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snapdist - gives a measure for the distance between two snapshots, with
and without sliding in time to minimize the distance.
snapdist computes the 3-dimensional Euclidean distances
between the positions of corresponding particles in the two snapshots. It
reports five of these N values: the minimum value, the first quartile,
the median, the third quartile, the maximum value; in this order, on a
single line on the screen. On the second line it reports similar but smaller
values, obtained by a linear shift of all particles in one of the two snapshots
along their velocity vectors, in such a way as to minimize their combined
3N-dimensional distance in positions. In other words, the distance vector
between the two snapshots in 3N-dimensional configuration space is decomposed
into two components, one parallel to the 3N-dimensional velocity vector
of one of the systems, and one perpendicular. The perpendicular distance
serves as a basis for the max/min and quartile information on the second
line of output, and on the third line the amount of time sliding is given:
dt = drpar/v , where drpar and v are the absolute values of the 3N-dimensional
vectors of the parallel component of the distance between the snapshots,
and the velocity of one the snapshots, respectively.
Another way of comparing
snapshots is described in snapcmp(1NEMO)
The following parameters
are recognized; they may be given in any order.
An extended version could use acceleration information
as well, either to provide information in velocity space, or to make a
quadratic (rather than linear) sliding in time possible to give a more
accurate decomposition in distances along and perpendicular to worldlines.
- Input data
is read from snap-file1, which must be in SnapShot format.
- Input data is read from snap-file2, which must be in SnapShot format.
10-Jun-88 V1.0: created PIET
8-jul-89 V1.1: updated for get_snap() PJT
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