How to implement Similarity for custom sorting by field ( or by docID)?

[Che Dong]

Hi All: 
In lucene 1.3 rc1 release included a Similarity for custom scoring. Is it possible 
implement a docID based or (some field value based )scoring beside DefautlSimilarity? 

Thanks.

Che, Dong
http://www.chedong.com/tech/lucene.html