Fig. 2

The histogram of the color image shown in Fig. 1. The dark gray color with RGB Value of (68, 68, 68) approximates the shadow area, which composes 21.2% (the frequency of this color is displayed on the top right) of the whole image. (Satellite map from Amap Software Co., Ltd, (2023) No.4047, Beijing, China. Image source: DigitalGlobe&spaceview. URL link: https://www.amap.com/).