| Description | I have volume V under matrix M2. Then I decide to tweak the result M2(V), which is displayed on the screen. (When I decide to display V, I of course see M2(V).) I move M2(V) under M1, the tweaking matrix. So now I have in the data module:
+M1
+M2
V
That is: M1(M2(V)). And, I see M1(M2(V)). I decide to harden M2. Then my display changes! I didn't expect that. Here's the reason: the volume V is now under M2', which is now untransformed (free). But M2' is, it turns out, M2 x M1, not M1 x M2. That's why the display changed!
I'd like to think that this is a bug???? |
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| Additional Information | The Python code below defines M1 and M2 in a simpler scenario without a volume.
#################################################################
M1 = vtk.vtkMatrix4x4()
M1.DeepCopy([ 1, 0, 0, 0,
0, 0, 1, 0,
0, 1, 0, 0,
0, 0, 0, 1])
M2 = vtk.vtkMatrix4x4()
M2.DeepCopy([1, 0, 0, 0,
0, 1, 0, 0,
0, 1, 0, 0,
0, 0, 0, 1])
M1xM2 = vtk.vtkMatrix4x4()
vtk.vtkMatrix4x4.Multiply4x4(M1, M2, M1xM2)
print "M1xM2", M1xM2
1 0 0 0
0 1 0 0
0 1 0 0
0 0 0 1
M2xM1 = vtk.vtkMatrix4x4()
vtk.vtkMatrix4x4.Multiply4x4(M2, M1, M2xM1)
print "M2xM1", M2xM1
1 0 0 0
0 0 1 0
0 0 1 0
0 0 0 1
M1ltn = slicer.vtkMRMLLinearTransformNode()
M1ltn.SetName('M1')
slicer.mrmlScene.AddNode(M1ltn)
M1ltn.SetAndObserveMatrixTransformToParent(M1)
M2ltn = slicer.vtkMRMLLinearTransformNode()
M2ltn.SetName('M2')
slicer.mrmlScene.AddNode(M2ltn)
M2ltn.SetAndObserveMatrixTransformToParent(M2)
In Slicer:
(1) Drag M2 under M1 in Data editor (or use Transforms module).
(2) Harden M2, so as to bake left multiplication with M1 onto M2
(3) Now M2 is M2 x M1, which is incorrect (?). It should be M1 x M2. |
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