Understanding Engineering Drawings: A Survey

Mechanical design and manufacturing information for 3-D solid objects has been eeectively conveyed through a set of annotated orthographic projections and optional cross-sections. This forms the basis of engineering drawings, which solve the problem of unambiguously representing a 3-D object on a 2-D plane. In this paper we address the inverse problem: given an engineering drawing of an object, construct the object's 3-D representation. To enable automatic recognition, the paper line drawings are initially scanned, and yield images which are inherently noisy. The 3-D objects themselves can have surfaces that are planar, spherical, or cylindrical. We examine the stages of drawing generation and formulate the drawing interpretation problem. Most 3-D reconstruction algorithms have assumed that the vertex coordinates and line and arc endpoint coordinates are known accurately and without error, and that no annotation exists in the drawing. In practice, however, scanned drawings are noisy and contain annotation interwoven with the geometry lines. Current bottom-up rule-based systems do not utilize prior knowledge of the constraints imposed on 3-D object models, neither do they model the document degradation. Moreover, no performance evaluation of the systems for varying noise levels and object complexities has been carried out. We compare the merits and drawbacks of the strategies employed in key works in the area of CAD model interpretation from engineering drawings, and propose research directions to enhance the practicality of paper engineering drawings-to-CAD conversion systems.