Matrix fun!

Here's a few snippets I wrote when learning how to interact with matrices via Maya's Python API.

Note: You can get the local and world matrices directly from attributes on a dag node. You can also use the xform command.

In the example image, pCube1 is a child of pSphere1. If the position of both objects in the scene are random, we can find the local and world matrices for pCube1 and the world-space and local-space positions (translate, rotate, scale) of pCube1 with the example bits of code below. The channel box will only give us local or relative values to the objects parent.

Finding a world matrix from a local matrix

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import maya.OpenMaya as om import math transformFn = om.MFnTransform() # Create empty transform function set mSel = om.MSelectionList() mSel.add('pSphere1') mSel.add('pCube1') dagPath = om.MDagPath() mSel.getDagPath(0, dagPath) # Get the dagPath for pSphere1 transformFn.setObject(dagPath) # Set the function set to operate on pSphere1 dagPath transformationMatrix = transformFn.transformation() # Get a MTransformationMatrix object matrixA = transformationMatrix.asMatrix() # Get a MMatrix object representing to local matrix of pSphere1 mSel.getDagPath(1, dagPath) transformFn.setObject(dagPath) transformationMatrix = transformFn.transformation() matrixB = transformationMatrix.asMatrix() # Get a MMatrix object representing to local matrix of pCube1 # Multiply local matrices up from child to parent worldMatrixB = matrixB * matrixA translation = om.MTransformationMatrix(worldMatrixB).getTranslation(om.MSpace.kWorld) # <-- Not sure on this parameter. All I care about is the 4th row in the matrix. rotation = om.MTransformationMatrix(worldMatrixB).eulerRotation() print '\n' print 'pCube1.translate ->', translation.x, translation.y, translation.z print 'pCube1.rotate ->', math.degrees(rotation.x), math.degrees(rotation.y), math.degrees(rotation.z)

We could just call the inclusiveMatrix() method on a dagPath object to retrieve the world matrix (as is done below) but lets just say that for umm...some unknown reason you only had the local matrix to work with.

Finding a local matrix from a world matrix

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import maya.OpenMaya as om import math transformFn = om.MFnTransform() # Create empty transform function set mSel = om.MSelectionList() mSel.add('pSphere1') mSel.add('pCube1') dagPath = om.MDagPath() mSel.getDagPath(0, dagPath) # get dagPath for pSphere1 worldMatrixInverseA = dagPath.inclusiveMatrixInverse() # get the world inverse matrix for pCube1 parent (pSphere1) mSel.getDagPath(1, dagPath) worldMatrixB = dagPath.inclusiveMatrix() # Get the world matrix for pCube1 localMatrixB = worldMatrixB * worldMatrixInverseA # Multiply pCube1's world matrix by it's parents world inverse matrix translation = om.MTransformationMatrix(localMatrixB).getTranslation(om.MSpace.kWorld) # <-- Not sure on this parameter. All I care about is the 4th row in the matrix. rotation = om.MTransformationMatrix(localMatrixB).eulerRotation() print '\n' print 'pCube1.translate ->', translation.x, translation.y, translation.z print 'pCube1.rotate ->', math.degrees(rotation.x), math.degrees(rotation.y), math.degrees(rotation.z)