Extraction of line objects from piping and instrumentation diagrams using an improved continuous line detection algorithm

Digitizing image-format piping and instrumentation diagrams (P&IDs) consists of a step for detecting the information objects that constitute P&IDs, which identifies connection relationships between the detected objects, and a step for creating digital P&IDs. This paper presents a P&ID line object extraction method that uses an improved continuous line detection algorithm to extract the information objects that constitute P&IDs. The improved continuous line detection algorithm reduces the time spent performing line extraction by edge detection that employs a differential filter. It is also used to detect continuous lines in the vertical, horizontal, and diagonal directions. Additionally, it processes diagonal continuous lines after performing image differentiation to handle short continuous lines, which are a major cause of misdetection when detecting diagonal continuous lines. The P&ID line object extraction method that incorporates this algorithm consists of three steps. The preprocessing step removes the diagram's outline borders and heading areas. Second, the detection step detects continuous lines and then detects the special signs that are needed to distinguish different types of lines. Third, the postprocessing step uses the detected line signs to identify detected continuous lines, which must be converted to other types of lines, and their types are changed. Finally, the lines and the flow arrow detection information are merged. To verify the proposed method, an image-format P&ID line extraction system prototype was implemented, and line extraction tests were conducted. In nine test P&IDs, the overall average precision and recall were 95.26 % and 91.25 %, respectively, demonstrating good line extraction performance.