Function representation

Function Representation (FRep or F-Rep) is used in solid modeling, volume modeling and computer graphics. FRep was introduced in "Function representation in geometric modeling: concepts, implementation and applications" as a uniform representation of multidimensional geometric objects (shapes). An object as a point set in multidimensional space is defined by a single continuous real-valued function of point coordinates ${\displaystyle f(x_{1},x_{2},...,x_{n})}$ which is evaluated at the given point by a procedure traversing a tree structure with primitives in the leaves and operations in the nodes of the tree. The points with ${\displaystyle f(x_{1},x_{2},...,x_{n})\geq 0}$ belong to the object, and the points with ${\displaystyle f(x_{1},x_{2},...,x_{n})<0}$ are outside of the object. The point set with ${\displaystyle f(x_{1},x_{2},...,x_{n})=0}$ is called an isosurface.

...
Wikipedia