[cctbxbb] Attitude of rotation, Python to C++

[Graeme.Winter at Diamond.ac.uk]

Thanks Ralf, good to know the difference was real.

It has to be said, spotting this was much easier where transferring code from Python to C++ was not far from trivial, which is a massive help!

Thanks,

Graeme


________________________________
From: cctbxbb-bounces at phenix-online.org [cctbxbb-bounces at phenix-online.org] on behalf of Ralf Grosse-Kunstleve [rwgrosse-kunstleve at lbl.gov]
Sent: 13 January 2012 06:43
To: cctbx mailing list
Subject: Re: [cctbxbb] Attitude of rotation, Python to C++

Hi Graeme,

Ouch... this is an unfortunate mixup. I derived the C++ code from cgkit many years ago without checking. I'm changing it now to be compatible with http://en.wikipedia.org/wiki/Rodrigues%27_rotation_formula
and the Python code in scitbx.matrix.
cctbx rev. 14644, labelit rev. 1923.
Thanks a lot for pointing out the inconsistency.

Ralf


On Thu, Jan 12, 2012 at 4:03 AM, <Graeme.Winter at diamond.ac.uk<mailto:Graeme.Winter at diamond.ac.uk>> wrote:
Hi Folks,

Just started moving code from Python to C++ - and seeing a 40x speedup which is nice - however I noticed something odd.

scitbx.matrix.col.rotate(axis, angle)

has the reversed attitude to

scitbx::vec3<double>.rotate(scitbx::vec3<double>, double angle)

viz:

this python code


           s = (self._ub * hkl).rotate(self._axis, angle)
           q = (s + self._s0).normalize()

           # check if diffracted ray parallel to detector face

           q_dot_n = q.dot(detector_normal)

           if q_dot_n == 0:
               continue

           r = (q * distance / q_dot_n) - self._detector_origin

           x = r.dot(self._detector_fast)
           y = r.dot(self._detector_slow)

           if x < self._limits[0] or y < self._limits[2]:
               continue
           if x > self._limits[1] or y > self._limits[3]:
               continue

           observed_reflection_positions.append((hkl, x, y, angle))


gives the same result as


 scitbx::vec3<double> s, q, r;
 double x, y, q_dot_n;

 /* this is rather weird - the attitude of rotation is back to front in
    the c++ code */

 s = (ub * hkl).rotate(axis, - angle);
 q = (s + s0).normalize();
 q_dot_n = q * normal;

 if (q_dot_n == 0) return false;

 r = (q * distance / q_dot_n) - origin;

 x = r * fast;
 y = r * slow;

 if (x < limits[0]) return false;
 if (y < limits[2]) return false;
 if (x > limits[1]) return false;
 if (y > limits[3]) return false;

 prediction[0] = x;
 prediction[1] = y;


where I had to use -angle. Code is linked into same routines - used identically. With -angle I get numerically identical results (which is nice)

Anyone got any clues on this one?

Thanks,

Graeme
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