[vtkusers] odd vtkImageData interpolation behavior

[David Feng]

I noticed some odd behavior when interpolating a vtkImageData object.  
Rather than make a 2D image and rotate it to the XZ plane, I made it a 
3D image with dimensions (x,1,y).  When I call FindCell simply using the 
world-space pixel coordinates, the interpolated values were not what I 
expected.  Taking a closer look at the function, I noticed the following:

return loc[2] * (dims[0]-1)*(dims[1]-1) +
          loc[1] * (dims[0]-1) + loc[0];

...where loc is the interpolated image-space pixel coordinate and dims 
are the dimensions of the image.  The return value is supposed to the 1D 
array index within the image.  With this formula, a pixel location of 
(0,0,5) and (0,0,0) have the same 1D index for an image such as mine (XZ 
instead of XY, with dimensions (x,1,y)).   I haven't looked more 
closely, but indexing into a 3D should actually look like this, if I'm 
not mistaken:

return loc[2] * dims[0] * dims[1] +
          loc[1] * dims[0] + loc[0];

I'm not sure why the dimensions are being decremented in the original 
code.  It would seem to make the indexing off by several pixels for each 
row and slice.  When I made the change, I started getting the values I 
expected.  A simple way to test this is to use a vtkProbeFilter to probe 
an XZ image using itself as the input sample points -- without the 
change, my input is different from my output, which should not be the 
case.   Am I missing something?

Thanks,
David