## 1 Introduction
The authors suggest geometry as the best candidate for data embedding in a 3D model compared to another attribute such as shininess, surface or vertex colors, surface or vertex normal vectors, per-vertex texture coordinates, or texture images. They have shown that the embedding of a watermark can be achieved by action on the surface vertices (e.g., inserting or displacing vertices) or on the surface topology (connectivity). Such watermark embeddings are used for copyright notification or protection, theft deterrence, and inventory of 3D polygonal models [15]. The need for centralized 3D cultural heritage archives was stressed through different projects described in [16]. Authors listed several such projects, two of them created by the Stanford University: the 3D archive of [PERSON]'s sculpture [17] and the Forma Urbis Romae [18]. The first project refers to the digitization of ten sculptures, including [PERSON]'s David and the second is a giant marble map of Rome constructed in the early 3 rd century. In both projects, the 3D models and raw data are distributed only to the established scientists due to the contract with the Italian authorities. The 3D content is available to the general public in a reduced form through a new remote display system named \"ScanView\". They also presented a 3D reconstruction of the Roman Forum, which is developed at UCLA [19]. The authors showed two approaches to protect 3D models: watermarking 3D model and a trusted 3D graphics pipeline. In this pipeline, a ScanView prevents theft of a 3D model's geometry by providing the user with only a low-resolution 3D model version. When the user requests a view of the model, a high-resolution 3D model is rendered on the server, which is in the same position as the low poly model, and its view is sent to the user. To analyze the protection of 3D models, we have researched on the Internet a number of techniques for sharing 3D models in a safe and secure way. One of the ways that are often mentioned is watermark [14; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30]. In this research, we will use an example of a sculpture to describe and present our conceptual solution for the protection of 3D models created by 3D digitization of cultural heritage (i.e., 3D models in general) from misuse and copying. The sculpture shows the upper part of a human figure (head, neck, and part of the torso) cast in plaster. The object is dominated by golden-yellow color, but darker shades of brown are also present. The emphasis was on the digitization of the head/face of the sculpture, with dimensions: width \(\times\) height \(\times\) depth = 203 mm \(\times\) 254 mm \(\times\) 203 mm. Our idea is to add masses with clay in a color that corresponds to the color of the sculpture to the parts of the sculpture. The curator (sculptor) from the Gallery of Malica srpska, which is a representative cultural institution, supported our idea as a meaningful solution, and his proposal was to add these masses to a lock of hair (on the left side), the earlobe (on the left side), and the cheekbone (on the left side), i.e., the parts of the sculpture where those masses would be unnoticeable to an average observer. Moreover, for the same reason, the amount of clay applied was taken into account. Attention was also focused on avoiding any damage to the sculpture both during the application of the clay and in the process of digitization. In this research, the emphasis was on the use of contactless methods for digitization to avoid unintentional damage to the sculpture. We used photogrammetry, because of relative simplicity and the need for minimal hardware and software resources, as well as satisfactory accuracy. In addition to photogrammetry, another 3D digitization technique we used is 3D digitization with structured light (fringe projection method). The reason for this is that this technique is not time-consuming and provides models with a high level of detail.