Surface continuity is the term used in Alias to describe how surface patches meet.

## What is surface continuity?

Here, a fillet blend is used as an example:

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:

### G1 continuity

As we progress up the numbers on continuity, keep in mind that the previous number(s) before must exist in order for it to be true. In other words, **you cant have G1 continuity unless you at least have G0 continuity**. In a sense, it’s a prerequisite. *G1* or *Tangent continuity* or *Angular continuity* implies that two faces/surfaces meet along a common edge and that the tangent plane, at each point along the edge, is equal for both faces/surfaces. They share a common angle; the best example of this is a fillet, or a blend with Tangent Continuity or in some cases a Conic. In the examples below, you can see what this could look like on both curves and surfaces.

### G2 Continuity

*G2 Continuity* or *Curvature continuity* or *Radial continuity* implies two faces/surfaces meet along a common edge, are tangent, and the rate of curvature change at each point along the edge is equal for both faces/surfaces. The transition across the edge is therefore curvature continuous. This is the minimum math requirement for Class A Surface. Another way to describe this is in a situation where a reflection is cast upon the surfaces and you would not be able to tell where one patch ends and the other begins. The examples of this in CATIA V5 would be a connect curve (with curvature continuity, or in the surface terminology in the GSD Workbench, a blend surface with curvature continuity or a fill surface that meets another). In the examples below, you can see what this could look like on both curves and surfaces.

### G3 Continuity

There is also a *G3 continuity,* which follows the same process as its predecessors but controls the rate of the curvature along the curve as it transitions from one curve or surface to the other. G3 is looking for balance on the rate of curvature – in other words, that the max value of the curvature hits its peak about the middle of the transition area. It is a bit outside of the scope of this blog post, but look for something in a future post on it, as well as how to check for these conditions in CATIA V5.