## 1 Introduction
Empirical methods, such as One-Slope [16], Dual-Slope [16], COST-231 [16] or Okumura-Hata [17], are based on a large number of measurements but may be inaccurate in the case of atypical spaces. For the application of RFID deployment in an indoor area, these methods lack spatial information and energy changes due to multipath propagation, which is very important for defining the tag readability area [15]. Among the deterministic methods, Method of Moments (MoM) [18], Finite-Difference Time-Domain (FDTD) numerical method [19] and Finite Integration Technique (FIT) numerical method [20] are mentioned. These methods are accurate but are computationally complex and time-consuming [15]. As a compromise, the now widely used deterministic method ray tracing [15; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30], more precisely, ray tracing (physics) (RT), is based on geometrical optics (GO). It is usable as an approximated method to estimate the intensity of an electromagnetic field [21]. In this method, electromagnetic waves are treated as rays. The principle is based on emitting a certain number of rays from a source and tracking them. These then interact with obstacles in the form of refraction, diffraction and reflection depending on the material of the object, and so attenuation occurs at the obstacles [21]. In an indoor environment, the material represents a large part of the influence on attenuation. In RT simulations, the material properties of all objects within the simulation are often taken into account based on the knowledge of the dielectric constant and permittivity in the range of operating frequencies [31]. For instance, the authors in [32] discuss the calibration of ray tracing models based on measurements of permittivity and conductivity in the indoor environment. The limitations of this method are mainly the number of rays emitted and the size of the surfaces on which the rays hit and on which the resulting intensity is subsequently calculated and correlated. In the first case, the calculation time increases with an increasing number of rays, and in the second case, inaccuracy in the calculation may occur due to the different number of incident rays [33]. RT is a frequency-independent and widely used method for 2D and 3D radio coverage prediction. Therefore, it is often implemented as a feature in software tools. Some of the frequently used tools include Altair WinProp [34], Wireless InSite [35] and Altair Feko [36]. The authors of [37] use a combination of Altair Feko and Altair WinProp to predict the coverage of an indoor RFID infrastructure. The authors of [21] study the coverage of UHF-RFID tags at 433 MHz in an indoor environment using Altair WinProp. The authors of [22]use Wireless InSite as a 3-D ray-tracer to evaluate the proposed tag placement algorithm for THz RFID. The papers are categorized in Table 1.  
An analog of the physical ray tracing method is encountered in the domain of computer graphics as a technique for generating images in computer graphics. It belongs to the group of so-called global illumination algorithms, further referred to as ray tracing (graphics) (RTg). This method can be considered as a possible approach to predict the coverage of the radio signal in the graphics domain. The equation that describes the propagation of light in a 3D scene (i.e., the rendering equation) was published by [PERSON] in 1986 in [38]. The aim of this paper is the verification of the use of the RTg algorithm for the simulation of radio signal propagation in a 3D indoor environment for UHF RFID technology. The investigation of this approach is the main contribution of the paper because the RTg algorithm solves the global illumination problem in the domain of computer graphics by default. In the simulation, the effect of reflections and refractions, where some parameters, such as the shape of the 3D environment and the influence of materials, are considered.