I am missing some context here. Why does a tile get dirty?
In gimp parlance, a tile is 'dirtied' whenever its pixel data gets written to (okay, that's a bit ambiguous with the tile ref system -- that could mean either when a write-able reference is added to it or when that reference is removed again upon the write completion).
-- Adam D. Moss . ,,^^ adam@xxxxxxxx http://www.foxbox.org/ co:3 busting makes me feel good kthx bye