Types, Segments) plus Global Settings. On Sections, pick the Path (edges or sketch entities; Chain Selection picks adjacent ones automatically) and a Global Diameter; you can select sections on the path and change their position, angle and diameter individually. On End Types choose Open (a surface), Square (closes the end) or Spike (extends a point one segment beyond the end). On Segments, a Density slider adds or removes segments globally, and you can type a segment count near a selected point. Global Settings include Display Mode, Curve Tolerance and Display Curve Splits. Pipe also handles networks of curves. Use it for tubing, wires, cables and anything that follows a curve.

Face#

Create > Face creates a single T-spline face or appends one to an existing body. Choose a Mode (Simple: click each vertex; Edge: pick an edge to define the first two vertices; Chain: pick connected edges) and a Number of Sides (Four Sides, or Multiple Sides where you click the first vertex again to close). Object Snap and Offset can snap new vertices to nearby solid, surface or mesh bodies. Use it to build exactly the topology you want from scratch, which is an advanced technique.

Extrude#

Create > Extrude creates or modifies a T-spline body from a profile (sketch curves, profiles or faces). Options: Distance, face Spacing (Uniform, or Curvature for more faces where the curve is tightest), Angle, Direction (One Side, Two Sides, Symmetric), Front Faces, and Keep Creases. Uncheck Profile Chain Selection to stop tangent geometry being selected automatically. To grow new faces from existing cage faces or edges, hold Alt while dragging a manipulator in Edit Form: that inserts new geometry. This is how you extrude a face to create a limb, a boss or a handle.

Revolve#

Revolves a profile around an axis (a linear or circular object). Set Type (Full for 360 degrees, or Angle), Direction (One Side, Two Sides, Symmetric), face Spacing (Uniform or Curvature), optionally Weld (merges coincident vertices, for example at the axis), and Symmetry (None, or Circular with Symmetric Faces). Good for vases, bottles and domes. Make the profile an open curve to get an open surface of revolution that can later be thickened.

Sweep#

Sweeps a profile along a path. Choose the profile and path, set the number of Faces around and along the sweep, the face Spacing (Uniform or Curvature), and the Orientation (Perpendicular or Parallel to the path). The distance is entered as a decimal percentage of the path length. Compared with the Solid sweep, this produces editable cage faces you can sculpt.

Loft#

Lofts between two or more profiles (sketches, edges or faces). Per-profile end conditions are Free, Direction, Tangent, Smooth, Sharp and Point Tangent (which ones appear depends on the profile geometry). Guide Type is Rails or Centerline. Tangent Edges can Merge or Keep edges between tangent faces. Closed connects the first and last profiles; Length Spacing and Width Spacing (Uniform with a Faces count, or Curvature with a Deviation) control face counts. Profiles should have compatible shapes. If the T-spline does not match the profiles well, increase the face count.

Options reference#

Tips and pitfalls#

Tip: Think "how many faces are in the final shape?" Primitives with more faces give more control but resist smooth shaping. Start with half what you think you need.

Tip: Use Mirror symmetry on the Box when modeling something with a left/right plane, such as a head or a mouse. You edit one half and the other follows.

Tip: When starting from a sketch, keep sketch profiles simple, with few segments. A Form Extrude from a sketch with many tiny line segments gives a dense, hard-to-edit cage.

Warning: Primitives with an even number of faces behave differently from odd (general T-spline guidance, not an Autodesk statement). For example, a Box with an even number of faces in a direction has a center edge loop on the symmetry plane; an odd count has a center face. Pick based on what you need: an edge loop for creases, a face for pushing a feature out of the center.

Warning: A Plane or open Pipe yields open edges. Finish Form produces a surface from such a form, not a solid. Use Thicken or close the form if you need a solid.

Tip: Rather than distorting a sphere into a torso, begin with the primitive closest to the final topology. A Cylinder with few height faces is a better start for a bottle than a sphere.

Topology decisions that matter later#

Face counts and the even/odd rule#

With mirror symmetry across a centre plane, the face count in that direction decides what sits on the plane. An even count (2, 4, 6) puts an edge loop on the plane, which is what you want for a crease along a centre line or a seam that stays sharp. An odd count (3, 5) puts a face across the plane, which suits a feature that rises out of the middle such as a nose or a button. Choose before creating, because changing it later means rebuilding topology.

Star points you get for free#

Every primitive carries a fixed set of star points. A Box has eight corners where three faces meet. Quadball and Sphere place their star points differently, and Quadball is often preferred for heads and joints (general guidance). Cylinder caps and sphere poles can carry high-valence points; a star point at a visible spot can produce pinching. Autodesk's help says star points should only be used where necessary and are best placed on flatter parts of the surface. Move star points to flat, low-curvature regions of the final shape.

Worked example: a bottle in three ways#

Target: a 200 mm tall, 70 mm diameter bottle with a 25 mm neck.

The revolve gives the most accurate result in the least time. Choose by asking which primitive already has the symmetry and ring structure of the final shape.

Create-from-profile commands in more detail#

• Extrude (Form): accepts a sketch profile, or selected faces and edges on an existing form. Use the Faces and Distance values to control how many rows are created. One row (Faces = 1) yields the lightest cage; add more only where the shape bends. Extruding an edge of an open plane creates a wall, which is how you grow a fin from a single edge.
• Revolve: the profile may be open or closed. An open profile makes an open surface. Divide the angle into at least 8 faces for a smooth circle, because T-spline circles at 4 divisions look rounded-square.
• Sweep: set the profile and path and the face counts. A path with a hard corner needs extra segments there, otherwise the form cuts across the corner.
• Loft: profiles need similar shapes. The Closed option connects the last profile back to the first, producing a ring or a loop like a torus.
• Pipe: turns a sketch path into a tube. Pick the path with a smooth sketch spline rather than a polyline with many tiny segments, since each sketch segment tends to add a cage segment.

Choosing sketch quality for Form profiles#

Form converts sketch geometry into cage points, so the sketch directly sets cage density. A 40 segment polyline traced from a drawing yields a hopelessly dense cage. Draw the profile with the fewest possible curve segments: a spline with 5 to 7 fit points, or lines and tangent arcs. If you need an accurate silhouette, build the form coarse first and refine it with Edit Form.

Common failures and fixes#

Practice exercise#

Make three starting forms for the same 200 mm tall, 70 mm diameter bottle and compare effort.

1. A Cylinder (70 mm diameter, 200 mm high) with 8 faces around and 6 height faces. Shape the neck and shoulder by scaling rings. Record the number of operations you needed.
2. A Revolve of a bottle silhouette sketch with 8 divisions. Record the number of operations to get the same shape.
3. A Box 60 x 60 x 200 mm with 2 x 2 x 4 faces and Mirror symmetry on both planes. Round it by moving vertices toward the axis.
4. For each, switch to Control Frame display, count star points, and check the shape from the front view against a 25 mm neck diameter with Measure.
5. Finish all three and compare their face counts in the browser.

Expected result: the revolved bottle is easiest and most symmetric, the cylinder is acceptable, the box takes the most work and shows irregular face sizes. Write down which one you would choose next time and why; the answer is almost always the primitive whose symmetry matches the final shape.

Related#

• Form workspace overview
• Edit Form
• Symmetry and patterns