[Insight-users] Going from 3D Image to 4DImage: How to deal with Directions/Spacing

[Kent Williams]

I am running into this with DWI Datasets, which are a series of 3D
volumes, but as it happens, they're packed in a 3D image.

I want to write out a 4D NRRD file.

Just packing in the pixels is no problem -- it's sufficient to just
memcpy from the 3D image buffer to the 4D image buffer.

But Direction Cosines are trickier -- do I just do something like this?

3D:

Dir1.0 Dir2.0 Dir3.0
Dir1.1 Dir2.1 Dir3.0
Dir1.2 Dir2.2 Dir3.2

4D:

Dir1.0 Dir2.0 Dir3.0 0
Dir1.1 Dir2.1 Dir3.0 0
Dir1.2 Dir2.2 Dir3.2 0
0        0        0       1

And spacing is probably OK like this

3D: [ X Y Z ] to 4D: [X Y Z 1]