Automatic Title Block Location in Technical Drawings

This paper presents a method that automatically detects the location of the title block in scanned technical drawings. Numerous applications could benefit from such a tool. For example, it is useful to locate and zoom on the drawing title block, which can help in indexing technical drawings. As the title block is normally on the right-bottom corner of the drawing, it can also permit turning the drawing in the correct reading direction. Our method of title block location is based on signal measurements computed on the drawing parts where the title block can be present. The usual drawing format norms are used to reduce the number of possible title block locations. The method has been validated on a large database of technical drawings. 1 Motivation and position of the problem This paper presents a method that automatically detects the location of a title block in scanned technical drawings such as mechanical or architectural drawings. Numerous applications, especially in the industrial market, could benefit from such a tool. Some examples are: • Viewing tool for technical drawings: if the title block is normally on the right-bottom corner, once located, the drawing can be turned in the correct reading position. This is especially important for scanned drawings coming from microfiches, where it is not uncommon to find drawings upside-down. • Indexing of technical drawings: in order to fill up a drawing database with the information contained in the title block, the user has to locate the title block and to zoom on it. Title block locating could also be the first step in automatic indexing of technical drawings, the following steps being the extraction of the indexing information itself. • Automatic folding of printed technical drawings: it is indeed useful to fold the drawing with the title block on the visible part. This paper is organised as follows: section 2 describes technical drawings with respect to the norms and usage in industry. It then extracts some characteristics of technical drawings, which will be used as parameters of our method. In section 3, we cover the state of the art in table and title block analysis. Section 4 presents our method. Section 5 describes our experiments and results we obtain. Section 6 concludes this paper. 2 Description of Technical Drawings An example of a technical drawing is presented in figure 1. Technical drawings follow some rules and recommendations (as International Standard Organisation ISO norms). They use standard paper formats. [ISO75] lists those formats for technical documents and provides their sizes. Usual paper formats are A and B series. According to the ISO 5457 [ISO80], Borders enclosed by the edges of the trimmed sheet and the frame limiting the drawing space shall be provided with all sizes. Concretely, a drawn frame should limit the drawing space. Figure 1: Example of Technical Drawing All technical drawings must contain a title block to allow their identification. According to ISO 5457, the position of the title block should be within the drawing space (...). The identification portion of the title block shall be at the right-hand bottom corner of the title block when seen in its normal direction of viewing, and have a maximum length of 170 mm. According to ISO 7200 [ISO84], the title block is a table-like form, composed of various rectangular fields (see figure 2). The standards, while giving some precise prescriptions to follow, leave some freedom. Moreover, the industry usage shows that those standards are not always respected, especially for old drawings. Figure 2: Examples of Title Block However, the following rules hold: • Standard paper formats are used (A4-A0 for European drawings). • A drawn frame limits the drawing space. • The title block is located in the drawing space and in one of the corners of the drawing. For A0 to A3 format, this corner is the right-down one, the drawing being in landscape position. For A4, the corner is the right-down one, the drawing being in portrait position. • The title block is a looking-like table of width over 100mm and of height included between 40mm and 300mm. • From our experience, the height of one of the title block fields is usually comprised between 5 mm and 30 mm. These heights permit to write inside the fields.