# Performance Evaluation of Ground AR Anchor with WebXR Device API
## 4 Results and Discussion
### WebXR with Checked Pattern  
Figure 6 shows an image captured through the device after loading the actual model and the 3D model in the WebXR environment. The image was captured when moving to the left (2) and right (3) from the image at (1). When an object was loaded based on the anchor (4) shown in Figure 4a,b, the difference between the position of the real model and the 3D model was checked. In order to calculate the difference between the anchor and model loading accuracy of the WebXR Device API, the image captured only in the real model was considered as ground truth. The captured image had a resolution of 1440 \(\times\) 3088. To calculate the difference image, an evaluation was performed using MATLAB software (Ver.R2020a). Because most calculated images had the same pixel value when the difference image was extracted, the background was identified as black (RGB #000000); the parts where the difference between the ground truth and the WebXR image was a non-zero value in the image. The result derived using Equation (1) is expressed through the difference image in Figure 7; it shows the difference image measured at positions 1, 2, and 3. The results of the experiment show the difference between the ground truth and WebXR images, which shows the accuracy of the WebXR anchor. Figure 7 shows the image with the background removed to obtain the mean absolute error (MAE) of the image captured at positions 1, 2, and 3. MAE is the amount of the error in the measured value; it is the difference between the \"measured value\" and the \"real value\". In this study, the actual value was designated as the ground truth and the measured value was designated as WebXR. In addition, MAE represents a qualitative measure of the accuracy of the statistical estimation; the lower the number, the higher will be the accuracy. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **GT-WR** & **GT-SR** \\ \hline X-axis (cm) & 0.21 & 0.73 \\ \hline Y-axis (cm) & 0.32 & 0.38 \\ \hline Z-axis (cm) & 0.51 & 0.55 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Standard deviation of the comparison of ground truth and WebXR. Figure 6: eXtended Reality image in each direction. In this study, the MAE is a value between 0 and 255. After obtaining images of the real model and 3D model, the background is removed to calculate the mean absolute error of the model. Figure 8 shows images where the background is removed in each direction to obtain the MAE. A binary image is created from the image in which the background is removed. Then, the average absolute error is calculated. Table 3 shows the mean of the difference image. Experimental results are rounded to the fifth decimal place. In weak reflection ground, through experiments conducted for positions 1, 2, and 3, mean differences of 0.1926, 0.7154, and 0.9611, respectively, were obtained. In addition, when normalized as a percentage, results of 0.0756%, 0.2805%, and 0.3770% were confirmed. In strong reflection ground, through experiments conducted for positions 1, 2, and 3, mean differences of 0.2833, 1.3664, and 1.1958, respectively, were obtained. In addition, when normalized as a percentage, results of 0.1110%, 0.5358%, and 0.4689% were confirmed. Figure 8: Background removal image for MAE. Figure 7: Comparison of difference images in WebXR with a checkered pattern. Figure 9 shows the intersection image of the real model images and the mask image of the union image in each direction to obtain the match rate between regions. In this paper, only a part of the mask image obtained through the experiment of the match rate between regions is expressed as shown in Figure 9. The number of pixels in the non-black (RGB #00000) part of each image is extracted. The match rate between regions is calculated using the extracted number of pixels. The match rate uses the ratio of the number of pixels in the intersection image and the number of pixels in the union image. Table 4 shows the match rates between the intersection image and the union image.