|   | 1 |  initial version  | 
As Steven pointed out OpenCV doesn't have a global image descriptor. However you can use the local descriptors to form a local one. Please have a look at a previous answer of me at: http://answers.opencv.org/question/8677/image-comparison-with-a-database/#8686 , where you'll find more information how to achieve that. Good luck!
|   | 2 |  No.2 Revision  | 
As Steven pointed out OpenCV doesn't have a global image descriptor. However you can use the local descriptors to form a local one. Please have a look at a previous answer of by me at: http://answers.opencv.org/question/8677/image-comparison-with-a-database/#8686 , where you'll find more information how to achieve that. Good luck!