We present a data structure for three-dimensional fields C1 continuous in the modelling space. Regular grids storing the field values discretely are combined with a triquadratic approximation filter to define volume objects. This association of a grid and an approximation/interpolation filter allows the field to be defined by a C 1 continuous real function and the surface to be directly visualised from its own equation. We show how accurate and high quality interactive visualisation is obtained during the modelling process, and we explain why the visualisation is faithful to the object definition. We also describe, as an example of application of our data structure, how advanced Boolean operators realised with soft or "functionally controlled "transitions are performed under the influence of an interactive modelling tool.