My original posting on this question generated quite a few replies,
for which many thanks.
I found the most useful pointer to the required algorithm was the link
to Mark Gerstein's site given by Ian,
and I have now programmed this in a C++ class using Clipper routines,
which I can make available if anyone wants it.
There are many programs which will give the direction of the rotation
axis, but defining a point lying on the axis, such that the minimal
translation is just along the axis (ie a screw) is a bit more elusive.
Thanks
Phil
On 29 Jul 2008, at 12:30, Ian Tickle wrote:
Phil
What I suggested works only if the point x is transformed onto itself,
i.e. there's no screw component.
The general solution is here:
http://bioinfo.mbb.yale.edu/geometry/screw-axis/
There may be a neater way of deriving this in the general case using a
homogeneous matrix & co-ordinates but I haven't worked it out yet!
Cheers
-- IAn
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Phil Evans
Sent: 29 July 2008 09:11
To: CCP4BB@JISCMAIL.AC.UK
Subject: Rotation axis
If I've go a superposition transformation (x' = Rx + t), as
it happens
from a superposition in ccp4mg, how do I get the position &
direction
of the rotation axis (to draw in a picture)?
I know that any (orthonormal) transformation can be represented as a
rotation about an axis + a screw translation along that axis
I'm sure I've done this before ...
thanks
Phil
