## 3 Experiment on Texture Perception with an Eye-Tracker  
### Stimuli and Apparatus  
Stimuli: This research investigated the influences of global and local features on eye-movement patterns in texture visual-similarity perception. Therefore, textures that were globally and locally similar had to be synthesized for use in this research. We synthesized the textures by separately controlling local (grain) and global (skeleton) parameters using the PGPC texture model. Here, global parameters refer to the directionality and density of skeletons, and local parameters refer to the shapes of the grain. The synthesized textures, their grains, and their skeletons are shown in Figure 1. The different grains of Textures 1 (Figure 1a) and 2 (Figure 1e) generated different local features. The skeleton of Texture 1 was created with horizontal strength (Figure 1b), and the skeleton of Texture 2 was created with diagonal strength (Figure 1f). Both skeletons had the same density, but different directionality. Synthesized textures (Figure 1c,d) were cropped into disks to reduce the visual effects of the horizontal and vertical borders of the texture frames. Using the two synthesized textures, we derived six texture stimuli and arranged them into two scenes, as shown in Figure 2. Synthesized Textures 1 and 2 were labeled as Textures A and B in Scene 1. Standard Texture S was derived by rotating Synthesized Texture 2 by 45\({}^{\circ}\) counterclockwise (Figure 2a). Textures in Scene 2 are presented at twice the magnification (200% zoomed in view of Scene 1) of the respective textures in Scene 1 (Figure 2b). Thus, Texture S in both scenes had the same global features (directionality) as Texture A and the same local features (grains) as Texture B.  
Figure 1: Synthesized textures. Texture 1 (**a**) grain and (**b**) skeleton and (**c**) Synthesized Texture 1; (**d**) Synthesized Texture 2 and Texture 2 (**e**) grain and (**f**) skeleton. Figure 2: (**a**) Scene 1 and (**b**) Scene 2 (200% zoom of Scene 1) in the experiment. The locations of Textures A and B are randomly on the left or right. We synthesized the two other textures on the basis of the same idea as in the above synthesis procedure. The grains used for synthesis were the same as those in Figure 1a,e. However, the skeletons used for synthesizing two textures had the same directionality (with horizontal strength), but different density. The synthesized texture scenes were Scenes 3 and 4, as shown in Figure 3. The density of the skeleton was twice higher in Texture A than that in Texture B in Scenes 3 and 4. A total of four scenes were generated in this experiment (the synthesized textures are available for research upon request). In this experiment, to counterbalance the influences from the order of trials (Scenes 1 and 3 used a similar synthesizing process, so the display of the front trial appeared to influence the subsequent trial) and the presentation in the left and right visual field (some participants may have had attention-related disorders such as spatial hemineglect, so the location of Textures A and B may influence the visual attention), we took two strategies: (1) in both tests, the display orders of two scenes were alternated, and (2) in each scene, the locations of Textures A and B were randomly on the left or right. viewing distance. In order to evaluate the absolute position where local and global features had the same domination, two equations were proposed in [17], as follows:  
\[ \begin{split} D_{1,P_{A}=0.5}&=2.2814\times 2\times d_{1} \\ D_{2,P_{A}=0.5}&=3.3473\times 2\times d_{2}, \end{split}  \tag{3}\]  
where \(d_{1}\) denotes the diameter of the texture in Scene 1, \(d_{2}\) denotes the diameter of the texture in Scene 2, \(D_{1,P_{A}=0.5}\) is the viewing distance at which the probability of selecting Texture A of Scene 1 was 50%, and \(D_{2,P_{A}=0.5}\) is the viewing distance at which the probability of selecting Texture A of Scene 2 was 50%. By the above equations, we estimated the viewing distances for different visual stimuli.