How can I overlay graphics on particle simulations? For instance if I am representing a liquid or gas with tons of little points how can I make those points look like a liquid or gas? Take Nvidia Flex for instance, they simulate liquids and gases with tons of small points but can render them as realistic liquids and gases instead of small balls.
For rendering of gases, I think the usual approach is to simply render each particle as a tiny disc. Gases don't really coalesce into surfaces like liquids do, so this should produce acceptable results. You could perhaps apply a light blur over the gas layer afterwards to soften it and hide the fact that it is made of discrete elements.
Liquids, on the other hand, tend to coalesce together to form droplets and smooth surfaces, so you need to derive a surface from the particles somehow. One way to do this is to use Metaballs, which also display this behavior and can be tweaked to suit different liquids and particle densities. By interpreting each particle as a metaball, you will have an implicit equation representing your liquid surface. To render this implicit surface you can then use an algorithm like Marching Tetrahedra to convert it to triangles, or utilize Ray Marching to directly render it. (Ray marching can be easily done in realtime in a fragment shader these days.) You can also use this approach for gases if you want a somewhat softer look.
A good reference for game physics is this chapter 4 describes the basic free deformable surfaces (nurbs and bspline are of course cited and treated enough well) fluid and gases are instead treated in chapter five (basically the author derivate simplified model of navier stokes equations, suitable for real time applications).
So actually i guess what i've written commenting your post was correct, i.e. combine the physical deformation of the point that controls the shape of the surface with a ffd computations.
The book i cited should provide somewhere/somehow on the web source code, and itself it cites some example involving the technique i cited. If that is what you need let me know for a more specific explanation.