Rotation In Quaternion Format

[maher-d]

Hi, I'm just learning scripting and Python
I would like to print out an object's rotation transform in quaternion format (either xyzw or wxyz).
So far I 've got... (with a pCube1 in the scene)

CODE
import maya.cmds as cmds
rotations = cmds.getAttr( 'pCube1.rotate' )
print rotations

outputs...euler rotation
[(0.0, 0.0, 0.0)]

As far as I can tell, the quaternion solution seems to involve using something called MQuaternion() ,described in the API sdk. While Mel is pretty beginner friendly...I find the API sdk really cryptic. What would the equivalent script to above look like using Python and the API to output a quaternion. Or is there infact a Mel command solution?

Hope you can help, Thanks

[joojaa]

Its basically just c++ with python as host language (thats why its hard, its made byprogrammers to programmers). I've tried to document the inner thinking here quite well so you get the idea.

CODE
import maya.OpenMaya as OpenMaya

#make  a object of type MObject
obj=OpenMaya.MObject()

#make a object of type MSelectionList
selList=OpenMaya.MSelectionList()

#add something to it
#you could retrieve this form function or the user selection
selList.add("pCube1")

#fill in the MObject
selList.getDependNode(0,obj)

#check if its a transform
if (obj.hasFn(OpenMaya.MFn.kTransform)):
  #then we can add it to transfrom Fn
  #Fn is basically the collection of functions for given objects
  xform=OpenMaya.MFnTransform(obj)
  #now the api requires pointer types
  x = OpenMaya.MScriptUtil().asDoublePtr()
  y = OpenMaya.MScriptUtil().asDoublePtr()
  z = OpenMaya.MScriptUtil().asDoublePtr()
  w = OpenMaya.MScriptUtil().asDoublePtr()
  #we are ready to ask the values
  xform.getRotationQuaternion(x,y,z,w)
  #convert them back to normal python floats
  x = OpenMaya.MScriptUtil().getDouble(x)
  y = OpenMaya.MScriptUtil().getDouble(y)
  z = OpenMaya.MScriptUtil().getDouble(z)
  w = OpenMaya.MScriptUtil().getDouble(w)
  #and print it out
  print ("QuatRotation is: "+str(x)+" "+str(y)+" "+str(z)+" "+str(w))

Please note asking for the values via getAttr gives you the channels wheras this gets the values fome th actual transformation matrix. As such theese are computed out and the getAttr values are allways what maya stores for users to see ans set.

[maher-d]

Thanks so much! The comments are very clear, and I really appreciate your time. This is something I can play with and see where I get. Clearly the API commands require a deeper understanding of things. Still, it's something I want to get comfortable with. BTW, I discussed this problem with a friend and looked at an alternate solution too...though not as elegant, that is to convert the euler to quaternion in script. Somehow it seems backwards if the values already exist deep down. I'll post this code just for reference and contrast. Thanks again.

CODE
import maya.cmds as cmds
import math

def matrix_to_quat(m):
    w = math.sqrt(1.0 + m[0] + m[5] + m[10]) / 2.0
    w4 = w * 4.0
    x = (m[6] - m[9]) / w4
    y = (m[8] - m[2]) / w4
    z = (m[1] - m[4]) / w4
    return [x, y, z, w]

matrix = cmds.xform('pCube1', q=True, m=True)
quat = matrix_to_quat(matrix)
print 'position:', matrix[12:15]
print 'quaternion:', quat

[joojaa]

even the api does a conversion (tough it may allready have been done by the time the script runs if you sue quat inetrpolation for curves). But you may find that when you start adding up on this that the api has a lot of fuctions that you may need. Since the functions run c code underneath ist much much faster on the long run when you add things up on pure python.