[chimera-dev] Xform for rotation about a point

[Thomas Goddard]

> From: "Lavu Sridhar" <slavu at bcm.tmc.edu>
> To: "Thomas Goddard" <goddard at cgl.ucsf.edu>
> Subject: Rotation about arbitrary point
> Date: Tue, 7 Jun 2005 16:44:35 -0500
> 
> Hi, Tom,
> 
> I want to rotate a chimera model about 
> an arbitrary point - (ax,ay,az). 
> 
> Basicallly, what I am looking for is 
> something like following:
> 
> xform.rotate(axis, angle, about_point)
> 
> Is that possible?
> 
> Thank you.
> 
> Lavu

Hi Lavu,

  There is not an Xform constructor to rotate about a specified point
although it would be nice and I'll suggest it to Greg Couch
(gregc at cgl.ucsf.edu) who maintains the Xform code.

  Here is how you currently would create a rotation about a center (cx,cy,cz):

    >>> center = chimera.Vector(cx, cy, cz)
    >>> axis = chimera.Vector(ax, ay, az)
    >>> angle = 123.4  # in degrees
    >>> xf = chimera.Xform_translation(center)
    >>> xf.rotate(axis, angle)
    >>> xf.translate(-center)

    # To apply transform to point p to get rp use:
    >>> p = chimera.Point(px, py, pz)
    >>> rp = xf.apply(p)

This transform acts on a point by first translating by (-cx,-cy,-cz) then
rotating about axis (ax,ay,az) by angle, then translating back by (cx,cy,cz).
That achieves the rotation you want about (cx,cy,cz).  I'd suggest making
a function to do this if you need it at several places in your code.

     Tom