[Freesurfer] voxel to ras transform written by mri_convert -c

Hi there,

How is the rightmost column in `mri_convert -c` calculated?

I have the following fslhd (relevant fields shown only) in a NIFTI image:

``` dim1 208 dim2 300 dim3 320 qto_xyz:1 0.799690 0.022176 0.001352 -86.687996 qto_xyz:2 -0.021442 0.783122 -0.162051 -62.728546 qto_xyz:3 -0.005816 0.161952 0.783414 -174.620270 qto_xyz:4 0.000000 0.000000 0.000000 1.000000 ```

After `mri_convert -c`, mri_convert reports the following (relevant fields shown only) in the MGZ image:

``` voxel to ras transform: -1.0000 -0.0000 0.0000 128.0225 -0.0000 0.0000 1.0000 -101.4184 0.0000 -1.0000 -0.0000 102.4140 0.0000 0.0000 0.0000 1.0000 ```

Shouldn't the right-most column be: 128.5 -128.5 128.5 1 The numbers are (256+1)/2=128.5

Best, Tashrif