## 4 Underwater Image-Enhancement Experiments and Result Analysis
### Ablation Studies  
To quantify the contribution of each component, we ran an ablation study that yielded four simplified versions of the WWSF pipeline: one without the multiscale Retinex with color restoration module (No-MSRCR), one without the gamma-correction step (No-\(\gamma\)), one without the CLAHE-based contrast enhancer (No-CLAHE), and one that substitutes ordinary, unweighted wavelet fusion for the weighted stage (Plain-WF). Each variant, together with the complete WWSF model, was scored with the objective image-quality metrics, the UIQM, AG, and EN (Table 2); the visual output of the model variants is shown in Figure 5a-f.  
A visual comparison of the ablated outputs in Figure 5 reveals distinct impacts of each component. The No-MSRCR variant enhances edge sharpness to some extent; however, due to the absence of the multiscale Retinex with color restoration module, the overall tone appears grayish and the local contrast is subdued. This highlights the essential role of the MSRCR module in suppressing underwater color casts (e.g., bluish-green bias) and restoring the intrinsic color of submerged objects. The No-\(\gamma\) version improves color saturation, yet fails to appropriately compress the overall luminance, leading to  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Ablation Model**} & \multicolumn{3}{c}{**Figure 1**} & \multicolumn{3}{c}{**Figure 2**} & \multicolumn{3}{c}{**Figure 3**} & \multicolumn{3}{c}{**Figure 4**} \\  & **UIQM** & **AG** & **Entropy** & **UIQM** & **AG** & **Entropy** & **UIQM** & **AG** & **Entropy** \\ \hline No-MSRCR & 1.172 & 4.166 & 6.695 & 0.815 & 5.451 & 7.100 & 0.946 & 3.737 & 7.312 & 0.575 & 4.157 & 7.293 \\ No-GAMMA & 1.276 & 6.228 & 7.346 & 1.264 & 7.727 & 7.508 & 1.000 & 3.665 & 7.628 & 1.021 & 4.657 & 7.686 \\ No-CLAHE & 1.295 & 6.027 & 7.420 & 1.176 & 7.968 & 7.188 & 1.005 & 3.168 & 7.256 & 1.000 & 3.942 & 7.424 \\ Ordinary Fusion & 1.271 & 6.173 & 7.280 & 1.167 & 7.968 & 7.671 & 1.039 & 4.361 & 7.555 & 0.989 & 5.320 & 7.544 \\ Full model & 1.460 & 12.474 & 7.366 & 1.477 & 15.510 & 7.726 & 1.201 & 8.731 & 7.594 & 1.229 & 8.524 & 7.577 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Quantitative assessment of ablation studies.  
Figure 4: Line graphs of metrics for experimental comparison results: (**a**) UIQM; (**b**) AG; (**c**) Entropy.  
overexposed regions. This emphasizes the compensatory effect of gamma correction in adjusting the dynamic range and suppressing localized glare.  
In the No-CLAHE version, while the colors are vivid, the fine structural textures appear less prominent. This indicates that the CLAHE module is critical for enhancing local contrast and edge definition, particularly under uneven illumination and turbid water conditions where hydraulic structural details may otherwise remain obscured. The Plain-WF result approximates the output of the complete model but lacks significant texture reinforcement, suggesting that the introduction of a weighted fusion mechanism effectively strengthens scale-specific texture cues--beneficial for highlighting microstructural features such as concrete spalling and cracks.  
By contrast, the complete WWSF model achieves a well-balanced enhancement across color, brightness, and texture domains. The MSRCR module provides global illumination compensation and color fidelity; gamma correction suppresses local overexposure; CLAHE enhances local detail and clarity; and the weighted wavelet fusion aggregates multiscale information to preserve structural integrity. This collaborative effect among modules is particularly advantageous in underwater environments with complex lighting and significant scattering, delivering visually reliable inputs for downstream hydraulic defect identification tasks.  
Table 2 confirms these observations: the full model attains the highest or second-highest scores for nearly every metric, demonstrating that each module makes a positive, measurable contribution to the overall performance.