[Insight-users] Understanding why the direction of the transform mapping is from fixed to moving image

[Anja Ende]

Hello everyone,

I was going through the ITK software guide and came across this bit:

"If we have used the Transform that maps coordinates from the moving
image physical space
into the fixed image physical space, then the resampling process could
not guarantee that every
pixel in the grid of the fixed image was going to receive one and only
one value".

I am having a bit of problem understanding why this would be the case.
So, we have a transform (the inverse of the transform from
fixed->moving) that will map each point in the moving image to each
physical point in the fixed image. I am not sure how the unambiguity
of the fixed image pixel not receiving one and only one value arises.

Would anyone be kind enough to explain this a bit?

many thanks,

Anja