Theory Builder Title

Introduction to Continuity Terminology

Continuity is the term used in Alias to describe how surface patches meet. Here, a fillet blend is used as an example:

Examples of Surfaces with G0 to G3 Continuity

How CVs Control Continuity

Continuity is achieved by controlling the CVs at the edge of the aligned curve or surface. A higher continuity level requires more CVs to be aligned.

Here is an example of both a curve and a surface being matched at different continuity levels:

How CVs control continuity

Continuity Between Two Curves or Surfaces

A circular (G1) fillet is specified simply by a radius value. But because G2 and G3 fillets aren't circular, then they need a different way of specifying their size.

The first technique is Radius which keeps a fixed radius (allowing a variable width), and then specifies the size in two ways:

Specifying Radius Fillets

The second technique is Chord which keeps a fixed width (allowing a variable radius), and again there are two ways to specify the size of the fillet:

Specifying Chordal Fillets

Form Factor

The main determinant of shape is the continuity level chosen (G1, G2 etc.), but there is some flexibility within that provided by the Form Factor. This controls the concentration of CVs towards or away from the centre of the blend, making the curve sharper or flatter.

Specifying Form Factor

Colinear and Non-Colinear Alignment

The easiest way to get surfaces matching smoothly is when you have:

These are called colinear surfaces, and when using the Object Edit → Align tool then the Alignment Type : Edge should be used:

Illustration of colinear surfaces being Aligned

As the design progresses and you need to build more complex transitions, you will have non-colinear situations where you are:

When using the Object Edit → Align tool, the options should be set to : Alignment Type : Project.

Illustration of non-co-linear surfaces being Aligned

Other tools like Bi-Rail, Square and Freeform Blend will work in non-co-linear situations, but typically produce heavier surfaces with more spans, which may be unevenly spaced.

Continuity Tolerances

Continuity is calculated to a certain tolerance or accuracy. This can be set in the Construction options window and are usually chosen to match the CAD system you expect to be exporting your data to.

Tolerances Option Window

Tools that Achieve Continuity

List of tools that can achieve continuity


Example of the Align tool used on an automotive wheel arch

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