Creating custom affines for positioning

In prepositioning and fine positioning, the soft tissues are very simplified which can lead to interpolation artefacts.

A possibility to improve the results is to add custom affine functions to refine locally the deformation. To do that:

define groups in your favorite pre-processor with node numbers corresponding to the locations where you want the affines. Name the groups custom_affine__ adding after the name of the PIPER entity. For example: custom_affine__Skin_of_left_free_lower_limb or custom_affine__Skin_of_body_proper. Here is an example for the Child pedestrian model in Ls-Dyna:

*SET_NODE_LIST_TITLE
custom_affine__Skin_of_left_free_lower_limb
$#     sid       da1       da2       da3       da4    solver      
   9920070       0.0       0.0       0.0       0.0MECH
$#    nid1      nid2      nid3      nid4      nid5      nid6      nid7      nid8
  20000955  20000563  20000557  20001160  20000770  20000750  20000840  20000894
  20001237  20001207  20000810         0         0         0         0         0
*SET_NODE_LIST_TITLE
custom_affine__Skin_of_body_proper
$#     sid       da1       da2       da3       da4    solver      
   9920071       0.0       0.0       0.0       0.0MECH
$#    nid1      nid2      nid3      nid4      nid5      nid6      nid7      nid8
  30000137  30000132  30000001  30000497  30000624  30000382  30000489  30000632
  30006548  30000252  30006239  30000237  30006839         0         0         0

Add the groups in an include file that PIPER reads (e.g. groups in affines_namedmetadata.k, and modify main file as in Child_model_affine.dyn to include it.
change the PIPER metadata file (.pmr) to list these groups as generic metadata (i.e. whose function will be based on the name of the groups. Hence, it is very important to name the groups correctly). (example file here: Child_model_custom_affines.pmr)

<genericmetadata name="Auto">
    <keyword kw="*SET_NODE_LIST_TITLE">
        <id>9920070 9920071</id> <!-- custom affines -->
    </keyword>
</genericmetadata>

check the box to enable the custom affines in the parameters before starting the positioning modules (settings_affine)
start a positioning module. You can check if the affines were interpreted correctly by enabling them in the display box (show_affines)

When you position, then affines are used to interpolate the soft tissues, which can help mitigating some of the artefacts. Below is an example of the Child pedestrian model (from left to right) raising a leg without the affines in the pre and fine positioning, and after adding custom affines. Surface artefacts are reduced.

If you want to use affines, you should experiment on how many to use and where to place them (the example above is just to show the feature and should not be used as a guideline).