Section 3 Shape Grammars Semantic Interpretation of Architectural Drawings Olubi Babalola Georgia Institute of Technology, USA Charles Eastman Georgia Institute of Technology, USA Abstract The paper reviews the needs and issues of automatically interpreting architec- tural drawings into building model representations. It distinguishes between recognition and semantic interpretation and reviews the steps involved in de- veloping such a conversion capability, referring to the relevant literature and concepts. It identifies two potentially useful components, neither of which has received attention. One is the development of a syntactically defined drafting language. The other is a strategy for interpreting the semantic content of ar- chitectural drawings, based on the analogy of natural language interpretation Keywords Semantic Interpretation, Drawing Understanding 166 2001: ACADIA Olubi Babalola Charles Eastman Semantic Interpretation of Architectural Drawings Part I: An Overview and Survey and building operation. Yet, given the amount of data archived in paper as well as non-intelligent 1 Introduction CAD formats, it is clear that some means of auto- Semantic interpretation of architectural drawings matically migrating drawings from these formats has the potential for simplifying the process of into intelligent model representations would save conversion of archived drawing information in millions of man hours. paper or old CADD formats into the newer and 1.2 Current view of the drawing recognition intelligent model-based CADD representations problem which posses vastly greater capabilities. The fol- Semantic recognition of architectural drawings lowing section addresses some of the pertinent from paper-based input is a potentially vast re- questions, such as the difference between archi- search area. Small but relevant segments of the tectural and other recognition. process have been separately tackled from the perspective of different drawing recognition do- 1.1 Justification for Research mains. The most relevant effort, tackled from a Interest in extending the functionality of CAD directly architectural viewpoint, is the Knowledge beyond simple drafting and editing through the Based Interpretation of Architectural Drawing addition of model-based design support and system, KBIAD [CJ90] and has a focus somewhat analysis intelligence is almost as old as CAD re- similar to that of this paper. The KBIAD research search itself. Progress in these extensions hinge sought to develop a generalized method for draw- on the evolution of CAD systems from drawing- ing interpretation, and demonstrated this using oriented representations to model-oriented rep- an architectural floor-plan example. A number of resentations, which identify each element in the ongoing efforts have also been directed towards design, associating its geometry and other prop- semantic reconstruction of drawing information erties and relations. A growing number of archi- using layer-separated CAD input. tectural CAD products of this sort exist. While each uses its own internal representation, most of Outside of architectural drawing recognition, these intelligent CAD systems interface with a there is a large body of drawing recognition re- standard building model, for exchange and inter- search from other domains, such as office docu- facing with special applications. A standard build- ments, schematic drawings, and mechanical draw- ing model is a public-domain open standard for ings. Many issues are common across these do- representing the useful information about a build- mains, especially in the early stages of their re- ing, defined as a composition of objects at differ- spective processes, providing a useful body of rel- ent levels of aggregation. evant research sources, techniques and occasion- ally ready solutions to borrow. Non-architectural Considerable progress has been made in the de- domains also overlap similarly, and it is conceiv- velopment of a standard building model. The able that discoveries in architectural recognition Industry Alliance for Interoperability group (IAI) may in turn influence approaches in other do- is about to release the fifth version of its Industry mains. Foundation Classes (IFC 2.X), an industry-led effort to develop a standard building model. Simi- The objectives of drawing recognition vary widely, larly, The Standard for the Exchange of Product ranging from the conversion of raster-scanned model data under the International Standards drawings into layered CADD documents, to gen- Organization (ISO-STEP) has developed ex- erating the input for a specific analysis applica- change technologies that have been used to de- tion, to the identification of the represented ob- velop a range of standard exchange models in jects, along with their various properties. Our in- building, steel structures, precast concrete, and terest in this paper focuses upon interpreting non- other industry domains [CE99]. Eventually, re- layered CAD drawings for input to building prod- searchers in the building industry expect that such uct models, which offer a common base for all a standard building model will become the basis other uses. for most work in design, contracting, fabrication 2001: ACADIA 167 Section 3 Shape Grammars 1.3 An overview of issues addressed in the paper tive is to arrive at the beginnings of a theory of This paper has two parts. The first defines the architectural drawing understanding, in the be- various aspects of the drawing understanding lief that such formalization will help in bridging problem and the multiple issues that must be ad- some of the existing gaps in the process. dressed for product model interpretation. A rough 2 Drawing Recognition Input and Process process model is provided both as a means of dem- Model Outline onstrating relationships and a possible sequence 2.1 Detail Description of Architectural Drawings of the various activities, as well as providing some A variety of drawing-types are employed in ar- structure for discussion of our survey of related chitecture at different stages in the design pro- research. The purpose of the survey is to identify cess, ranging from unstructured sketches, to sche- work and concepts specifically relevant to the field matic designs, through to a richer and more for- of architectural drawing recognition, including malized representation used in communicating techniques, principles and/or algorithms. construction information to a contractor [GN97]. The second part of the paper explores how rich The latter, called construction drawings, is the semantics may eventually be extracted from ar- focus of this paper. chitectural drawings, using natural language pro- Architectural construction drawings are graphi- cessing as an analogy. We identify the difference cally complex, depicting an assemblage of sub- between drawing recognition and drawing under- assemblages and parts. A typical architectural floor standing, pointing out the importance of seman- plan for a 3 bedroom residence may run in excess tics in understanding. We discuss some of the of 3500 graphical entities, while a drawing for a major problems involved, drawing parallels with machine part may consist of less than 300. We the resolution of similar problems in natural lan- define complexity here in terms of the number of guage understanding, and in the process identi- geometric entities involved in depicting the ob- fying several useful areas of research. The objec- Figure 1. Architectural Drawing Samples 168 2001: ACADIA Olubi Babalola Charles Eastman Semantic Interpretation of Architectural Drawings ject, as well as the nature of the spatial relation- · Views are orthographic 2D, 3D parallel projections ship between these entities, such as connections, (isometric or oblique), and occasionally 3D perspec- tive views. overlap, containment and nesting. Figure 1 illus- · Multiple views must be interpreted to derive all 3D trates this complexity with sample architectural shape and detail aspects of the design; Views required floor plans from a construction drawing. for complete object-description typically reside on mul- tiple drawing sheets. We summarize some general properties of draw- · Views represent building assemblages. Distinct isolated ing relevant for the task of interpretation below building parts are seldom represented separately. (some of these properties are graphically presented · A view within a drawing often depicts multiple sys- in Figure 2.) tems (architectural, electrical, structural, MEP) to in- dicate their placement or relationships. As a result, a · Architectural drawings are typically a mixture of pre- view often is a complex overlay of different types of sentation format and semantic content, with a bordered systems. sheet, with a panel detailing general drawing content, version and ownership, individually labeled and scaled · Floor plans are the central organizing view in contract views in each of the sheets. drawings, with sections and other views keyed on the plan. · Drawings consist of a variety of symbol types, ranging from text through more restricted text notation types. Abstract symbols consisting of text, graphic symbols or a combination are used in conjunction with scaled geometry representing physical object parts. These properties of architectural construction drawings are unique and not exactly matched by drawings in other domains. 2.2 The Drawing Recognition Process A block diagram of the recognition process is il- lustrated in fig 3. It will be used as a backdrop for discussion