Problem with compose-dofs and approximate option

[iimog]

When I try to compose two dof files with the -approximate option, the result is not as expected. Attached are all the files from this short reproducible example.
compose-dofs-reprex.zip

mirtk register im1.nii.gz im2.nii.gz -parin my.cfg -dofout 1_to_2.dof.gz
mirtk register im2.nii.gz im3.nii.gz -parin my.cfg -dofout 2_to_3.dof.gz
mirtk compose-dofs 1_to_2.dof.gz 2_to_3.dof.gz 1_to_3.dof.gz
mirtk compose-dofs 1_to_2.dof.gz 2_to_3.dof.gz 1_to_3_appr.dof.gz -approximate

While the first two dof files have a "control point lattice" of 29x29 and the 1_to_3.dof.gz contains both transformations, the 1_to_3_appr.dof.gz has just a single transformation with a lattice of 1x1. Also, applying transform-image on im1.nii.gz with the 1_to_3.dof.gz has the intended result while applying it with 1_to_3_appr.dof.gz has almost no effect.

As I need a dof file with a single transformation inside, I use

mirtk convert-dof 1_to_3.dof.gz 1_to_3_single.dof.gz

This works as intended.

I think, this problem has previously been reported on Gitter by @karllandheer on Feb 19 2021 22:14

I'm using the latest version of mirtk, compiled today (mirtk rev 973ce2f, built on Aug 5 2022)

Outputs of `mirtk info`

❯ mirtk info 1_to_2.dof.gz 
Multi-level FFD:
  Global transformation:
    tx  =   0.0000  ty  =   0.0000  tz  =   0.0000
    rx  =   0.0000  ry  =   0.0000  rz  =   0.0000
    sx  = 100.0000  sy  = 100.0000  sz  = 100.0000  
    sxy =   0.0000  syz =   0.0000  sxz =   0.0000  
  Local transformation (passive):
    3D B-spline FFD:
      Number of DOFs:          2523
      Number of CPs (active):  441
      Number of CPs (passive): 400
      Extrapolation mode:      Const
      Control point lattice:
        Size:             29 x         29
        Spacing:    10.00000 x   10.00000
        Origin:  [ -63.92842 ,   33.27434 ,  -54.88671 ,    0.00000]
        X-axis:  [  -0.61921 ,   -0.66309 ,   -0.42057]
        Y-axis:  [   0.44986 ,    0.13942 ,   -0.88215]
        Z-axis:  [  -0.64358 ,    0.73544 ,   -0.21197]


❯ mirtk info 2_to_3.dof.gz
Multi-level FFD:
  Global transformation:
    tx  =   0.0000  ty  =   0.0000  tz  =   0.0000
    rx  =   0.0000  ry  =   0.0000  rz  =   0.0000
    sx  = 100.0000  sy  = 100.0000  sz  = 100.0000  
    sxy =   0.0000  syz =   0.0000  sxz =   0.0000  
  Local transformation (passive):
    3D B-spline FFD:
      Number of DOFs:          2523
      Number of CPs (active):  441
      Number of CPs (passive): 400
      Extrapolation mode:      Const
      Control point lattice:
        Size:             29 x         29
        Spacing:    10.00000 x   10.00000
        Origin:  [ -63.92842 ,   33.27434 ,  -54.88671 ,    0.00000]
        X-axis:  [  -0.61921 ,   -0.66309 ,   -0.42057]
        Y-axis:  [   0.44986 ,    0.13942 ,   -0.88215]
        Z-axis:  [  -0.64358 ,    0.73544 ,   -0.21197]


❯ mirtk info 1_to_3.dof.gz
Fluid FFD:
  Global transformation:
    tx  =   0.0000  ty  =   0.0000  tz  =   0.0000
    rx  =   0.0000  ry  =   0.0000  rz  =   0.0000
    sx  = 100.0000  sy  = 100.0000  sz  = 100.0000  
    sxy =   0.0000  syz =   0.0000  sxz =   0.0000  
  Local transformation (passive):
    3D B-spline FFD:
      Number of DOFs:          2523
      Number of CPs (active):  441
      Number of CPs (passive): 400
      Extrapolation mode:      Const
      Control point lattice:
        Size:             29 x         29
        Spacing:    10.00000 x   10.00000
        Origin:  [ -63.92842 ,   33.27434 ,  -54.88671 ,    0.00000]
        X-axis:  [  -0.61921 ,   -0.66309 ,   -0.42057]
        Y-axis:  [   0.44986 ,    0.13942 ,   -0.88215]
        Z-axis:  [  -0.64358 ,    0.73544 ,   -0.21197]
  Local transformation (passive):
    3D B-spline FFD:
      Number of DOFs:          2523
      Number of CPs (active):  441
      Number of CPs (passive): 400
      Extrapolation mode:      Const
      Control point lattice:
        Size:             29 x         29
        Spacing:    10.00000 x   10.00000
        Origin:  [ -63.92842 ,   33.27434 ,  -54.88671 ,    0.00000]
        X-axis:  [  -0.61921 ,   -0.66309 ,   -0.42057]
        Y-axis:  [   0.44986 ,    0.13942 ,   -0.88215]
        Z-axis:  [  -0.64358 ,    0.73544 ,   -0.21197]
  Affine post-transformation:
    tx  =   0.0000  ty  =   0.0000  tz  =   0.0000
    rx  =   0.0000  ry  =   0.0000  rz  =   0.0000
    sx  = 100.0000  sy  = 100.0000  sz  = 100.0000  
    sxy =   0.0000  syz =   0.0000  sxz =   0.0000  


❯ mirtk info 1_to_3_appr.dof.gz 
Fluid FFD:
  Global transformation:
    tx  =   0.0000  ty  =   0.0000  tz  =   0.0000
    rx  =   0.0000  ry  =  -0.0000  rz  =   0.0000
    sx  = 100.0000  sy  = 100.0000  sz  = 100.0000  
    sxy =   0.0000  syz =   0.0000  sxz =   0.0000  
  Local transformation (passive):
    3D B-spline FFD:
      Number of DOFs:          3
      Number of CPs (active):  1
      Number of CPs (passive): 0
      Extrapolation mode:      Const
      Control point lattice:
        Size:              1 x          1
        Spacing:     0.00000 x    0.00000
        Origin:  [ -63.92842 ,   33.27434 ,  -54.88671 ,    0.00000]
        X-axis:  [  -0.61921 ,   -0.66309 ,   -0.42057]
        Y-axis:  [   0.44986 ,    0.13942 ,   -0.88215]
        Z-axis:  [  -0.64358 ,    0.73544 ,   -0.21197]
  Affine post-transformation:
    tx  =   0.0000  ty  =   0.0000  tz  =   0.0000
    rx  =   0.0000  ry  =  -0.0000  rz  =   0.0000
    sx  = 100.0000  sy  = 100.0000  sz  = 100.0000  
    sxy =   0.0000  syz =   0.0000  sxz =   0.0000  


❯ mirtk info 1_to_3_single.dof.gz 
Multi-level FFD:
  Global transformation:
    tx  =   0.0000  ty  =   0.0000  tz  =   0.0000
    rx  =  -0.0826  ry  =  -0.0266  rz  =  -0.0061
    sx  = 100.0275  sy  =  99.9523  sz  =  99.9112  
    sxy =  -0.0525  syz =   0.0757  sxz =  -0.0178  
  Local transformation (passive):
    3D B-spline FFD:
      Number of DOFs:          2523
      Number of CPs (active):  841
      Number of CPs (passive): 0
      Extrapolation mode:      Const
      Control point lattice:
        Size:             29 x         29
        Spacing:    10.00000 x   10.00000
        Origin:  [ -63.92842 ,   33.27434 ,  -54.88671 ,    0.00000]
        X-axis:  [  -0.61921 ,   -0.66309 ,   -0.42057]
        Y-axis:  [   0.44986 ,    0.13942 ,   -0.88215]
        Z-axis:  [  -0.64358 ,    0.73544 ,   -0.21197]