Those of you who have been developing software for quite some time likely came across the adapter pattern. It's one of the classical patterns described in the famous Design Pattern's book by Erich Gamma et al. The concept is simple yet powerful allowing you to adapt one interface to another.

How different is computing an intersection between two curves and between two edges ? Not that much – you just need to minimize a function representing a distance between any two points along each. What about a distance between two wires ? Just the same. And what about an intersection between an edge and a wire ? Or between a curve and an edge? What if you need to build a tube (or pipe) surface along the curve or along the wire ? Would that make any difference? Nope, either.

OK, but how would your algorithm API look like ? Will it accept Geom_Curve, TopoDS_Edge and TopoDS_Wire all at once and in any possible combination? Wouldn't it be weird and unreadable ? What about efforts to maintain it ?

The adapter pattern is the answer. You just have an abstract class that would provide an interface, which the algorithm could use. And particular implementation (for curves, edges or wires) would be provided by subclasses.

That's exactly what is provided by Open CASCADE adapters – abstract Adaptor3d_Curve, implementation for curves - GeomAdaptor_Curve, for edges – BrepAdaptor_Curve, and for wires – BRepAdaptor_CompCurve. The pipe algorithm (GeomFill_Pipe) accepts adapters (not Geom_Curve) to construct a surface. This provides a good flexibility for you to feed your objects.

Here is a sample code:

TopoDS_Wire aSpineWire = ...;
TopoDS_Edge aGuide1 = ..., aGuide2 = ...;
Handle(BRepAdaptor_HCompCurve) aSpineAdaptor = new BRepAdaptor_HCompCurve (aWire);
Handle(BRepAdaptor_HCurve) aGuideAdaptor1 = new BRepAdaptor_HCurve (aGuide1), aGuideAdaptor2=new BRepAdaptor_HCurve (aGuide2);
GeomFill_Pipe aPipe (aSpineAdaptor, aGuideAdaptor1, aGuideAdaptor2, aRadius);
aPipe.Perform (aTol, anIsPolynomial);

In the code above, *_H* are just handle-based equivalents, subclasses of Adaptor3d_HCurve.

Another good example is Extrema that is used to compute distances between the curve (and surface) adapters. You can feed curves, edges or wires to calculate extremum distances (including intersections).

Unfortunately, Open CASCADE API is not consistent in terms of use of adapters and often abuses hard-coded types where adapters would have been a better choice. For instance, the very
GeomFill_Pipe abuses Geom_Curve in some constructors while Adaptor3d_Curve could be used.GeomConvert_ApproxCurve which approximates a curve with B-Spline could have accepted an adapter thereby providing powerful possibilities to create a single NURBS-curve from TopoDS_Wires, for example.

In the follow-up post I'll try to show some example of how you could create your own adaptor, but hopefully this one already sheds some light on the concept and will give you some food for thoughts and experimenting.

(to be continued...)