Hello Abbas,

I believe that SIFT in not a good choose to perform this task, maybe be useful use color or texture descriptor to identitify the lips.

I hope helped you...

Dear Danillo Roberto Pereira,

Thanks. 

Dear Abbas,

As Danilo stated you first should segment the lips with a 'classical' colour segnentation method, e.g. transforming your images in an HSI-colour space and after that applying a Hue ans Saturation segmentation. This will give you the segmented lips. After this pre-processing step you probably can use/apply 'sift' or better 'surf' to create a robust edge/corner feature vector. The rest depends on your concrete application.

Good luck,

Volker

Thanks