[vtkusers] about reslicing a volume

[David Gobbi]

On Mon, 4 Dec 2000, Fabrice Vincent wrote:

> Hi,
> 
> I  read a volume with vtkImageReader. I then would like to view one slice of the volume,  with a
> variable  orientation but always
> passing through a same point (say voxel  P( i,j,k)).
> 
> I think one way to do  this is to use  vtkImageReslice, but my  problem  is :
>   which slice (Zslice)    from the resliced volume is the one  passing through the point P ?

Hi Vincent,

If you only want one slice, then you should set the OutputExtent of
vtkImageReslice to cover only one slice, i.e. something like this:

reslice.SetOutputOrigin(originx, originy, 0.0) 
reslice.SetOutputSpacing(spacingx, spacingy, 1.0)
reslice.SetOutputExtent(XILOW,XIHIGH,YILOW,YIHIGH,0,0)

this will extract a slice that covers the plane:

xmin = originx + spacingx*XILOW,   xmax = originx + spacingx*XIHIGH
ymin = originy + spacingy*YILOW,   ymax = originy + spacingy*YIHIGH

where this plane lies at z = 0.  Note that you can choose originx,originy,
spacingx,spacingy and XILOW,XIHIGH, YILOW,YIHIGH to be anything you
desire.  It is often wisest to choose them so that the defined plane has
its center at coordinate (0,0,0).

So now you just have to find the geometrical transformation which,
when applied to the aforementioned plane, places it so that it
slices through your data at the desired location.  If the plane
has its center at (0,0,0), then your transformation to rotate by
the desired angles and then translate to the coordinates that you want
to be your center of rotation.

If the voxel you want to rotate around is i,j,k then the coordinates of
that voxel are

(xo + xs*i, yo + ys*i, zo + zs*k)

where (xo,yo,zo) = input.GetOrigin()
      (xs,ys,zs) = input.GetSpacing()
and 'input' is reslice.GetInput() i.e. whatever data you are using as
input to vtkImageReslice.  You will have set up the origin and spacing
when you originally read in the data.

Hope this helps.  The most important thing is to think about your images
in terms of (x,y,z) data coordinates, not as (i,j,k) voxel indices.  And
it is also important to be able to fluently convert between coordinates
and indices.

 - David

--
  David Gobbi, MSc                    dgobbi at irus.rri.on.ca
  Advanced Imaging Research Group
  Robarts Research Institute, University of Western Ontario