1 INTRODUCTION  
2 BACKGROUND  
3 SIMILARITY LEARNING USING SHALLOW STRUCTURES  
4 SIMILARITY LEARNING USING DEEP STRUCTURES  
5 INDEXING MECHANISM FOR EFFICIENT RETRIEVAL  
6 OPTIMIZATION METHODS OF RETRIEVAL  
7 DATASETS AND EXPERIMENTS  
8 CASE STUDY OF CURRENT INDUSTRIAL PRACTICES  
9 CHALLENGES AND FUTURE DIRECTIONS  
10 CONCLUSION  