# Reflectance Transformation Imaging as a Tool for Computer-Aided Visual Inspection
## 3 Dynamic Perceptual Assessment for RTI-Based Visual Inspection: Methodology
### Design of Experiment
#### 3.2.2 Psychometric Experiment  
As mentioned before, the approach chosen is based on the principle of the Pair Comparison (PC) method [51]. To reduce the number of image sequences to be evaluated and thus the duration of the experiment, we chose to extend the PC approach to a triple comparison, which made it possible to evaluate the three selected models simultaneously. This approach is detailed below. In addition, to include the dynamic aspects in the proposed methodology, which are of particular importance in the human perception mnechanisms, the psychometric experiments were conducted on rendering videos, for which the reference lighting paths were defined. To conduct the evaluation of the reference videos to the rendered videos using RTI data, the proposed extended version of the Paired Comparison method allowed us to compare the videos according to their resemblance, and due to the simultaneous viewing of the reference-reconstruction pairs, the PC method saved time and reduced the distance quantification. To build the reference videos, a dense acquisition was acquired for each surface in the two measurement scales (two magnification levels). For this experiment, 6 reference acquisitions were acquired (3 surfaces \(\times\) 2 measurement scales). The reference was then displayed simultaneously with the reconstructed videos associated with the DoE parameters and modalities on a divided screen as illustrated in Figure 5. The reference video was always in the top left, and the other three were randomly displayed. As the  
Figure 4: (**a**) Chosen light path. (**b-e**) Acquisition positions associated with parameter \(P_{2}\) modalities. reconstruction model can particularly alter the quality of the reconstruction, we chose to show simultaneously the reconstructions associated with the three models retained for each configuration of the parameters \(P_{2-4}\) of the experimental design. The videos were resized to fill 1/4 of the screen resolution (\(1440\times 2560\)) using bicubic interpolation. For each experimental design configuration, a video containing the four sub-videos (renderings) was generated frame by frame to ensure synchronization. The randomized positioning of the videos allowed us to avoid a potential experimental bias, which was confirmed by the obtained values presented in Table 1. The participants in the experiment then selected, among the three videos associated with the visual reconstructions obtained with the different values of the factors of the experimental design, which one seemed to them to be the closest to the reference video (systematically displayed in the top left). This preferential choice was made by clicking on the chosen video, and it was recorded by the system. The next video, corresponding to the next configuration of the experimental design, was then displayed. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Top Right** & **Bottom Left** & **Bottom Right** \\ \hline PTM (\%) & 4 & 8 & 6 \\ HSH (\%) & 37 & 35 & 39 \\ DMD (\%) & 59 & 56 & 55 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Participants’ preferred choices per reconstruction model and screen zone. Figure 5: User interface for the psychometric experiment (\(S_{1}\) sample). The acquired raw video sequence was presented in the top-left screen zone, and the three other areas were dedicated to the rendering videos using the three tested approximation models (PTM, HSH, and DMD), which were randomly positioned. Each experimental configuration was displayed twice in a random order for each subject to enable the quantification of the intra-subject variation. The experiment was divided into two parts: the training and the main session. For the training session, six sequences were chosen for their temporal and spatial perceptual information to stabilize the observers' opinion following the methodology presented in [50].