The Warp brush

The Warp brush is available by the two last modes of the Special Brush.
There are two types of warps: warp out and warp in.

It may be used to give the impression that a drop of water has fallen on the image. Combined with certain types of paper, it gives the impression that you are looking at the image through a window (see below).

Warp in at top, warp out at bottomOriginal imageUse of this tool on an image