Different outputs with rtkfdk when modifying --subsetsize

[SimonRit]

The following four commands produce different outputs:
rtksimulatedgeometry -n 180 -o g
rtkprojectshepploganphantom -g g --dimension 512 -o proj.mha
rtkfdk -p. -r ^proj.mha$ -o fdk16.mha --subsetsize 16
rtkfdk -p. -r ^proj.mha$ -o fdk32.mha --subsetsize 32
The maximum absolute difference between corresponding pixels of fdk16.mha and fdk32.mha is 5.1e-16.

[SimonRit]

I think the problem is that the ramp filter does a 3D FFT with different sizes depending on --subsetsize. This can be reproduced with the new option --subsetsize of the command line tool rtkramp:

rtkramp -p . -r ^proj.mha$ -o ramp16.mha --subsetsize 16
rtkramp -p . -r ^proj.mha$ -o ramp32.mha --subsetsize 32
diff ramp16.mha ramp32.mha

I'm closing this issue because I don't think it is really an issue, just numerical limitations.