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Name

snaprotate - rotate N-body snapshot file about principle axes.

Synopsis

snaprotate in=snap-file out=snap-file [parameter=value] ...

Description

snaprotate rotates the positions and velocities of an N-body snapshot file. A maximum of 3 rotations around either the x, y or z axes can be applied in any order. An Eulerian rotation is a rotation around the z, y and z axes respectively. Some common examples for projections of barred and unbarred galaxies are given in the examples below.

The rotation is counter clock-wise (i.e., mathematically positive, unless invert=f is used) as seen from the right-handed xyz-axes: a z rotation will rotate the x axis towards the y axis, a y rotation will rotate the z axis towards the x axis, and an x rotation will rotate the y axis towards the z axis.

As an example, for a disk in the X-Y plane, a positive Y rotation causes the -X axis to move upwards, towards the observer, i.e. becoming the near side.

An alternate way to specify rotations is using the spinvector= keyword, where all points are rotated around this vector.

Although by default all vectors are rotated by the same matrix, the keyword select= can be used to select any set of Position, Velocity or Acceleration.

Parameters

The following parameters are recognized; they may be given in any order.
in=snap-file
Input data is read from snap-file, which must be in snapshot format.
out=snap-file
Output data is written to snap-file.
theta=x-angle
Angles (in degrees) to rotate particles about the specified axes in the order= keyword. Up to 3 rotations can be applied. Default: none.
order=trans-order
Order to apply the rotations about the three coordinate axes. An Eulerian rotation would be order=zyz. Default is none. See also the spinvector keyword below for an alternate way to specify rotations.
invert=invert-flag
If true, invert specified transformation. Useful for rotating something back to original coordinate system. Default is false.
tscale=t-scale-factor
If used, this is the degrees per time-unit that the theta angles are multiplied with in order to calculate the actual rotation angle. This can give interesting effects if used for simulations. Default not used.
spinvector=x,y,z
Use this vector if you know the vector to rotate around instead of a set of euler angles. Only one angle theta= is needed. Positive rotation is a counterclockwise rotation as seen down the spin vector towards the origin. The vector does not need to be normalized. **note this option still disables various other options such as tscale, invert
select=vec1,vec2,...
Vector variables in a snapshot that should be rotated. Valid options are: pos, vel, and acc. Exact match required for now. [Default: pos,vel,acc]

Examples

For ’normal’ unbarred spiral galaxies the following example shows how to project them into their normaly sky view, taking the sense of rotation into account: 

   set pa=170             # M51, use pa=200 for M33
   set inc=22             # M51, use inc=30 for M33
   set sign=1             # M51, use sign=-1 for M33
   set sinc=‘nemoinp "-$sign*$inc"‘
   mkdisk - 1000 sign=$sign |\
      snaprotate - - theta=$sinc,$pa order=yz |\
      snapgrid - velfie.ccd mom=-1
   nds9 velfie.ccd        # view the velocity field using ’ds9’.fi
With sign=1 it will create a counter clockwise (CCW) rotating disk and project it using -$inc,$pa, For a clock-wise (CW) rotating disk $inc,$pa, hence the somewhat awkward looking expressing to compute sinc. An alternative desription could be to allow the inclination to be negative as well (which observers do not do), where inc<0 means CCW, and inc>0 means CW rotation.

Suppose you have a barred galaxy model, bar.dat, with the galactic plane at z=0 and the bar along the y axis ("position angle=0"), and want to project this (such that z=0 will be the sky plane, and the positive z-axis pointed towards the observer) with given final position angle ($pa), inclination ($inc) and angle from line of nodes to bar ($phi) the following rotations will do the job: 

    set pa=39
    set inc=-45
    set phi=-34
    snaprotate bar.dat n7476.dat "atand(tand($phi)/cosd($inc)),$inc,$pa"
zyz
    snapplot n7476 psize="z>0?0.03:0.0"
where psize was choosen such that ‘‘near’’ (z>0) particles were plotted a bit bigger to accentuate the near side of the model. Flipping the sign if $inc will flip the near and far side.

Geometry


rotation    dPA_near    J_z    sign(J_z)    sinc
CW    +90    away    < 0    inc
CCW    -90    towards    > 0    -inc

Roll-pitch-yaw

Another common way to describe motion (e.g. in an aircraft, robotics) is the Yaw-Pitch-Roll (RPY) convention.

See Also

snapshift(1NEMO) , snapscale(1NEMO) , snapspin(1NEMO) , snapshot(5NEMO)

Author

Joshua E. Barnes,

Peter Teuben

History


xx-jan-88    V1.0 original                       JEB
3-oct-88    V2.1 eulerian angles (order=zyz)    PJT
17-feb-89:      V2.1a new get+snap macros redef’s    PJT
27-nov-89:    V2.1b cosmetic defv[]                 PJT
15-nov-90    V3.0 theta is now an array keyword    PJT
1-jun-92    documented the rotation manner bit more precise         PJT
11-jun-92    V4.0 rotation now defined in math sense                 PJT
21-nov-98    V5.0 added tscale= keyword    PJT
18-nov-05    V5.1a added select=, also implemented select=acc    PJT
22-sep-08    V6.0 added spinvector=    PJT
18-jan-22    fix sense of rotation example    PJT

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