## 2 Materials and Methods
### Image Acquisition and Processing  
#### 2.2.1 Image Acquisition  
In this study, a systematic approach was used to collect water-sensitive test paper image data to ensure the reliability and representativeness of the experimental results. The water-sensitive test paper model 20301-1N produced by Syngenta Crop Protection AG (CH-4002, Basel, Switzerland) was used in this study, which is widely acclaimed for its stability and sensitivity in agricultural spraying applications.  
In order to simulate the actual agricultural environment, a T50 agricultural drone (model: 3 WWDZ-40B) from DJI Innovative Technology Ltd. (Shenzhen, China) was used  
Figure 1: Overall flow chart.  
for the spraying operation. This model UAV (see Table 2 for model parameters) ensures uniform and controlled exposure of water-sensitive paper during spraying. During the spraying process, in order to reduce the non-uniformity of water droplet distribution, the test was conducted in an environment with low wind speed and stable wind direction to reduce the effect of wind on water droplet distribution [18]. All WSP samples were obtained from a field trial conducted on 15 July 2024 in the commercial orchard of Beituja, Youlan Town, Nanchang County, Nanchang, Jiangxi Province (116\({}^{\circ}\)4\({}^{\prime}\)1\({}^{\prime\prime}\) N, 28\({}^{\circ}\)33\({}^{\prime}\)45\({}^{\prime\prime}\) E). The experimental design is shown in Figure 2. Three fruit trees were selected for sampling, and a zonal sampling strategy was used to assess the spray distribution. The whole tree was divided into three classes along the height and width of the canopy, and 10 sampling points were uniformly distributed within each contour level, and the water-sensitive paper was on the sampling surface facing the direction of the UAV spray at each point, and the three fruit trees were selected with a spacing of 3-5 m. A total of 480 samples of valid WSPs were collected in the experiment. Fog-droplet-image acquisition was carried out in the laboratory using an industrial camera to image the collected WSPs samples. The industrial camera model is RER-USB16 MP01-AF70, the specific model parameters are shown in Table 3, and a fixed lens was used for the image acquisition. During the image acquisition, uneven light, too strong light, or too weak light will affect the visibility of the fog droplets on the WSPs, which will affect the contrast and clarity of the image. For this reason, the equipment was used in a closed environment and equipped with an artificial light source to ensure the lative stability of light intensity and the quality of the captured images. A total of 420 images of WSPs were collected.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Item** & **Parameter** \\ \hline Total Weight & 40 KG \\ Overall Dimensions & 2800 \(\times\) 3085 \(\times\) 820 mm \\ Number of Nozzles & 4 \\ Spraying Width & 8 m \\ Maximum Chemical Capacity & 40 L \\ Operating Speed & 0–5 m/s \\ Operating Height & 1.5–3.5 m \\ \hline \hline \end{tabular}
\end{table}
Table 2: 3 WWDZ-40B model plant protection UAV product parameters.  
Figure 2: Test sample collection. The UAV was operated at a flight speed of 3–5 m/s and at a flight height of 1.5–3 m above the crop canopy. WSPs were placed on the leaves, and WSPs were placed on the sampling plane facing the direction of the UAV spray at each point.