### 2.6.5 Sphere Sweep Object

A Sphere Sweep is the envelope of a moving sphere with varying radius, or, in other words, the space a sphere occupies during its movement along a trajectory in space. This shape was first introduced to raytracing by J.J. van Wijk. Sphere Sweeps are modeled by specifying a list of single spheres which are then interpolated. The current implementation allows for three kinds of interpolation:

- Interpolating the input data with a linear function, which means that the single spheres are connected by straight tubes.
- Approximating the input data using a cubic B-Spline function, which results in a curved object.
- Approximating the input data using a cubic spline, which results in a curved object.

A code example:

    sphere_sweep {
      linear_spline,
      4,
      <-5,-5, 0>, 1
      <-5, 5, 0>, 1
      < 5,-5, 0>, 1
      < 5, 5, 0>, 1
    }