# A Display-Adaptive Pipeline for Dynamic Range Expansion of Standard Dynamic Range Video Content
## 4 Results and Evaluation
### HDR-to-xHDR Conversion: Subjective Evaluation
Ten non-expert participants, aged between 21 and 65, were involved in our subjective experiment. The experiment was conducted in a dimly lit room, adhering to ITU-BT.500-14 recommendations. To minimize participant fatigue, we divided the experiment into three separate sessions. Figure 18 shows the results of our subjective experiment. To clarify the data representation in the graph, we assigned numerical values to each possible user selection: 0 for HDR, 1 for both (indicating no preference), and 2 for xHDR. This numerical representation was chosen to provide a clearer visualization of user preferences and their variability across different video sequences. The bars in the graph indicate the interquartile range to filter outliers in responses, while the diamond symbol (\(\diamond\)) marks the average preference for each video sequence. Figure 19 displays the total selections, reflecting the participants' overall preference for a video sequence type or their perception of equal quality in both sequences. \begin{table}
\begin{tabular}{l l} \hline \hline
**Pipeline Step** & **Parameters Values** \\ \hline \multirow{4}{*}{Dynamic range expansion} & \(L_{\text{w,max}}=4000/6000\) (4000 cd/m\({}^{2}\) peak luminance) \\  & \(m_{\text{i}}=0.214\times 300/\text{max\_lum}\) (Mid-level in) \\  & \(a=1.2\) (Contrast settings) \\  & \(d=1.05\) (Highlights settings) \\  & \(p=0.2\) (Dampening factor) \\ \hline \multirow{4}{*}{Highlights boosting} & Parameters of the fast guided-filter to transfer details on the \\  & binary mask: \\  & \(r=20\) (Local window radius) \\  & \(\epsilon=10^{-2}\) (Regularization parameter) \\  & \(s=4\) (Subsampling ratio) \\  & Parameters to apply the expansion mask: \\  & \(M=2000/6000\) (2000 cd/m\({}^{2}\) for highlights boosting) \\  & \(\alpha=2.0\) (Highlights boosting control) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Parameter values used for HDR-to-xHDR conversion. Figure 18: Results of our subjective experiment with numerical data representation for clarity. User preferences are indicated as 0 for HDR, 1 for equivalent (no preference), and 2 for xHDR, facilitating an easier understanding of variability across video sequences. The graph’s bars depict the interquartile range employed to exclude potential outliers in user responses. The subjective assessment results highlight a distinct preference among users for xHDR video sequences generated through our HDR-to-xHDR conversion pipeline. Our key findings include the following:  
* Among various video sequences tested, participants predominantly favored only four of the original HDR video sequences over our up-converted xHDR version, as demonstrated in Figure 18.
* A closer investigation into specific sequences where the HDR version was preferred revealed that the xHDR versions suffered from temporal artifacts. These artifacts primarily arose from the highlights boosting step, where the generation of unstable expansion masks over time led to inconsistent brightness levels in certain frames. This inconsistency resulted in a flickering effect in the final xHDR video sequences. * Similar to previous findings (LDR-to-HDR conversion pipeline), we noted that users perceive similar quality between HDR and xHDR in certain video sequences. Specifically, in six video sequences, all participants unanimously agreed on the similar quality of both types of video sequences, i.e., the HDR and its xHDR version. * Our previous findings contrast with our LDR-to-HDR pipeline, where similar quality perceptions were mostly confined to dark, low-contrast sequences. However, in the current assessment, not all sequences perceived as equal in quality were characterized by being dark and low contrast. In fact, we found that some dark sequences with high contrast were transformed into xHDR videos of comparable quality to their HDR counterparts. This outcome is likely due to the expansion operation's dependence on features derived from a tone-mapped version of the HDR input, which can be a darker and lower-contrast version of the original image.