"invisible" was not the right word, sorry <img src="smileys/smiley9.gif" border="0" align="middle" /> I meant something like... "masked".
For example: I have a sprite, I want it to be "masked" by default (for the player, it would just be invisible) and smoothly unmasked when I put a scope over it, just like the example above but "reverted".
In a game, it could be, I don't know... Maybe some "invisible" ghosts the player would have to reveal with a magical light.
Know what I mean? I think my description is not clear enough...