[MINC-users] mincresample and slice-based scaling

[Alex Zijdenbos]

Hi Andrew,

I ran a few more tests:

1) mincresample -float, to generate float output
2) convert the input volume to float first
3) combination of the two options above

in all cases the result is the same expanded output range. Also in all
cases the float output volume has the slice scaling ranges set (that may be
fixed in a subsequent MINC releases I assume, following the discussion on
minc-development).

So I would assume that points towards the actual trilinear interpolation,
not discretization, no?

-- A



On Wed, Mar 20, 2013 at 4:03 PM, Andrew Janke <a.janke at gmail.com> wrote:

> Hi Alex,
>
> My guess is that this is a combination of discretisation (of your
> chosen data type) and the macros hidden deep within MINC that do the
> tri-linear interpolation. Probably more of the former than the latter
> though.
>
> Does the same problem happen when you use a float/double volume?
>
> In any case the discretisation is very minimal and I'd say within
> expected values given the range of a short (5 places of precision)
> matches the error you are seeing.
>
>
> a
>
> On 21 March 2013 03:43, Alex Zijdenbos <zijdenbos at gmail.com> wrote:
> > Hi all,
> >
> > I came across something somewhat unexpected (at least to me), and was
> > wondering if anybody would be able to clarify this:
> >
> > I have a volume with a real range [0,255] (signed short, the valid
> > range=full range of data type, no slice scaling). When I mincresample
> that
> > using a linear xfm and some new starts and steps, I end up with a global
> > range (from mincstats) of [-0.0002770616653,255.0002071]. I am used to
> > seeing over- and undershoots with tricubic interpolation, but I was
> always
> > under the impression that trilinear by definition should maintain the
> > extrema. When I add the -keep_real_range flag to the mincresample call,
> the
> > resulting range is the expected [0,255] (again from mincstats); so this
> > essentially fixes the problem. But why would the voxel range change when
> > using trilinear interpolation? Precision issues? Slice scaling issues?
> > Extrapolation instead of interpolation perhaps?
> >
> > -- A
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