There is a more in depth explanation of the Houdini part in this PDF: http://valvesoftware.com/publications/2011/gdc_2011_grimes_nonstandard_textures.pdf
Valve doesn't paint Flowmaps as far as I understood that paper. They are importing level geometry and using the geometry normals as influences on a forcefield simulation in houdini (probably plus a general flow direction). the massive advantage of doing it that way is that you can batch through large amounts of flowmaps at…
I did a bunch of experimentation with this stuff. A few things I tried: 1) Wrote a tool for max that would push verts on a grid plane and move them away from colliding verts. That gives you a base "flow" around geo (like rocks in a pond) 2) Wrote an external tool that takes a mask image, and generates a 2d flow offset map…