ring-diff arrays are not set appropriately for blocksoncylindrical

[KrisThielemans]

See comments in #1006, e.g. #1006 (comment)_

[markus-jehl]

This can be closed, right?

[KrisThielemans]

I think so, but cannot check at the moment. @danieldeidda do you remember?

[danieldeidda]

not sure, the comments say that this was going to be done in another PR but there was no error related to this as that change was reverted

[KrisThielemans]

This was about the initialisation of segment_axial_pos_to_ring1_plus_ring2, in particular

STIR/src/buildblock/ProjDataInfoCylindrical.cxx

Lines 291 to 293 in 4142d59

	const float ring1_plus_ring2_float =
	2*ax_pos_num/get_num_axial_poss_per_ring_inc(s_num)
	-2*m_offset[s_num]/ring_spacing + (get_scanner_ptr()->get_num_rings()-1);

. The use of m_offset is unsafe for a blocks/generic scanner (due to irregular sampling). I believe that the end result of the calculation would still be fine, but this would need to be checked.

For instance for span=1 (which is currently used for blocks/generic), get_ring_pair_for_segment_axial_pos_num(ring1, ring2, seg, axial_pos) can be tested as
seg = ring1 - ring2 (or maybe reversed) and axial_pos = min(ring1, ring2).

[markus-jehl]

This was tested for the NeuroLF scanner geometry, and the function indeed returns the correct results: for seg=5 and axial_pos=5 it returns 5 and 10, and for seg=2 and axial_pos=14 it returns 14 and 16.