A BSTRACT

1 Introduction

2 Document Image Enhancement Tasks  
2.1 Binarization  
2.2 Deblurring  
2.3 Defading  
2.4 Denoising  
2.5 Shadow Removal  
2.6 Watermark Removal  

3 Datasets

4 Metrics

5 Document Image Enhancement Methods

6 Open Problems and Future Directions  
6.1 Overexposure and underexposure correction tasks  
6.2 Defading task  
6.3 Super-resolution task  
6.4 Binarization task  
6.5 OCR performance evaluation  

7 Conclusion