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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 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.
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.

See Also



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.

See Also

snapcmp(1NEMO) , snapcmphist(1NEMO)


Piet Hut



Update History

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

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