Is there a way in MEL or Python in Maya to get one object's position in the coordinate system of another object? I have a camera in a scene that may be rotated in any direction and am trying to measure the distance in its local Z axis to the vertices of various objects in the scene. This obviously needs to be fast, since it will likely be run thousands of times across the scene.
In Maxscript the command would be something like
" in coordsys $camera "
but I have yet to find something like this in Maya. If there's no direct command to do this, does anyone have a way to calculate it using matrix math?
There is no one liner similar to the MXS idiom -- and no easy way to do it in mel. However in Python you can do this fairly easily.
First you need to get the matrix for the coordinate system you want as an MMatrix, which is part of the OpenMaya api. Then get the position you want to check as an MPoint, which is another api class. Here's the cheap way to get them (there are faster methods but they're much wordier):
from maya.api.OpenMaya import MVector, MMatrix, MPoint
import maya.cmds as cmds
def world_matrix(obj):
"""'
convenience method to get the world matrix of <obj> as a matrix object
"""
return MMatrix( cmds.xform(obj, q=True, matrix=True, ws=True))
def world_pos(obj):
"""'
convenience method to get the world position of <obj> as an MPoint
"""
return MPoint( cmds.xform(obj, q=True, t=True, ws=True))
Once you have the matrix and the point, the relative position is simply point times the inverse of the matrix:
relative_position = world_pos('pSphere1') * world_matrix('pCube1').inverse()
print relative_position
# (0.756766, -0.0498943, 3.38499, 1)
The result will be an MPoint, which has 4 numbers (x, y, z and w); the 4th will always be 1 so you can just ignore it, although the math needs it to account for scales and shears.