resampling issue

[qezlou]

Describe the bug
Hi Yu,

It happens a lot that I get this error when trying to resample a mesh, so I end up changing the final grid from the one I really need (not quite happy with that.)

Can we replace this with an interpolation or do you think it would be crazy ?

I should confess I do not quite understand how the current scheme is resampling.

Steps To Reproduce

Converting a mesh of shape (2089, 2089, 75) to (209,209,8)

Version

In [3]: print(pmesh.__version__)
0.1.56

I was actually using nbodykit, but the error seems more relevant to this package.

[rainwoodman]

Chatted offline and general undividing resample() is not very well supported. The current scheme of resample is to transform the image to fourier space, then zero pad / truncate, then transform back.

.downsample and .upsample does resample in real space(and does the interpolation you mentioned). There are 2 ways of interpolation, treating the source pixels as particles / treating the destination pixels as particles. One is downsample and the other is upsample, but I don't immediately recall which is which. :/