可以更改iPhone上某些像素的alpha值吗？

Is it possible to change just a portion of a Sprite's alpha in response to user interaction? A good example of what I mean is iFog or iSteam, where the user can wipe "steam" off the iPhone's screen. Swapping images out wouldn't be feasible due to the sheer number of possibilities where the user could touch and move...Is it possible to change just a portion of a Sp