Dear Ran,
The non-reference volume is the input volume to the registration command.
It is used to determine the centre of rotation for the decomposition, as a rigid/affine decomposition is not unique but depends on where the rotations (and scales/skews) take the origin to be located. When you specify the volume it takes the centre of this volume to be the rotation. Consequently, the translation values change to reflect this choice of origin (as the same rigid body transformation about different origins implies different translations).
All the best,
Mark
On 13 Dec 2012, at 23:13, Ran Xu <[log in to unmask]> wrote:
> Dear Mark,
>
> I am using the avscale command to decompose an affine matrix to get the translation and rotation angles. What does the 'non-reference volume' mean? Is it a corresponding dwi volume? The translation results are different with and without 'non-reference volume' input. Which translation result should I use? I think the avscale command shouldn't change the original translation part, am I right?
> Thank you so much! Happy holidays in advance!
> Ran
>
> For example, if my original affine matrix corresponding to my vol0000.nii is like this. If I use two ways of avscale, they seems to give me the same rotation, but not the same translation.
> 1.0162 0.0002 0.0079 -2.1265
> -0.0035 1.0297 0.0027 -1.2068
> -0.0078 -0.0134 1.0600 1.3993
> 0.0000 0.0000 0.0000 1.0000
>
> [fsl@localhost affine_matrix]$ avscale --allparams aff_fsl_1.txt
> Rotation & Translation Matrix:
> 0.999965 0.003344 0.007720 -2.126500
> -0.003444 0.999909 0.013011 -1.206800
> -0.007675 -0.013037 0.999886 1.399300
> 0.000000 0.000000 0.000000 1.000000
>
> [fsl@localhost affine_matrix]$ avscale --allparams aff_fsl_1.txt vol0000.nii.gz
> Rotation & Translation Matrix:
> 0.999965 0.003344 0.007720 0.241926
> -0.003444 0.999909 0.013011 1.721465
> -0.007675 -0.013037 0.999886 2.092240
> 0.000000 0.000000 0.000000 1.000000
>
>
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