Access to Cells in VTK.js

Hi All,

My question is: for a given IJK data cell position in vtkImageData, how can
I get access to:
- voxel value,
- voxel bounds (position of all 8 vertices of a voxel)
- all voxels belonging to a given vtkImageSlice.

More detailed description of why I need this: 
I am working on a tool for annotations and segmentations of MRI images using
vtk.js with vtkImageData to represent MRI volume. I tried to use
vtkImageSlice for both original image (grayscale) and overlay (color), but
since there is no transparency for background voxels in vtkImageSlice (i.e.
there is opacity property but no alpha), I had to use a different approach. 

Now, I am using "my own" transparent canvas as segmentation/annotation layer
and vtkImageSlice to represent original image. 
To establish position of annotation I use picker. Using 
- picker.getPickedPositions()[0] 
- picker.getCellIJK() 
I can get both world position and cell position for a picked point. This
works great, including interactions like zooming, pan, windowlevel, slicing
etc. (see picture).
<http://vtk.1045678.n5.nabble.com/file/t342677/annotationTool.png> 

However, I would like to draw on my canvas not only points (as in the
picture), but also full rectangles covering voxels. To do that I need to
know position of 4 vertices of voxel in a given plane and its value (I want
to mark it with different color regarding the value).

Thanks
Andrzej



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