...the meta-node. The central idea is that you need to something like a boolean intersection between the coast line mask and the foam pattern so that only bits that are opaque in both will be retained. To achieve that the values of the two need to be mapped into an appropriate value range, multiplied, and mapped back into a standard transparency map range... More in the tutorial. By the way, I forgot to comment on Dave's suggestion above that the mask can be done using the paint material tool in v7.4/v7.5. To a certain degree that's true, but I think this tool is still in its infancy and not flexible enough to generate a nice mask. I didnt just paint the mask with a brush (although that was my first experiment). In fact I didnt use a brush/pencil at all.  I used tools like select by color, grow selection, color fill, and gaussian blur so that there is actually a distance (to shore) dependence. I expect vue's painting will evolve in the future, and it will be really nice to be able to do it all in one application.

"I paint that which comes from the imagination or from dreams, or from an unconscious drive. I photograph the things that I do not wish to paint, the things which already have an existence."
Man Ray, modernist painter
http://artpearl.redbubble.com/