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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 in1=**snap-file1
**in2=**snap-file2
*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
3*N*-dimensional distance in positions. In other words, the distance vector
between the two snapshots in 3*N*-dimensional configuration space is decomposed
into two components, one parallel to the 3*N*-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 3*N*-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.
**in1=***snap-file1*
- Input data
is read from
*snap-file1*, which must be in SnapShot format.
**in2=***snap-file2*
- Input data is read from
*snap-file2*, which must be in SnapShot format.

snaplist(1NEMO)
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.
snapcmp(1NEMO)
, snapcmphist(1NEMO)
Piet Hut

~src/piet/clib/snapdist.c

10-Jun-88` ` ` ` V1.0: created ` ` ` ` PIET
8-jul-89` ` ` ` V1.1: updated for get_snap()` ` ` ` PJT

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