[Numpy-discussion] nd_image.affine_transform edge effects

[James Turner]

Thanks for the explanation, Travis.

It now looks like there are 2 distinct issues getting mixed up here,
however. First, there is the issue of the mirror symmetry of the spline
algorithm affecting the data towards the edges, as described by Peter and
Travis (this is probably also what Anne is referring to). It is certainly
useful to know about this, but I think this effect *within* the data
boundaries must be relatively subtle compared with what I'm noticing.
Second, there is a reflection 1 pixel *outside* the bounds of the original
data, instead of the blank value I would expect. I think the code is just
calculating one too many rows. I talked to Peter about this a bit more and
he now agrees that it might be a real bug. I'll attach the most relevant
part of his email below, along with my question (quoted).

This doesn't solve the problem of fixing the code, of course, but I think
it's useful to agree what the behaviour should be.

Cheers,

James.

-----

 > Actually, just going back to your last email, it looks like the mirrored
 > values in the output are *outside* the bounds of the input data (by 1
 > pixel), not just close enough for the interpolation window to overlap the
 > edge. In the example from my last email, the value of 1.899 in the 5th
 > column corresponds to a data point beyond the final 1.0 of the input data.

You are right, that values is actually interpolating from a point just outside 
the boundary of the input. Probably that should have been -1, so that you could 
call a bug. Why it calculates that value (and 2 for order=0) I am not sure...