b) Stone and masonry structures or components widely exist in histor- ical buildings and cultural heritages [13,14]. There is a propensity for

86 J. Xu et al.  /  Automation  in Construction 76 (2017)  85–96

structures and components of such materials to have detailed en- gravings, which may be on complex curved surfaces [11]. To manu- ally reproduce or reconstruct these components is an arduous task; the number of artisans in China, for example, who have skills and ex- perience to undertake this work is limited and even decreasing, and the cost to manufacture templates for their construction activities is an expensive undertaking. New adaptable automatic restoration methods are encouraged for architectural conservation work.

Yet with the advancement and development of digital technologies, such problems can be overcome. Three-dimensional (3D) scanning im- aging systems and photogrammetric shape measurement systems are capable of measuring space dimension of existing structures and arte- facts. As-built surface information can be acquired without physical contact, while digital photos and other camera or vision based systems can be used for monitoring [15,16], recognition [17,18], localization [19] and tracking [20,21]. Combined with 3D printing technology, which is an automatic manufacturing process without templates, the potential and increasing applications of 3D scanning for restoring or reconstructing historical heritage buildings for people to enjoy is un- bounded [22-24]. However, there is limited research that have used these technologies to restore key components of historical buildings. This paper presents a novel digital process for reproducing a whole or- namental component of a historical building using a combination of 3D laser scanning and cement mortar-based 3D printing technology. Ce- ment mortar is used for that purpose, as is characterizes the physical properties of stone or rock and performs harmoniously with the original stony material in color, tone, texture, form and scale; this material con- forms to the requirements embedded in conservation ethics [8].

2. Related work review of 3D scanning and 3D printing

2.1. Digital restoration of artefacts: 3D scanning

Three dimensional scanning is a non-contact, non-destructive tech- nology that digitally captures the shapes of physical objects using lasers, lights or x-rays. A 3D scanner creates ‘point clouds’ of data from the sur- face of an object; it is a way to capture a physical object's exact size and shape as a digital 3D representation that is stored in a computer [25]. The application of 3D scanning of handicrafts and cultural artefacts assumes an important place among documentation and analytical tech- niques used for heritage objects (e.g [26].). In fact, the digital conserva- tion and interpretation of cultural heritage has attracted considerable attention in computer graphics, geometric modeling, virtual reality and general computer science communities (e.g., [27–29]). Examples of 3D scanning are replete now within the normative literatures since the emergence of this technology in the 1990s [30,31]. For example, the Stanford University and the University of Washington digitized the sculptures and architecture works produced by Michelangelo using 3D scanning technology  [32].

The success of heritage 3D scanning has resulted in an array of com- mercial applications being developed such as the MatterPort [33] that can digitize objects and rooms and Faro large volume 3D laser scanning technology that can be used for measuring buildings and creating City- scapes [34]. A 3D scan enables objects to be brought into a virtual work- room, which means that its physical integrity is assured. In addition, such scans provide an ability to ensure quality control during its physi- cal restoration and reverse engineering [29,30]. In the case of quality control, parts and assemblies can be compared to the design specifica- tion and checked for tolerances. The identity and origin of an object can be compared to other known similar artefacts to ensure that resto- ration process can be replicated [35]. The ability to create multiple cross sections and 3D overlays of data has only been enabled by 3D scanning; replicas of original shapes can be produced.