[Insight-users] ImageWriter does not write Orientation (Direction Cosines)

[Gordon Kindlmann]

hello,

> I'm a bit confused about how the NRRD IO works. Why does it rescale the
> direction cosines at all? I would prefer the assumption that the user 
> has
> set them correctly...

Nrrd is representing a single matrix, the transform from index space to 
world space.  At a later point I may switch using the "spacings" field 
in conjuction with the "space direction" field, so that the "space 
directions" can be true unit vectors, but to me this seemed simplest 
for now (given nrrd's existing use of a per-axis min, max, spacing, and 
centering).

> Say I set the following direction cosines:
>
> 0.286406 0.118831 0.950711
> 0.948371 -0.176269 -0.263669
> 0.136249 0.977143 -0.16318
>
> then the NRRD IO writes:
>
> space directions:
> (0.24613,0.10212,0.817017)
> (0.815007,-0.151481,-0.22659)
> (0.681244,4.88571,-0.815902)
>
> if I read them with the NRRD IO, I read:
>
> 0.286406 0.948371 0.136249
> 0.118831 -0.176269 0.977143
> 0.950711 -0.263668 -0.163181

Hmm.  Let me investigate this.

thanks,
Gordon