I picked this up on the pigsfly forum. I’ve yet to try it but it looks like a very cool technique. ( I’ve be looking around for a cheap way to do smoke FX. )

The graphicsrunner thread has a detailed explanation.

(in games )”…The standard technique for animated water is scrolling normal maps. The problem with this is that it looks unnatural as water does not uniformly move in one direction. So Valve came up with the idea of using flow maps ( based on a flow viz paper from the mid 90s ). The basic idea of flow maps is that you create a 2D texture that you will map to your water. And this map will contain the flow directions that you want the water to flow, with each pixel in the flow map representing a flow vector. This allows you to have varying velocity ( based on length of the flow vector ), and varying flow directions ( based on the color of the flow vector ). You then use this flow map to alter the texture coordinates of the normal maps instead of scrolling them. …”

Teck Lee Tan has released a free tool for creating flow maps. FlowMap painter is a stand alone application that enables you to easily paint flow maps for real-time textures. Find out more and watch a video on polycount

mattiasvancamp has written a flow map plugin for 3dsMax