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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.