New question #449020 on Yade: https://answers.launchpad.net/yade/+question/449020
I'm trying to output some data from clumps for use in an external program (SolidWorks), and I've not been able to reproduce the orientation of the clumps in YADE. As a bit of background, I've seen that the quaternion can be converted into an axis-angle representation, according to this document: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.468.5407&rep=rep1&type=pdf I've noticed during testing that the following code produces unexpected results: ``` from yade import qt import numpy as np def clamp (val, minval, maxval): if val < minval: return minval if val > maxval: return maxval return val O.bodies.append(sphere([0,0,0],0.2,fixed=True,wire=True)) #Reference Sphere sphId = O.bodies.append(sphere([0,0,0],0.2,fixed=False,wire=True)) #Rotating Sphere arccos = np.arccos def Rotate(): global O,clumpId,pi,sphId #O.forces.addRot(sphId,Vector3(pi/10,0,0)) #Rotate about x O.forces.addRot(sphId,Vector3(0,pi/10,0)) #Rotate about y #O.forces.addRot(sphId,Vector3(0,0,pi/10)) #Rotate about z O.engines=[ ForceResetter(), PyRunner(iterPeriod=1,command='Rotate()'), NewtonIntegrator() ] O.step() for i in xrange(0,10): q = O.bodies[sphId].state.ori.normalized() ang = 2*arccos(clamp(q[0],-1,1)) print('\t%g\t%g\t%g\t%g'%(ang,q[1],q[2],q[3])) O.step() ``` It prints the expected angle to rotate with and the vector to rotate about. I would expect that for each of the cases (x,y,and z), it would just change the angle (1st output) and the second three outputs would be the constant i,j, and k unit vectors respectively. When rotationg about the x axis, that appears to be the case, as this is the output: 3.14159 0 0 1 2.82743 0 0 0.987688 2.51327 0 0 0.951057 2.19911 0 0 0.891007 1.88496 0 0 0.809017 1.5708 0 0 0.707107 1.25664 0 0 0.587785 0.942478 0 0 0.45399 0.628319 0 0 0.309017 0.314159 0 0 0.156434 But rotating about the other two axes does not produce the same results: Rotate about y axis: 3.14159 0 0 1 3.14159 0.156434 0 0.987688 3.14159 0.309017 0 0.951057 3.14159 0.45399 0 0.891007 3.14159 0.587785 0 0.809017 3.14159 0.707107 0 0.707107 3.14159 0.809017 0 0.587785 3.14159 0.891007 0 0.45399 3.14159 0.951057 0 0.309017 3.14159 0.987688 0 0.156434 Rotate about z axis: 3.14159 0 0 1 3.14159 0 0.156434 0.987688 3.14159 0 0.309017 0.951057 3.14159 0 0.45399 0.891007 3.14159 0 0.587785 0.809017 3.14159 0 0.707107 0.707107 3.14159 0 0.809017 0.587785 3.14159 0 0.891007 0.45399 3.14159 0 0.951057 0.309017 3.14159 0 0.987688 0.156434 What is going on here? If I'm rotating a sphere from a default orientation about a principal axis, shouldn't the last three outputs on each line be the unit vector of the axis, as is the case with the x-axis? Also, why does the default quaternion include a rotation of pi about the z axis? Regardless of whether this is a bug or something wrong with my understanding, can someone help me figure out a workaround so I can obtain a useful axis-angle representation of a sphere or clump? -- You received this question notification because your team yade-users is an answer contact for Yade. _______________________________________________ Mailing list: https://launchpad.net/~yade-users Post to : [email protected] Unsubscribe : https://launchpad.net/~yade-users More help : https://help.launchpad.net/ListHelp
