solid modelling systems

The popularity of solid modelling systems has grown rapidly ever since their introduction more than a decade ago. For a time it appeared that solid modelling would be the only necessary form of modern 3D CAD. Recently, an increasing number of users have found that there are limitations in solid modelling and that surface modelling functionality is becoming more and more important in many applications.

Surface modellers build a model by working a "skin" over the component being developed. They have the advantage that they can usually model any shape that a designer can imagine, but their disadvantage lies in the fact that they have historically been difficult to use.

Surface modellers have a hierarchy of geometrical objects that the user must know about - surfaces with internal control-curves, each of which has control points plus tangent vectors or knots and weights; plus trim curves, each of which has its own points, etc. Furthermore, there are extra commands involved in linking or stitching individual surfaces together in a smooth way to form the complete, continuous "skin" over the model.

Solid modellers have the advantage of being easier to understand since they often only involve working with a single class of object - the solid. This solid can be manipulated by only three basic editing commands - the Boolean operations of subtraction, addition, and intersection. Modern solid modellers provide a longer list of edit operations than just the three Boolean operations, by combining them in various ways as "features". Generally they are still much easier to learn and use than surface modellers. The ability to change a parameter and have the rest of the model update automatically makes solid modellers ideal for conceptual design where designers must try out various "what-if" scenarios.

Despite their simplicity and the ability to parameterise, solid modellers are by no means perfect for every engineering application. Solid modellers are not very satisfactory at handling complex geometries with free-form, sculptured surfaces. Such shapes are essential for any product where aesthetics or ergonomics are an important consideration.

An increasing interest in surface modelling is also coming from product design companies that produce complex parts for their clients. These include industries such as packaging, footwear, ceramics and toys, as well as housings for electrical goods and aerospace and automotive components. Surface modelling remains the only method that allows geometry to be modified interactively. This is a key factor in any application where the appearance of the product is of greater importance than functional requirements. Manufacturing companies are also realising the problems that result when models are just approximations with complex details left out. Virtual prototyping and digital mock-ups need accurate geometry to work effectively.

Hybrid modellers are being developed that offer the speed and ease of use associated with solid modelling, together with the power and design flexibility of surface modelling.

Solid Modelling	Surface Modelling
Easy to learn/use	More flexible in modelling complex geometry
Parametric/associative capabilities	Interactive modelling capabilities
Quicker creation and updating of assemblies	Quicker creation and updating of complex components and tooling
Excellent for creating functional models	Excellent for creating aesthetic or ergonomic free-form models