Is it possible to have water ripples react to one another when they collide? I am trying to do a Matrix/Independance Day type shield effect, and the results look good with regular ripples as a displacement and compound blurs, but if they reacted with each other the effect would be much more convincing.
Not in particleIllusion -- there are no interparticle collisions.
If you're generating essentially cocentric rings which you then use as your displacement map, if you keep the range of colors (greyscale) at 50% max, then when you combine ripples, you'll get the interference patterns you want.
So instead of rendering ripples that go full white on black, make them go to 50% (grey) instead, so when you add 2 of these ripple images on top of each other, you'll get some areas that add up to full white.
That makes perfect sense, on small hitch would be that when you create a displacement map or compound blurs (I'm using AE) the background color needs to be 128,128,128. I could do alphas, but I can never get the nice defined edges that I see in PI.
Actually, I know exactly what I will do, I'll make them exactly as you suggested, but against a black background, and then in AE, simply use Knoll Unmult, the best free plugin ever made. Then it's a simple precomp and it's done.