What does translation mean in registration?

[Xiaojie Guo]

Hi, all

I have a pair of fixed image and moving image. And the transform is itk::AffineTransform, which includes translation, rotation, isotropic scale. In ITK-SNAP, it's like:

My question is, what does translation mean?

I found the source code of ITK-SNAP. Here it is:

// ----------------------------------------------------------------------

// Pinv: a matrix, SVD result of AffineMat(3x3)

// R: a matrix, SVD result of AffineMat(3x3)

// crot: the center of rotation

// cimg: the center of moving image

Vec3 v_tran =  Pinv * (b - cimg) + cimg + R * crot - crot;

// ----------------------------------------------------------------------

I don't understand what this translation is. Is it one of the followings:

1) The translation from the fixed image center to the moving image center

2) The translation from the fixed image origin to the moving image origin

3) Something else?