[Paraview] vtkTransmitImageDataPiece question

[Jeff Becker]

Hi. I'm trying to run a Python Programmable source across four nodes 
(started with mpirun -np 4 pvserver). My script contains the following 
code, but the output looks wrong, so I'm not sure if the extents are 
right.  The partitioning among nodes is correct (using processId 
filter). Also I'm careful to set Output Data Set Type to vtkImageData.

import vtk
import vtk.util.numpy_support as ns
contr = vtk.vtkMultiProcessController.GetGlobalController()
nranks = contr.GetNumberOfProcesses()
rank = contr.GetLocalProcessId()

...
executive = self.GetExecutive()
outInfo = executive.GetOutputInformation(0)
updateExtent = [executive.UPDATE_EXTENT().Get(outInfo, i) for i in 
xrange(6)]
imageData = self.GetOutput()
imageData.SetExtent(updateExtent)
myout = ns.numpy_to_vtk(bmag.ravel(),1,vtk.VTK_FLOAT)
myout.SetName("B field magnitude")
imageData.GetPointData().SetScalars(myout)
tp = vtk.vtkTrivialProducer()
tp.SetOutput(imageData)
tp.SetWholeExtent(0, mesh.MX-1, 0, mesh.MY-1, 0, mesh.MZ-1)
xmit = vtk.vtkTransmitImageDataPiece()
xmit.SetInputConnection(tp.GetOutputPort())
xmit.UpdateInformation()
xmit.SetUpdateExtent(rank, nranks, 0)
xmit.Update()

For completeness, here is my requestInformation script (mesh has 
dimensions, and coordinate arrays for each of X,Y,Z)

executive = self.GetExecutive()
outInfo = executive.GetOutputInformation(0)
outInfo.Set(vtk.vtkAlgorithm.CAN_PRODUCE_SUB_EXTENT(), 1)
outInfo.Set(executive.WHOLE_EXTENT(), 0, mesh.MX-1, 0, mesh.MY-1, 0, 
mesh.MZ-1)
xspacing = (mesh.xcoords[-1] - mesh.xcoords[0])/mesh.MX
yspacing = (mesh.ycoords[-1] - mesh.ycoords[0])/mesh.MY
zspacing = (mesh.zcoords[-1] - mesh.zcoords[0])/mesh.MZ
outInfo.Set(vtk.vtkDataObject.SPACING(), xspacing, yspacing, zspacing)

This is what I've been able to come up with after spending much time 
reading documentation and examples, but I've obviously missed something. 
Please advise. Thanks.

-jeff