A simplified particle flow algorithm generates branching
structures starting at the recorded tip positions. Additional
starting points are defined within the estimated volume of the
tree crown. A vertical stack of boxes approximates the tree
crown and bounds particle flow and creation. The step length
of a particle's flow varies with the distance to the nearest
trunk point. The direction of particle motion is a combination
of vectors with one pointing to the nearest trunk point and
the other pointing to a constant pre-defined attractor point.