Vector fields may be defined on planar grids or curved surfaces. They are
always associated with a point set, e.g. the vertices of a surface, such each
surface vertex is the base point of a single vector of the vector field.
Different visualization methods allow to display vector fields as a collection
arrows, as integral curves, or as LIC images (Line Integral Convolution).
The centers of the potential are the marked surfaces of the underlying
grid. Keep pressing key 'm' while dragging a small rectangle to mark vertices
inside the rectangle as additional centers. Pressing 'u' allows to deselect
vertices. Keep pressing 'p' and continuously drag a single center.
