## 2 Materials and Methods
### Design of the Fuzzy-Control Anti-Cybersickness Intelligent System (FCACIS)  
#### 2.2.1 Fuzzy Sets  
As explained in Section 2.1, during 3D virtual store navigation, perceptual conflicts may occur due to the user's personal factors (including sex, age, visual ability, and susceptibility), hardware interfaces (including displays and controllers), exposure time, movement speeds, and angular rotation velocities, thereby inducing cybersickness. On the basis of these five main factors that induce cybersickness, five input fuzzy sets and one output fuzzy set are established, which are defined as follows: fuzzy set \(\mu P_{m}\) (\(p\)), where \(m\) represents the \(m\)-th membership function of the personal characteristic variable (\(P\)); fuzzy set \(\mu D_{n}\) (\(d\)), where \(n\) represents the \(n\)-th membership function of the hardware interface variable (\(D\)); fuzzy set \(\mu T_{x}\) (\(t\)), where \(x\) represents the \(x\)-th membership function of the exposure time variable (\(T\)); fuzzy set \(\mu V_{y}\) (\(v\)), where \(y\) represents the \(y\)-th membership function of the movement speed variable (\(V\)); fuzzy set \(\mu A_{z}\) (\(a\)), where \(z\) represents the \(z\)-th membership function of the angular rotation velocity variable (\(A\)); and the output fuzzy set \(\mu R_{k}\) (\(r\)), where \(k\) represents the \(k\)-th membership function of the cybersickness symptom value rank variable (\(R\)). Figure 1: Architecture of the virtual store’s intelligent anti-cybersickness design. Fuzzy set membership functions for personal characteristics  
The membership function of the personal characteristic fuzzy set \(\mu P_{m}\left(p\right)\) is constructed by integrating the sub-variables of the personal characteristics, including age, sex, visual ability, and susceptibility, and three membership functions are included: \(\widetilde{P}_{1}\), \(\widetilde{P}_{2}\), and \(\widetilde{P}_{3}\). The definitions of each membership function are as follows:  
1. \(\widetilde{P}_{1}\): Good personal characteristics (Good), which have a slight effect on cybersickness. 2. \(\widetilde{P}_{2}\): Fair personal characteristics (Fair), which have a normal effect on cybersickness. 3. \(\widetilde{P}_{3}\): Poor personal characteristics (Poor), which have a significant impact on cybersickness. The personal characteristic value definition range is [0, 1], where \"0\" means that the induction of cybersickness is very low and \"1\" means that the induction of cybersickness is very high. In this interval, 0.25, 0.5, and 0.75 are crisp values that are used to construct trapezoidal membership functions (\(\widetilde{P}_{1}\) and \(\widetilde{P}_{3}\)) and triangular membership functions (\(\widetilde{P}_{2}\)). The function includes the following:  
1. Statistical characteristic data  
When the user enters the 3D virtual store, in addition to performing character selection before starting, he or she inputs personal statistical characteristic data: age and sex. 2. Visual ability data  
Before entering the 3D virtual store, the user must complete a visual ability test. The test items include the stereoscopic aberration test and stereo motion test. This study used stereoscopic images for testing. The user wore red and green glasses and tested stereoscopic acuity at a certain test distance (80 cm from the screen). Before the test began, the user sat in front of the screen, with the lines of sight of both eyes parallel to the center of the screen. First, the user performed the \"stereoscopic aberration test\", which included a 300 s angle and a 35 min angle. A dot pattern appeared on the test screen, and the user had to identify the three-dimensional shapes on the screen. In the \"stereo motion test\", motion cues and aberration cues were detected separately. In the motion cue test, the user identified the positions of squares jumping toward the left and right on the basis of the dot pattern appearing on the screen.