Good to see you on the front page, Varun. :- )

It's possible to do this somewhat efficiently beyond two balls with GLSL and lots of uniforms (or a UBO), since metaballs from the graphics perspective are really just distance fields.

If you want more than a few balls, you can do it in two passes: one to produce the distance field, and one to threshold it.

As an added benefit, it's straightforward to generalize these approaches to any two-dimensional continuous function.