| Method | Description |
| Geometric-based modification methods | Perform geometric transformations on images, including flipping, rotating, cropping, deforming, scaling, etc. |
| Color-based modification methods | Change the content of the image itself, including noise, blur, color change, erasure, fill, etc. |
| Machine learning methods | Use machine learning methods to generate images purposefully [35] [36] [37] , such as using GAN [38] (generative adversarial networks) to generate confrontation images to expand the training set. |