**2. Materials and Methods**

#### *2.1. Samples*

The samples were composed of 180 defective Fuji apples and 50 healthy Fuji apples. Apples with different degrees of defects and healthy apples were picked in November 2021 from a commercial orchard in Beijing. Before capturing apple images, mud points on the apples' surface were washed to avoid mistaking them as defects.
