I have C++ code with a continuous scalar function in three variables and I would like to draw a surface that represents where this function is zero. I've had home success that gives me partial surfaces and I have an idea for starting at a point on the surface and tracing out the rest but I wondered if there is a library that can already do this for me.
Try marching cubes. VTK has an implementation. See also Efficient implementation of marching cubes cases with topological guarantees, which contains a paper and an implementation.