We describe a two-stage approach for interpreting line drawings of curved objects. In the first stage, the user enters a natu-ral line drawing of a polyhedral template; this is automatically interpreted as the corresponding polyhedral object. In the second stage, the user enters freehand curves; by relating these to the template, a curved object can be constructed.

Automatic creation of B-rep models of engineering objects from freehand sketches would benefit designers. A subgoal is to take a single line drawing (with hidden lines removed), and from it deduce an initial 3D geometric realisation of the visible part of the object. Junction and line labels, and provisional depth coordinates, are important components of this frontal geometry.

A tool which can quickly interpret line drawings (with hidden lines removed) of engineering objects as boundary representation CAD models would be of significant benefit in the process of engineering design. Inflation of the drawing to produce a frontal geometry, a geometric realisation of that part of the object visible in the drawing, is an important stage of this process. Previous

Engineering design would be easier if a computer could interpret initial concept drawings. We outline an approach for automated interpretation of line drawings of polyhedra, and summarise what is already possible, what developments can be expected in the near future, and which areas remain problematic. We illustrate this with particular reference to our own system, RIBALD, summarising the published state of the art, and discussing recent unpublished improvements to RIBALD. In general, successful interpretation depends on two factors: the number of lines, and whether or not the drawing can be classified as a member of special shape class (e.g. an extrusion or normalon). The state-of-the-art achieves correct interpretation of extrusions of any size and most normalons of 20–30 lines, but drawings of only 10–20 lines can be problematic for unclassified objects. Despite successes, there are cases where the desired interpretation is obvious to a human but cannot be determined by currently-available algorithms. We give examples both of our successes and of typical cases where human skill cannot be replicated.

Freehand sketching is an important part of the conceptual design process, and the increasing number of recent sketching applications shows a developing awareness of this importance. We aim to provide an automated tool to turn engineers' freehand sketches into CAD models. This would allow engineers to spend their time more productively and to be more creative. one natural