[Paraview-developers] Extracted selection moves with choice of extracted block (Paraview 4.1)

[redkite]

Hi all,

I'm writing a plugin which attaches boundary conditions to voxel data 
for FE analysis. We wish to represent our data as a multi-block in order 
to allow points/conditions to be defined on the surface of each block.

Let me know if there's another way -- but using the 'Select Points On' 
option with 'Extract Selection' button to define a set of points seems 
to have some strange behaviour with respect to multi-blocks. In the 
Paraview interface, under the Information tab, selecting and extracting 
points in this way generates a multi-block dataset but my plugin filter 
(which operates on the extracted object) actually receives 
vtkUnstructuredGrid data.

Stranger still (it seems to me), if the point selection is made across 2 
blocks, instead of receiving a multi-block with 2 blocks, my 
'add-boundary-condition' filter is called twice with different chunks of 
of vtkUnstructuredGrid data. Since these arrive in no particular order, 
it is difficult to know how best to reassemble them. A static member 
might do it, but would also get mixed with other conditions. Clumsy, but 
it might be made to work.

I then thought using the extract block filter might make things easier, 
but the extracted selection of points seems to move around with the 
choice of block. Here's an example to illustrate:-

1) Load the can.ex2 example and apply the 'Extract Block' filter.
2) Navigate to the 'Block indices' menu and switch on 'Unnamed block ID 
1' so you can see the semi-circular can object.
3) Choose 'vtkCompositeIndex' in the 'Solid Color' drop-down.
4) Click the 'Select Points On' selection button and draw a rectangle of 
points on the can.
5) Click the 'Extract Selection' button.
6) Go back to the 'Block indices' menu, switch off 'Unnamed block ID 1' 
and turn on 'Unnamed block ID 2'
7) Adjust the opacity if the selection is hidden, but you should the 
selection move between blocks as you move from ID 1 to 2 and back (if 
both are on the selection is hidden). Making another selection & 
extraction on the rectangular block object will move to the can, and so on.

Maybe I need to write another extraction procedure which acts as a 
source ie. cannot then be altered? Or has this perhaps been fixed in the 
next release?

regards,
Richard

ps I see this behaviour both on Ubuntu 12.10 and Windows 7.