using om.MVector.rotateBy

[sam williams]

Hello !

im trying to use a rotateBy method of the MVector class. I want to use the one that allows you to rotate a vector based on a given axis and angle. I have an axis and an angle but im really clueless about how to actually write out the code

om.MVector.rotateBy( axis, angle)   but the reference book says i need to use, kXaxis,kYaxisk,Zaxis,kWaxis to specify the axis.

here is ref manual: [url]http://download.autodesk.com/us/maya/2009help/API/class_m_vector.html#5fced20440d20185407aa5707ea1801c[/url]

could anyone help me out;)

thanks alot guys, 

Sam :scream:

[Pedro Bellini]

Hi,
on the axis,angle function you should use one of MVectors defaults, as the doc says..
so I think what you want would be something like:

import maya.OpenMaya as om

vecX = om.MVector(1,0,0)
print vecX.x

# if we want to rotate the vecX on the Y axis by 180 degrees
rotVec = vecX.rotateBy(om.MVector.kYaxis, 3.1415926)
print rotVec.x

Keep in mind that you can also use Euler and Quaternion data to rotate the vector

[sam williams]

thanks alot mate, very useful, 

Sam;)

[sam williams]

hey Pedro, as you can only specify x,y and z axis. Is there a way of using a specific vector to rotate around? so if instead of y being (0,1,0) can (-0.12542495647, 0.971342830439, -0.201895235331) be used to represent the y axis to use in the rotateBy method.

thanks, 

Sam

[Marcus Ottosson]

Back in the days, I wrote a Point and Matrix implementation for these purposes, maybe it can help you understand the math behind things. Today, I believe PyMEL provides similar functionality out of the box.

• https://gist.github.com/mottosso/7fa7a8ae6f5fad963b92

Especially see the rotate() method of Point.

Best,
Marcus

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[sam williams]

sweet, thanks alot Marcus 

Sam 

[Pedro Bellini]

Hi Sam, to answer your last question
if you want a different axis to rotate the vector around I probably would use MQuaternions, like

import maya.OpenMaya as om
from maya import cmds
import math

cube = cmds.polyCube(w=10)[0]
dag = om.MDagPath()
sel = om.MSelectionList()
sel.add(cube)
sel.getDagPath(0,dag)

#
transformFn = om.MFnTransform(dag)
vec1 = om.MVector(0,1,0) # rotation in Y vec
angle = 30
radAngle = math.radians(angle)
quat = om.MQuaternion(radAngle, vec1)
transformFn.setRotation(quat)

vec2=om.MVector(1,1,0)
quat2=om.MQuaternion(radAngle, vec2)
transformFn.setRotation(quat2)
# totates 30 degress about the [1,1,0] vector

vec3 = vec2.rotateBy(quat)
print vec3.x, vec3.y, vec3.z
# vec 3 is the vec2 [1,1,0] rotated 30 degrees in vec1 [0,1,0] axis
# therefore vec3 y value should maitain unchanged while
# vec3 x value becomes cos(30)
# vec3 z value becomes -sin(30)

vec4 = vec1.rotateBy(quat2)
print vec4.x, vec4.y, vec4.z
# vec 4 is the vec1 [0,1,0] rotated 30 degrees in vec2 [1,1,0] axis

## note that if you try to rotate vec1 on quat, or vec2 on quat2 the resulting vector will be unchanged
## since you are rotating a vector 30 degrees on its own axis...
## like:
vec5 =vec1.rotateBy(quat)
print vec5.x, vec5.y, vec5.z
vec6 = vec2.rotateBy(quat2)
print vec6.x, vec6.y, vec6.z

# cheers