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##

FieldLines

The `FieldLines' representation computes lines that trace the result
of integrating the motions of massless particles advected by the
volume gradient vectors associated with each location in a volumetric
dataset. VMD computes the volume gradient map when a volumetric dataset
is initially loaded, and the particle advection routines use a simple
trilinear interpolation of the volume gradients are along with a fast
(but simple) application of Euler's method to advect the particles at
each integration step. The user-adjustable gradient magnitude control
affects which points within the volumetric dataset are considered
candidates for field line seeds. The resulting set of seed points
are the initial points from which particles begin advection/integration.
The min and max length controls affect the minimum and maximum length of
the resulting field lines that will be selected for display. Field lines
shorter than the minimum or longer than the maximum are not displayed.
Similarly, field lines that collide with a critical point in the dataset
early in their integration are discarded.

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